Effects of Changing a Fork on Suspension Geometry

[GoFastGus]

Hi all, I've done some cursory reading on bike suspension geometry, but I still need to learn some more about what it means to convert a bike from rigid to hardtail. I know that some of the factors at play include Axle to Crown Length (A2C), Sag, and Head Angle.

My latest Ebike project is a 2020 Framed Minnesota 2.0, size M (18"). Here's the bike's geometry.
The fork length is 480mm, and the offset is 38mm. By my math, this makes the A2C ~481.5mm. The bike's front fork is rigid. I'm strongly considering DOT tires for this build, which I imagine will act as less of a shock absorber compared to standard fat bike tires. Ergo, I'd like to replace my rigid fork with a suspension fork.

This is the guide I found about fat bike suspension forks, which is where I'm getting a lot of my information about suspension theory.

My two options at the moment are the:

Name:	Manitou Mastodon Comp Gen 3	Wren inverted fatbike fork
Axle to Crown Length (A2C):	524mm	530mm
Fork Length (FL):	~522mm	~528mm
Hub:	15x150mm	15x150mm
Offset:	44mm	45mm
Stanchion:	34mm (not explicitly listed, gleaned from here)	36mm
Travel:	100mm	110mm

If you have other suggestions for comparable fat bike suspension forks, I'd love to hear them.

Now, to the main point of this post: how would changing out this fork affect my suspension geometry?

To figure out how long the fork will be when assembled on the bike, I need to figure out how much sag is going to be introduced. I have limited data for either fork's sag, and I'm trying to extrapolate from what I have. If the sag is linear, then at 20% sag, the Mastodon has an A2C of 504mm and the Wren has an A2C of 508mm.
The above article says that the majority of fat bike frames aren't designed for a sag A2C of above 511mm. I have no independent verification for this claim generally or for my specific bike; I'd love more context or info if anyone has it. I have no conceptual knowledge about sag in relation to bike & rider weight, center of gravity, suspension travel, etc. What does more get me, what does less get me?

Head angle is also a concern of mine, since it factors into rider position along with fork length. Sheldon Brown says the head angle change (in degrees) is represented by the formula: arcsin((old length(mm)-new length(mm))/1000). The stock head angle is 70deg. This means that the head angle using the FL measurement is 67.6deg for the Mastodon and 66.8deg for the Wren. Using a measurement at 20% linear sag (an educated guess from other models, but probably within a few mm), the Mastodon is 68.5deg and the Wren is 68.2deg. According to Sheldon Brown, if the sag measurements are correct, then the change in head angle is on the high end of acceptable. Is this correct, or should I be doing something different in my research or math?

According to this image, getting a longer fork that decreases my head angle would put my seating position further back and higher up. How would that change in weight distribution and center of gravity affect my handling either when accelerating or decelerating or when cornering?

In summary:
I have a fat bike, I want to make it go fast. Suspension seems like a good idea for that.
Adding suspension to a rigid bike's gonna change some geometry, like Axle to Crown length and Head Angle. What's that gonna do to the ride?
I have no clue about total weight or weight distribution for this project yet, but I know sag's gonna factor in there somewhere. How's that gonna work?

 

[amberwolf]

Actual effect would need to be determined experimentally, by riding under your conditions at your speeds, with the new configuration.

In my limited experiences with a few years of experimenting wtih CrazyBike2 (an LWB remote-steer semirecumbent), especially during hte time I was trying to make it work as a racing bike for the Death Races: At slower speeds, a more vertical headtube angle can help stability, but at higher speeds it can be detrimental. A less vertical headtube angle may work the opposite way.


In each geometry, there will also be some condition / speed combination that can cause a "death wobble" where feedback in the steering vs other things causes increasing steering oscilations until a rider can lose control, sometimes actually having hte handelbars yanked out of their hands. That last isn't common in bicycle applications, but isn't uncommon in motorcycles or even some mopeds.



What does "fast" mean to you in actual speed terms?


ATM I can only recommend that you pick a geometry that would be stable for your wheelbase and other conditions at the speeds you will ride at.

In general I find that longer wheelbases make higher speeds more stable.



BTW, if your system will have a front hubmotor, the suspension fork may be subject to forces it wasn't designed for, and may have assorted issues, including excessive stiction during motor operation (or regen braking, if any), Something to consider regarding bump absorption while motoring along.


Also, I can't access your linked "geometry" file. If you attach it to your post then anyone reading hte post can access it.

 

[GoFastGus]

Actual effect would need to be determined experimentally, by riding under your conditions at your speeds, with the new configuration.
At slower speeds, a more vertical headtube angle can help stability, but at higher speeds it can be detrimental. A less vertical headtube angle may work the opposite way.

That's what I was figuring, but like you said, I'll need to try it firsthand.

In each geometry, there will also be some condition / speed combination that can cause a "death wobble" where feedback in the steering vs other things causes increasing steering oscilations until a rider can lose control, sometimes actually having hte handelbars yanked out of their hands. That last isn't common in bicycle applications, but isn't uncommon in motorcycles or even some mopeds.

Re: death wobble in bikes v. mopeds/motorcycles: that's just about the higher speeds making the oscillations more powerful, right?

What does "fast" mean to you in actual speed terms?

50mph+?

ATM I can only recommend that you pick a geometry that would be stable for your wheelbase and other conditions at the speeds you will ride at.
In general I find that longer wheelbases make higher speeds more stable.

That makes sense, it sounds like I have more reading to do.

BTW, if your system will have a front hubmotor, the suspension fork may be subject to forces it wasn't designed for, and may have assorted issues, including excessive stiction during motor operation (or regen braking, if any), Something to consider regarding bump absorption while motoring along.

Not looking for FWD atm, but good to keep in mind!

Also, I can't access your linked "geometry" file. If you attach it to your post then anyone reading hte post can access it.

Sorry abt that, just re-attached it.

 

[neptronix]

Slacker angles have better steering characteristics when going high speed

Motorcycle tires will produce even worse drag than fatbike tires. Yes, they have some inherent suspension but the cost is high friction, so you need to throw more power at it to overcome that.

I've done over 50mph on a downhill bike and even with good geometry, about 50mph is the limit before the wheelbase of the bike ( versus a motorcycle ) starts to produce rickety handling

 

[GoFastGus]

Slacker angles have better steering characteristics when going high speed

Gotcha, so numerically lower angles are better for high speeds?

Motorcycle tires will produce even worse drag than fatbike tires. Yes, they have some inherent suspension but the cost is high friction, so you need to throw more power at it to overcome that.

Gotcha. My understanding is that rotating mass, puncture resistance, and coefficient of friction are some of the biggest difference between bike and motorcycle tires. Is ability to continuously absorb heat also a big difference between the two?

I've done over 50mph on a downhill bike and even with good geometry, about 50mph is the limit before the wheelbase of the bike ( versus a motorcycle ) starts to produce rickety handling

So wheelbase is going to be my bigger limitation for speed as opposed to head angle or fork suspension?

 

[neptronix]

The ability to absorb heat is going to be different, but you live in wisconsin and not phoenix, arizona, so the difference isn't big enough.

On my semi recumbent, with a 16 x 2.5" moto tire, i struggled to hit the low 30's mph on a 900w motor.
Moving to slick bike tire, made hitting 35mph possible and 33mph very easy. The bike was a chore to pedal with the single rear MC tire on it.

Newer MTBs have very slack fork angles like such. whereas 20 years ago the forks were closer to a 90 angle degree. head angle.

By putting a long fork on a bike that wasn't intended for one, you end upgoing somewhat in this direction, which could be good to reduce the steering drama at high speed.

So wheelbase is going to be my bigger limitation for speed as opposed to head angle or fork suspension?

Yeah. Check out the wheelbase of a motorcycle made for highway riding if you want a reference on approximately what you'd need to go well beyond 50mph.

But for a bike frame i'd buy the biggest one you could possibly fit.

I also don't recommend a 50mph capable build in the first place though, with the legal environment being what it is today.

 

[GoFastGus]

The ability to absorb heat is going to be different, but you live in wisconsin and not phoenix, arizona, so the difference isn't big enough.

Gotcha, thanks!

On my semi recumbent, with a 16 x 2.5" moto tire, i struggled to hit the low 30's mph on a 900w motor.
Moving to slick bike tire, made hitting 35mph possible and 33mph very easy. The bike was a chore to pedal with the single rear MC tire on it.

I didn't realize the rolling resistance was that dramatic. That's seriously worth considering.

Newer MTBs have very slack fork angles like such. whereas 20 years ago the forks were closer to a 90 angle degree. head angle.
By putting a long fork on a bike that wasn't intended for one, you end up going somewhat in this direction, which could be good to reduce the steering drama at high speed.
View attachment 380076

Does that change much if I downsize the back wheel? Going from 26 to 20" in the rear, for example, would lower the back end by ~3 inches, which would alter the centerline between the front and rear centers and the bottom bracket.

Yeah. Check out the wheelbase of a motorcycle made for highway riding if you want a reference on approximately what you'd need to go well beyond 50mph.
But for a bike frame i'd buy the biggest one you could possibly fit.

That's a good point, extra wheelbase isn't something you can buy off the shelf easily.

I also don't recommend a 50mph capable build in the first place though, with the legal environment being what it is today.

Cops, insurance, or smth else?

 

[neptronix]

Does that change much if I downsize the back wheel? Going from 26 to 20" in the rear, for example, would lower the back end by ~3 inches, which would alter the centerline between the front and rear centers and the bottom bracket.

Yeah that's a pretty dramatic change. I wouldn't go that far. You also get crank scraping on the ground issues.

Cops, insurance, or smth else?

Legal maximum for ebikes in the USA is 28mph and you'd be doing almost double that, so you'd stick out
I'd only try that if you were in a rural area.

Doesn't help that if have fat tires - looks primarily like a motorcycle on the basis of it's tires from the POV of a cop car. Doesn't help if it's doing 50 also

 

[GoFastGus]

Yeah that's a pretty dramatic change. I wouldn't go that far. You also get crank scraping on the ground issues.

Realistically I'm thinking about a 24" compared to a 26" wheel, but I need to do more reading about wheel size and motor winding and how they compliment each other. Would gaining 2" in the front and losing 1" in the rear significantly alter my geometry?

I'd only try that if you were in a rural area.

On the Madison isthmus, there's a well-connected bus system that you can bring your bike on. When I wanted to leave the isthmus with my previous build, I would ride the bus as far as I could towards my destination then bike the rest of the way, mostly on rural roads. I'm planning on using this upcoming build on the isthmus as little as possible, too much of a theft risk. I'd like to also take this bike on the bus to extend my range of travel, so it'll primarily be ridden in less crowded areas.

Doesn't help that if have fat tires - looks primarily like a motorcycle on the basis of it's tires from the POV of a cop car. Doesn't help if it's doing 50 also

Yeah, true that. Pedals and seating position might help a little with that, but those factors are also up for consideration for this build.

 

[neptronix]

Realistically I'm thinking about a 24" compared to a 26" wheel, but I need to do more reading about wheel size and motor winding and how they compliment each other. Would gaining 2" in the front and losing 1" in the rear significantly alter my geometry?

Yes, it may be too slack, but simulate it first. If you have a fork angle exceeding that of the pictured bike, and 1 1/8 headtube you may be pushing the strength of that headtube interface too far.

For simulating different windings, google the ebikes.ca motor simulator.

On the Madison isthmus, there's a well-connected bus system that you can bring your bike on. When I wanted to leave the isthmus with my previous build, I would ride the bus as far as I could towards my destination then bike the rest of the way, mostly on rural roads. I'm planning on using this upcoming build on the isthmus as little as possible, too much of a theft risk. I'd like to also take this bike on the bus to extend my range of travel, so it'll primarily be ridden in less crowded areas.

Not bad, i'd shoot for lower speed, much bigger battery, and maximum battery so you can avoid the bus altogether

How many miles is it?

 

[amberwolf]

Some serious things to consider for those speeds:

What happens if you hit a pothole, bump, or larger bit of something that falls off a truck you can't avoid?

Do you have a good enough suspension, designed to operate for your overall system/rider weight, and designed to operate for those speeds (reaction time, stiction, flexure), to absorb those impacts?

If not, either the wheel can fail, or the frame or fork, or you could be thrown into the air at those speeds, without wheels to land on (just your skin).


Do you have good enough brakes with sufficient heat dissipation in the calipers and the rotors to repeatedly stop from those speeds without losing braking ability? (fade) If they aren't designed to operate repeatedly and rapidly at those speeds with that amount of mass (you, bike, all the stuff on it, etc), they probably wont' do what you need them to.


Do you have enough traction on the tires to keep grip on the road while braking at those speeds? (skid means loss of control, skid on front wheel means loss of directional control so you hit whatever was in front of you that you couldn't stop in time for, at whatever speed you still have)

Are the tires thick enough so when you do skid they aren't simply sheared thru by the friction at those speeds?


Is your frame and fork strong enough to deal with or prevent the twisting, bending, etc., that happens in all those situations?

Are all the rear suspension pivots stiff enough to prevent the swingarm from twisting differently from the frame? (Is there a rear suspension? If not, impacts with road imperfections are rpobably going to break things at those speeds)

Is the fork a double-crown, so it is supported at the top of the headtube and the bottom? If not, all the forces are applied to the base of the steerer at the bottom of the headtube, and if the fork isn't designed to operate at those speeds under your riding conditions, eventually it may fail without warning and leave you with no front wheel still travelling (for a second or two) at those speeds until the front of the bike impacts the road and casts *you* off with the rest of the inertial leftover, at at least whatever speed you still had at that instant.

If it is double-crown, is the frame's headtube and attachment to triangle designed to deal with those forces? If not, the same failure can occur at that point instead.


Smaller wheels have harder times going over road imperfections, holes, etc. and transmit more of the shocks from those into the system. Do you have suspension to deal with that, and the wheels are designed and built to take it? If not, bigger wheels will roll over things better, but still have to be designed and built to take the abuse.


If you sit down to think about it, you can come up with other considerations for what your ride will have to deal with in your riding conditions, that you can then figure out if your ride *can* do, and what you will have to replace on it to make it safely capable of that.


There are reasons motorcycles designed for those speeds are built they way they are. If htey could get away with less, they would, becuase more costs more, and virtually no company spends more money on something they sell than absolutely necessary. (not ones that stick around very long, anyway).

 

[GoFastGus]

Yes, it may be too slack, but simulate it first.

What options are there for suspension simulation? My highest priority in a program would be ease of entry given my lack of an engineering background. I know that simulating out parts will save me money in the long run by helping me choose the right part the first time, but I'd still like something fairly affordable.

For simulating different windings, google the ebikes.ca motor simulator.

I'm currently looking mostly at the QS205 V3 hubmotor, which Ebikes.ca doesn't list in its motor simulator catalog. How would I get data like RMotor, LMotor, A0, and A1? I can't find them in any info published by QS motor.

How many miles is it?

Not sure exactly what you're asking. Bike's range of travel, total range of travel, something else?

 

[GoFastGus]

Some serious things to consider for those speeds:

Oh my gosh thank you for so many things to consider! I have so much more learning to do.

What happens if you hit a pothole, bump, or larger bit of something that falls off a truck you can't avoid?
Do you have a good enough suspension, designed to operate for your overall system/rider weight, and designed to operate for those speeds (reaction time, stiction, flexure), to absorb those impacts?
If not, either the wheel can fail, or the frame or fork, or you could be thrown into the air at those speeds, without wheels to land on

The fat tire bike I have right now is rigid, but the plan would be to convert it to hardtail. My biggest concern is hitting a bump or pothole with the rear wheel of a hardtail bike and all the force being absorbed by the wheel. Best case scenario, the tire pops and the wheel is saveable. Worst case, hub motor is damaged.

Full suspension mountain bikes were also an option that I was looking at, but I decided against them for a couple reasons. First is the price: a full suspension mountain bike sells for 3-4 times the price of a hardtail mountain bike on the used market around here. A fat bike is maybe twice the price of a mountain bike. Second is larger dropouts for wider tires. My logic was wider tires with a bigger, stickier contact patch would slow me down quicker. I know this is theoretically true, but I'm not certain if it justifies getting a bike with wider tires as opposed to one with better suspension. I'm not entirely convinced on this fatbike route, and I should consider what the suspension alternative would look like.

(just your skin).

I'm young and reckless but I take some precautions. Full body coverage was a requirement for the last bike, and a Moto jacket & chaps are on the Christmas list.

Do you have good enough brakes with sufficient heat dissipation in the calipers and the rotors to repeatedly stop from those speeds without losing braking ability? (fade) If they aren't designed to operate repeatedly and rapidly at those speeds with that amount of mass (you, bike, all the stuff on it, etc), they probably wont' do what you need them to.

For speeds like this, HUGE rotors and DOT fluid seem like essentials. The forks I linked claim 203mm diameter rotors are the biggest they can handle, but I'm curious as to what limits that. The top of the line bike DOT brakes are made for downhill bikes without much weight, so it might be worth thinking about if more thermal capacity is needed. Somebody mentioned a motorcycle brake fluid reservoir on a previous post of mine, which seems like a neat idea for that.

Do you have enough traction on the tires to keep grip on the road while braking at those speeds? (skid means loss of control, skid on front wheel means loss of directional control so you hit whatever was in front of you that you couldn't stop in time for, at whatever speed you still have)

This was exactly my argument for getting a fat tire bike: at higher speeds, assuming you can lock up the wheel, the thing that slows you down faster is a larger, stickier contact patch.

Are the tires thick enough so when you do skid they aren't simply sheared thru by the friction at those speeds?

How do I measure that? Is that TPI, or some other metric?

Is your frame and fork strong enough to deal with or prevent the twisting, bending, etc., that happens in all those situations?

Probably need to do some mathematical testing before I find out the hard way. Sheldon brown has a post about fork deformation, I'll start there.

Are all the rear suspension pivots stiff enough to prevent the swingarm from twisting differently from the frame? (Is there a rear suspension? If not, impacts with road imperfections are rpobably going to break things at those speeds)

Yeah, this was my big concern with a fat bike.

Is the fork a double-crown, so it is supported at the top of the headtube and the bottom? If not, all the forces are applied to the base of the steerer at the bottom of the headtube, and if the fork isn't designed to operate at those speeds under your riding conditions, eventually it may fail without warning and leave you with no front wheel still travelling (for a second or two) at those speeds until the front of the bike impacts the road and casts *you* off with the rest of the inertial leftover, at at least whatever speed you still had at that instant.

I haven't seen any double crown fatbike forks that support the headtube on both sides. Sounds worth investing in suspension designed to handle some punishment.

If it is double-crown, is the frame's headtube and attachment to triangle designed to deal with those forces? If not, the same failure can occur at that point instead.

So does moving from a single to a double crown fork only make sense if the manufacturer designed it with that in mind?

Smaller wheels have harder times going over road imperfections, holes, etc. and transmit more of the shocks from those into the system. Do you have suspension to deal with that, and the wheels are designed and built to take it? If not, bigger wheels will roll over things better, but still have to be designed and built to take the abuse.

I need to do more research, but I've heard some people use double-walled ebike wheels, some people used moped or motorcycle wheels with higher powered hub motors.

There are reasons motorcycles designed for those speeds are built they way they are. If htey could get away with less, they would, becuase more costs more, and virtually no company spends more money on something they sell than absolutely necessary. (not ones that stick around very long, anyway).

Yeah good point, there are no hardtail motorcycles for a reason.

 

[lnanek]

I haven't seen any double crown fatbike forks that support the headtube on both sides.

Eh? I'm sitting right next to an Ariel Rider X-Class with exactly that:

 

[neptronix]

What options are there for suspension simulation? My highest priority in a program would be ease of entry given my lack of an engineering background. I know that simulating out parts will save me money in the long run by helping me choose the right part the first time, but I'd still like something fairly affordable.

insert desired fork into frame without headset and look at the bike from a distance
Or maybe a block of wood could be sufficient. Something that closely models how much extra fork height you're about to add.

I'm currently looking mostly at the QS205 V3 hubmotor, which Ebikes.ca doesn't list in its motor simulator catalog. How would I get data like RMotor, LMotor, A0, and A1? I can't find them in any info published by QS motor.

Closest thing to that in the simulator is the all axle fatbike version ( 45mm wide stator )

Not sure exactly what you're asking. Bike's range of travel, total range of travel, something else?

Asking how many miles is the bus ride

 

[amberwolf]

Oh my gosh thank you for so many things to consider! I have so much more learning to do.

The fat tire bike I have right now is rigid, but the plan would be to convert it to hardtail. My biggest concern is hitting a bump or pothole with the rear wheel of a hardtail bike and all the force being absorbed by the wheel. Best case scenario, the tire pops and the wheel is saveable. Worst case, hub motor is damaged.

No, worst case is your wheel departs the bike and leaves you doing whatever it is that happens without one at those speeds. or the wheel and part of the frame departs the bike.... etc

Or no parts fail but the wheel bounces in the air hard enough to flip you off the bike and leave *you* travelling at those speeds without a bike or control over your direction.

etc.

Full suspension mountain bikes were also an option that I was looking at, but I decided against them for a couple reasons. First is the price: a full suspension mountain bike sells for 3-4 times the price of a hardtail mountain bike on the used market around here. A fat bike is maybe twice the price of a mountain bike. Second is larger dropouts for wider tires. My logic was wider tires with a bigger, stickier contact patch would slow me down quicker. I know this is theoretically true, but I'm not certain if it justifies getting a bike with wider tires as opposed to one with better suspension. I'm not entirely convinced on this fatbike route, and I should consider what the suspension alternative would look like.

I think you should consider the real motorcycle alternative. It is less likely to require a lawyer or hospital, and will almost certainly be cheaper.

Up to you, but I've seen enough realtime crashes (and/or the results thereof, with tarps or bags covering the various rider parts) and videos of crashes at those speeds to deter *me* from doing this.

I've crashed on my long low CrazyBike2 at <30mph at the deathraces a couple of times and was lucky that they were both side-skid washouts. . First one just ripped up my knee-shin pad when i skided off the track into the gravel. Seocnd one the rear slid out in a hairpin turn and i went across the track, didn't get run over by anybody else. The design of the seat saved my leg but i twisted my ankle almost 180 and fractured the leg above it, damaged muscles/tendons and have never walked the same since (between that and other problems I'm stuck with a cane).

while i'm posting about it, i found this post taht discusses the fork angle changes and whatnot that probably contributed to the second crash; might be of interest

2WD Semi-Recumbent Recycled-Parts Cargo eBike: "CrazyBike2"

amberwolf do you have a link to the cfl light mod you did some time back? running it off pack voltage. thanks :wink: lol forget it i see it in your sig cheers

endless-sphere.com

For speeds like this, HUGE rotors and DOT fluid seem like essentials.

Fluid is probably irrelevant if the calipers can get hot enough to boil it.

i'm not an expert on higher end bicycle brakes by any means, but what i have seen are not generally meant for repeated slowing from these speeds; DH downhill stuff might be an exception to that, but you also get pricy (marzocci dh forks for instance can cost a thousand dollars, and dh frames I don't even want to know).

200-203mm is not huge...it's a standard size. I use one on the front of SB Cruiser. At stops or even rapid slowdowns from 20mph, it gets so hot that if i have to do it a lot in traffic it turns blue and is far beyond the boiling point of water (if it's raining you can watch as water pouring over it flashes to steam). Works great to do the braking I need it to...but I doubt it's sufficient for your speeds.

The forks I linked claim 203mm diameter rotors are the biggest they can handle, but I'm curious as to what limits that.

forces perpendicular to the stanchions that can bend or break the stanchion, forces at the crown from the bending backwards caused by the bike continuiong forward while the contact patch is being stopped. etc.

Ther'es some pics in various threads about forks that failed from too high a braking force. I included one further down.

The top of the line bike DOT brakes are made for downhill bikes without much weight, so it might be worth thinking about if more thermal capacity is needed. Somebody mentioned a motorcycle brake fluid reservoir on a previous post of mine, which seems like a neat idea for that.

This was exactly my argument for getting a fat tire bike: at higher speeds, assuming you can lock up the wheel, the thing that slows you down faster is a larger, stickier contact patch.

How do I measure that? Is that TPI, or some other metric?

Probably need to do some mathematical testing before I find out the hard way. Sheldon brown has a post about fork deformation, I'll start there.

Yeah, this was my big concern with a fat bike.

I haven't seen any double crown fatbike forks that support the headtube on both sides. Sounds worth investing in suspension designed to handle some punishment.

They certainly exist. image search on the phrase: double crown fatbike forks - Google Search
Random image from that search

Or you can make one, finding crown clamps of the right width, a steerer taht fits them and your headtube, and stanchions from a regular width fork.
Random image from a websearch on fatbike double crown clamps

So does moving from a single to a double crown fork only make sense if the manufacturer designed it with that in mind?

Depends on the single crown fork you're planning on and whether or not it is designed to handle the forces from your speeds and road conditions (worst case thing you *might ever* hit at fastest speed you'd ever ride, because you have less time to react the faster you're going, and if you react too quickly when going fast you can easily crash).

Single crown fork (decent Suntour XCsomething) I broke in my only collision with a car, at walking speed:

A double crown would not have broken. It probably would have bent the stanchions below the bottom crown, and maybe the frame...but not broken off. If I had been riding down the road and this had happened from a pothole or whatever, the front of the trike would have instantly dug into the road and I would have been flipped end over end and probably crushed under the trike if I wasn't flung forward at the former speed I'd been riding at. :/

This fatbike fork failed, bending the crown a little at a time, *just* from repeated braking forces, at my low speeds:

Before

after

I need to do more research, but I've heard some people use double-walled ebike wheels, some people used moped or motorcycle wheels with higher powered hub motors.

Pretty much any hubmotor is going to come on a doublewall rim already, almost every bicycle wheel except on the cheapest of cheap will be at least doublewall, and there are stronger rims avialable.

If you go with MC wheels, or otherwise bulletproof those, you then transfer the stresses that might have damaged them to the motor axle for hubmtors, and the frame itself beyond that. (assuming no suspension)

Lots of threads and posts about using various kinds of wheels / rims / etc.

Basically, use parts intended for the usage you're going to put them. Any parts not designed for that may or may not be able to handle it, and when they can't, they fail. Things that fail at high speeds tend to leave the rider still going those speeds but without a bike under them, and that is not usually a happy ending.

I misuse plenty of stuff for purposes it wasn't meant for, overloading them with weight... but I'm only going 20mph at most, so when they break it's not a big deal...and I don't use stuff on the front that is going to give me unhappy results even at these speeds. If the rear breaks under weight it just drags me to a stop (I have broken axles, and frames, and crappy wheels (which is why I learned to build my own; they don't break without serious cause like head-sized chunks of debris falling off a truck that I can't avoid though rims can certainly be damaged from inches-deep potholes I might not be able to avoid due to traffic).

Yeah good point, there are no hardtail motorcycles for a reason.

There have been...

hardtail motorcycle - Google Search

 

[GoFastGus]

Eh? I'm sitting right next to an Ariel Rider X-Class with exactly that:
View attachment 380092

Cool fork! What I meant to say is I haven't seen any fat bike forks with a double crown construction available for retail purchase besides some cheap stuff on Amazon. If a good quality fat bike fork with a double crown is available, I'll strongly consider it if my frame's geometry accounts for it. Know anywhere that lists just that fork, or would I have to buy it off someone used?

 

[GoFastGus]

insert desired fork into frame without headset and look at the bike from a distance
Or maybe a block of wood could be sufficient. Something that closely models how much extra fork height you're about to add.

Now I understand what you're saying. I'll look into that, it'll take some fab work.

Closest thing to that in the simulator is the all axle fatbike version ( 45mm wide stator )

Gotcha, I'll look at that, thanks!

Asking how many miles is the bus ride

5 miles at most. So taking the bus there and back on a 20 mile round trip can increase my range 50%.

 

[amberwolf]

if my frame's geometry accounts for it.

Geometry doesn't care if the fork is single or double crown. Same results either way.



(Actually, if you have tall enough stanchions on a double crown, you can make a taller fork with one since they are clamped in not welded or interference-fit, etc., and youc an always make a shorter fork with a DC clamp setup by moving the stanchions up into the clamps further. So you can have a sort of variable / experimentally alterable geometry with a DC clamping fork vs a typical single-crown fork.)

Know anywhere that lists just that fork

Google has some results for "ariel rider fatbike fork", but you'd have to check them out to see if you can actually buy them there.

Google Search

www.google.com

and I'm sure there are even better searches you can come up with.

 

[GoFastGus]

No, worst case is your wheel departs the bike and leaves you doing whatever it is that happens without one at those speeds. or the wheel and part of the frame departs the bike.... etc
Or no parts fail but the wheel bounces in the air hard enough to flip you off the bike and leave *you* travelling at those speeds without a bike or control over your direction. etc.

Yeesh, that sounds pretty bad. Best to avoid if at all possible

I think you should consider the real motorcycle alternative. It is less likely to require a lawyer or hospital, and will almost certainly be cheaper.

An electric dirt bike would be comparable in price and a lot better designed than anything I could ever make. I imagine I would theoretically have to get a motorcycle license, register the bike, and insure it, but I'm not sure what the enforcement looks like for a motorcycle that's been converted to electric as opposed to a prebuilt electric dirtbike that's shipped in.

Up to you, but I've seen enough realtime crashes (and/or the results thereof, with tarps or bags covering the various rider parts) and videos of crashes at those speeds to deter *me* from doing this.

I've crashed on my long low CrazyBike2 at <30mph at the deathraces a couple of times and was lucky that they were both side-skid washouts. . First one just ripped up my knee-shin pad when i skided off the track into the gravel. Seocnd one the rear slid out in a hairpin turn and i went across the track, didn't get run over by anybody else. The design of the seat saved my leg but i twisted my ankle almost 180 and fractured the leg above it, damaged muscles/tendons and have never walked the same since (between that and other problems I'm stuck with a cane).

Good heavens, I don't imagine that was an easy recovery! I

while i'm posting about it, i found this post taht discusses the fork angle changes and whatnot that probably contributed to the second crash; might be of interest

2WD Semi-Recumbent Recycled-Parts Cargo eBike: "CrazyBike2"

amberwolf do you have a link to the cfl light mod you did some time back? running it off pack voltage. thanks :wink: lol forget it i see it in your sig cheers

endless-sphere.com

Fluid is probably irrelevant if the calipers can get hot enough to boil it.

i'm not an expert on higher end bicycle brakes by any means, but what i have seen are not generally meant for repeated slowing from these speeds;

Me neither, but that was my understanding. The brakes have the ability to lock up the tire, but not for any extended amount of time.

DH downhill stuff might be an exception to that, but you also get pricy (marzocci dh forks for instance can cost a thousand dollars, and dh frames I don't even want to know).

Worth reading about if there are actual performace differences in resistance to brake fade.

200-203mm is not huge...it's a standard size. I use one on the front of SB Cruiser. At stops or even rapid slowdowns from 20mph, it gets so hot that if i have to do it a lot in traffic it turns blue and is far beyond the boiling point of water (if it's raining you can watch as water pouring over it flashes to steam). Works great to do the braking I need it to...but I doubt it's sufficient for your speeds.

I know there's a 1.8mm and a 2.3mm thick version of those size rotors, I wonder if there's a significant difference in thermal capacity between the two.

forces perpendicular to the stanchions that can bend or break the stanchion, forces at the crown from the bending backwards caused by the bike continuiong forward while the contact patch is being stopped. etc.
Ther'es some pics in various threads about forks that failed from too high a braking force. I included one further down.

I'll look into that. If that's a pretty hard line, then no sense risking breaking my fork over more thermal capacity assuming I can lock the front wheel.

They certainly exist. image search on the phrase: double crown fatbike forks - Google Search
Random image from that search
View attachment 380090
Or you can make one, finding crown clamps of the right width, a steerer taht fits them and your headtube, and stanchions from a regular width fork.
Random image from a websearch on fatbike double crown clamps
View attachment 380091

At the moment, building my own looks like the better option; I can't find any reputable dual crown forks for retail purchase.

Pretty much any hubmotor is going to come on a doublewall rim already, almost every bicycle wheel except on the cheapest of cheap will be at least doublewall, and there are stronger rims avialable.

Any recommendations for what brands, materials, constructions, etc. to look for?

If you go with MC wheels, or otherwise bulletproof those, you then transfer the stresses that might have damaged them to the motor axle for hubmtors, and the frame itself beyond that. (assuming no suspension)

Yeah like the old Bing Crosby song, Something's Gotta Give.

Basically, use parts intended for the usage you're going to put them. Any parts not designed for that may or may not be able to handle it, and when they can't, they fail. Things that fail at high speeds tend to leave the rider still going those speeds but without a bike under them, and that is not usually a happy ending.

Seems like good advice. I gotta figure more out about the goals and conditions of this build, and from there I'll figure out the right parts given the usage.

 

[neptronix]

5 miles at most. So taking the bus there and back on a 20 mile round trip can increase my range 50%.

Man if you had a bike that does 37mph tops, you get some not fat tires, and get yourself a nice 2kwhrs, i bet you'd be able to do the entire trip on bike.

I had a leafbike 1.5kw on this thing and 1.2kwhrs of high discharge RC Lipo.. top speed of 45mph on a 48v batt, but if i kept it to 30-35 it would go crazy distances.

Here's an approximation of how many miles i'd get at that speed on really efficient tires:



A bigger motor would provide a little more range due to high efficiency, this is just a 35mm wide motor
A grin max45 is more efficient, 13lbs in weight ( lighter! ) but it has the caveat that >3000w would break the existing axle interface design, until they beef it up ( coming soon )
A MXUS 3kw is 18lbs, a bit less efficient than a QS, but could handle long stretches of 40mph, but more like 38mph continuously.
Or you could use A QS and just accept the enormous weight

A really big battery could be the ticket, batteries are half the size/weight versus what was available to us in the early-mid 2010's

 

[GoFastGus]

Geometry doesn't care if the fork is single or double crown. Same results either way.
(Actually, if you have tall enough stanchions on a double crown, you can make a taller fork with one since they are clamped in not welded or interference-fit, etc., and youc an always make a shorter fork with a DC clamp setup by moving the stanchions up into the clamps further. So you can have a sort of variable / experimentally alterable geometry with a DC clamping fork vs a typical single-crown fork.)

That's a strong argument for a dual crown fork, thanks!

Google has some results for "ariel rider fatbike fork", but you'd have to check them out to see if you can actually buy them there.

Google Search

www.google.com

and I'm sure there are even better searches you can come up with.

Thanks for the tip, I didn't see anything where I could buy just the fork. I'll keep looking for high quality dual crown forks, but I really like your suggestion of converting a single crown to dual.

 

[GoFastGus]

Man if you had a bike that does 37mph tops, you get some not fat tires, and get yourself a nice 2kwhrs, i bet you'd be able to do the entire trip on bike.
I had a leafbike 1.5kw on this thing and 1.2kwhrs of high discharge RC Lipo.. top speed of 45mph on a 48v batt, but if i kept it to 30-35 it would go crazy distances.

That's a good idea, lower voltage and higher capacity would make for a longer range bike.

A grin max45 is more efficient, 13lbs in weight ( lighter! ) but it has the caveat that >3000w would break the existing axle interface design, until they beef it up ( coming soon )

I was considering the max45, I'm not sure if the light weight makes it worth the cost. Is the axle design prone to breaking because it's fully rounded, or something else?

A MXUS 3kw is 18lbs, a bit less efficient than a QS, but could handle long stretches of 40mph, but more like 38mph continuously.

That would be a good option. The motor's dropouts are 142mm, which is 28mm too short for my application, so I might just put the torque arms on the inside if I went this route.

Or you could use A QS and just accept the enormous weight

I need to go back and look at the weight differences between everything.

A really big battery could be the ticket, batteries are half the size/weight versus what was available to us in the early-mid 2010's

Good point, I need to make a post about my plans for a battery.

 

[2old]

My advice is to search for an older downhill bike (that doesn't look trashed). There are used Rock Shox Domain (or the equivalent from Fox and SRAM), but they're $400+, or RST has lower priced Guide and Sigma dual crown forks and I think a steel single "Guide" fork that many use for dual motor bikes, and might suffice. They're made in Taiwan, so IMO good quality. Some of the bigger motors have 170 mm spacing which won't be compatible with most MTB's (except fat tires).

 

[99t4]

On the Madison isthmus, there's a well-connected bus system that you can bring your bike on.

All the buses in my area are bike-ready with front bike racks, slots for the tires with a spring-loaded j-shaped bar that secures the top of the front wheel. Weight limit is 55 lbs. While I have wrassled my 65+ lbs. ebike on the rack once or twice when I had a flat, it was not a good experience. Very inelegant high exertion manuever to get the tires up there into the tire slots, and even when locked in it would alarmingly wallow back and forth each time the bus hit a bump. I imagine the bus bike racks are similar on your area buses? What is your project going to weigh?

This was exactly my argument for getting a fat tire bike: at higher speeds, assuming you can lock up the wheel, the thing that slows you down faster is a larger, stickier contact patch.

Contact patch is measured by the L x W portion of the tire that is contacting the road surface. A larger diameter narrow width tire could have a larger contatact patch than a smaller fat tire. Now you start to see why knobby tread tires are not so capable on paved roadways (smaller effective contact patch). BTW, tire lockup (skidding) is not an effective method to stopping safely. Braking force right up to almost lockup is.