geometry critique

t_tberg · Mar 4, 2017

I had some parts laying around so naturally I justified a new build. I got a really cheap 29er frame, I read that some people said it has an oddly high bottom bracket so I thought that it would work ok with 19" moto wheel. The head tube angle seems reasonable, the bottom bracket seems to have good clearance but there almost no drop, does anyone know how this will effect the handling? I know high is not ideal for stability.. Hopefully I can build a battery pack that will fit behind the seat tube. Oh yeah, its a 26 inch wheel up front, the 19 by 2.2 rear tire ends up being a little over 24.5 inches.

View attachment 1

Lowberry · Mar 8, 2017

its a hard tail so its going to be rough... hope the forks al least have springs

Lowberry · Mar 8, 2017

what motor and controller do you have? are you going to use a bms or go RC style?

Drunkskunk · Mar 8, 2017

That looks like a non suspension frame, and that's why your geometry is wonky. You add a shock to the front, and it raises the front end. Not really a problem, if your goal is stability.

Higher center of gravity increases stability while lowering maneuverability, while lower center of gravity increases maneuverability but decreases stability. You'll carve trails less well than a normal geometry 29er, but be more stable at high speed.

t_tberg · Mar 9, 2017

Lowberry said:
its a hard tail so its going to be rough... hope the forks al least have springs

I have an air fork, rockshock solo air recon, the frame was 60 bucks so i couldn't justify a really nice fork. I plan of running it at a higher pressure to compensate for the added weight. All my bikes so far have been hardtail, I do entirely city riding and wide tires seem to provide enough comfort. Another aspect that appealed me to this frame is the longer wheel base (dare I say longtail-shorty?) Why do you recommend spring forks?

t_tberg · Mar 9, 2017

Lowberry said:
what motor and controller do you have? are you going to use a bms or go RC style?

The 12 FET controller from ES member powervelocity with BT module. I was considering going with a BMS with my RC lipo but I ran my last let of lipo raw -- balanced charged about ever dozen discharges for the first few months, since then I've just checked cell voltage periodically and they've never fallen out of balance. So I think I might do the same with these, but that means I'll need to make the individual packs accessible. I'll keep posting here to show everyone what I come up with..

The motor is from ES member evolutiongts, 1500W 35mm DD hub laced into a 19inch moto. I haven't done tests and I couldn't find the info but I believe is 12.7 Kv

t_tberg · Mar 9, 2017

Drunkskunk said:
That looks like a non suspension frame, and that's why your geometry is wonky. You add a shock to the front, and it raises the front end. Not really a problem, if your goal is stability.

It's a 26inch wheel and 100mm fork in a 29er frame so I figured that would bring the HTA closer to its intended angle (smaller tire OD somewhat compensates for the additional axel to crown length from the travel). Albeit the smaller rear wheel rear would slightly rise the front end slightly which is okay because I am actually looking for stability, this bike is exclusively for street riding/commuting. Another appeal to for this dirt cheap frame was the longer wheelbase, I'm looking for comfort and stability.

Drunkskunk said:
Higher center of gravity increases stability while lowering maneuverability, while lower center of gravity increases maneuverability but decreases stability. You'll carve trails less well than a normal geometry 29er, but be more stable at high speed.

I thought that a lower bottom bracket (and thus CG?) increased both stability and maneuverability.. Why would the two be inversely related?

Drunkskunk · Mar 9, 2017

t_tberg said:
I thought that a lower bottom bracket (and thus CG?) increased both stability and maneuverability.. Why would the two be inversely related?

Because Physics.

The higher center of gravity means a higher center of mass. That mass must move along a curve with a longer radius extending from the fulcrum, which is the ground. the longer that radius, the longer it takes to move the mass, the less the angle of displacement will be on any force that acts on that mass.

Or, a simple explanation. try to balance a broom by the end of it's handle on the palm of your hand. It's really easy, and you're able to keep it upright even if you move your hand around a lot.

Now try to balance a toothpick on it's tip in the palm of your hand. It's not so easy, and the toothpick reacts to the slightest movement of your hand.

It's the same with a bike's weight. the high center of gravity is easy to balance and takes a lot of movement to make it move. So it's stable, but needs a lot of effort/input to maneuver it. The low center of gravity takes much less effort to move but won't stay as stable.

Another way to think of it is that Stability is resistance of an object to be maneuvered.

High speed race bikes are built with their engines up high in the frame, so they stay more stable at high speed. Trials bikes, on the other hand, are often used while nearly standing still, so the motor and as much of the frame is as low as possible, so they can be maneuvered with great precision.

t_tberg · Mar 10, 2017

Drunkskunk said:
That looks like a non suspension frame, and that's why your geometry is wonky. You add a shock to the front, and it raises the front end. Not really a problem, if your goal is stability.

Higher center of gravity increases stability while lowering maneuverability, while lower center of gravity increases maneuverability but decreases stability. You'll carve trails less well than a normal geometry 29er, but be more stable at high speed.

I'm not looking to take this thing off road so thats fine. If you peruse through this site there are a few pictures of what people have done with this frame. I'm not saying that they correctly chose to put a suspention fork on it but the two builds feature either a suspension fork or suspension AC adjusted fork. http://forums.mtbr.com/29er-bikes/steel-29er-cheap-ever-heard-cfg-922435.html

chas58 · Mar 10, 2017

Boy, keep those fingers crossed.

Bottom bracket probably varies about 10mm. Usually it is around 70. But you are dropping it a lot. This will make the bike more stable, but pedal strike may be an issue. You can manage it, but don’t be surprised. All the latest mountain bike designs seem to like low BB to get the bike more stable these days – makes it a little easier to turn too.

I’m worried about what you are doing to the bike trail. The calculations are a little over my head, but a long fork can really mess things up. So can putting a bigger tire on the front.

A friend of mine took an old bike with a broken 80mm travel fork and put a newer 100mm travel fork on it. That thing was no fun to ride in the woods.
I have a bike that has very little trail. I have been riding it with a large wheel (55 mm tire) and a small rear tire (42mm). Its pretty twitchy. Putting matching tires on it did a lot to make the bike more stable.

I’m no expert, but increasing the fork length looks to me like it would move the contact patch of the front tire closer to the steering axis, thus reducing the bicycle’s trail, and really reducing the bike’s stability.

Moral of the story – a longer front fork (or larger front tire) is really going to affect your geometry.

motomech · Mar 11, 2017

Are those 4S bricks and do you intend to run 20S?
If so, you are going to have to "break" the pack to charge, so why not use 2 inexpensive 10S balance chargers.
You are not going to have much capacity (range) if that is the extent of your pack.

t_tberg · Mar 11, 2017

motomech said:
Are those 4S bricks and do you intend to run 20S?
If so, you are going to have to "break" the pack to charge, so why not use 2 inexpensive 10S balance chargers.
You are not going to have much capacity (range) if that is the extent of your pack.

I use a bulk charger for my current 15s pack, which is also 10Ah, I can increase the voltage on the charger and it's CC CV, has been working well with a server power supply. I have only broken my pack down one time in the past 6 months and that was just to check that they are still in balance. So yes, I do intend on running 20s with the 10Ah cells, probably not gonna go above 35 amps though. I've found that 90% charge on my 15s 10Ah pack is more than sufficient for my daily commuting purposes so if anything I'll have more range with more watt hours.

chas58 said:
Boy, keep those fingers crossed.

Unfortunately that's how it'll be.. I'm not intending on riding off road, I'll post a pic of my other bike and compare the ride feel once this one comes together..

chas58 · Mar 13, 2017

t_tberg said:
chas58 said:

Boy, keep those fingers crossed.

Click to expand...

Unfortunately that's how it'll be.. I'm not intending on riding off road, I'll post a pic of my other bike and compare the ride feel once this one comes together..

I'll be curious how it comes out.

I have a bike with slack geometry, that just handles too slow for me to work well as an ebike. Then I have one that is a little too twitchy. Both work fine unassisted. If you reduce your fork rake, you can add a bit of stability (but that means purchasing a new fork).

t_tberg · Mar 13, 2017

This post doesn't really pertain to the bike geometry so much, I just want to show off a little work that I did to fit an old rack that I had. I had to bend the rack to fit around the seatstays; I also used some pipe clamps to fix the rack to the chainstays. It turned out much sturdier than I expected; I was intending on just using the rack as a spot to mount the controller and a fender but now I'm thinking it could easily handle some panniers. Torque arm was also installed. I also picked up some entry level tecktro hydro brakes and rotor for $25' I've read that these brakes are pretty shitty but I'm ok with that considering I'll primarily use regen in the rear.

t_tberg · Mar 16, 2017

Finally looking like a bike!

flat tire · Mar 16, 2017

Drunkskunk said:
Higher center of gravity increases stability while lowering maneuverability, while lower center of gravity increases maneuverability but decreases stability. You'll carve trails less well than a normal geometry 29er, but be more stable at high speed.

This is completely backwards. What you may mean to say is that as you lower the front end relative to the CG, which transfers more weight forward and slightly reduces the rake and trail, the bike will be less stable and turn more aggressively. Raise the front end relative to the CG, and the opposite is true--the bike will be more stable, but turn less aggressively. This is racing motorcycle setup 101.

Overall, having a high CG will make the bike MORE maneuverable but less stable. See dual sport vs harley.

boytitan · Mar 16, 2017

Looks pretty nice so far I would at least put a fender on the front.

t_tberg · Mar 16, 2017

flat tire said:
Drunkskunk said:

Higher center of gravity increases stability while lowering maneuverability, while lower center of gravity increases maneuverability but decreases stability. You'll carve trails less well than a normal geometry 29er, but be more stable at high speed.

Click to expand...

This is completely backwards. What you may mean to say is that as you lower the front end relative to the CG, which transfers more weight forward and slightly reduces the rake and trail, the bike will be less stable and turn more aggressively. Raise the front end relative to the CG, and the opposite is true--the bike will be more stable, but turn less aggressively. This is racing motorcycle setup 101.

Overall, having a high CG will make the bike MORE maneuverable but less stable. See dual sport vs harley.

Ok, that makes more sense, I was trying to wrap my head around it the other way.. Either way this wheel and frame/fork combo doesn't fall into either extreme. Pedal strike might be the biggest problem considering the bottoms of the cranks are about a beer can minus a centimeter on this bike with no compression on the fork; it's exactly one beer can on my fully ridged kona but pedal strike hasn't been a problem on that bike..

t_tberg · Mar 16, 2017

Clearance for protective case.

boytitan · Mar 17, 2017

t_tberg said:
Clearance for protective case. View attachment 1

Id move the batteries inside the frame triangle. All the debris the wheel kicks up is right there.

t_tberg · Mar 17, 2017

boytitan said:
t_tberg said:

Clearance for protective case. View attachment 1

Click to expand...

Id move the batteries inside the frame triangle. All the debris the wheel kicks up is right there.

Don't gotta tell me, that's where my controller is mounted on my current bike. I will construct a fender/housing.

Lowberry · Mar 18, 2017

that looks nice, hey where did you get that tork arm? that looks nice

t_tberg · Mar 18, 2017

Lowberry said:
that looks nice, hey where did you get that tork arm? that looks nice

ES member dmwahl https://endless-sphere.com/forums/viewtopic.php?f=31&t=69073

t_tberg · Mar 19, 2017

I tired installing an old derailleur chain tensioner, it looks like this hanger is bent to shit, I could try bending it back by my tensioner still has lateral play, even when torqued down tight. So I think I'll try my luck with a half link chain

t_tberg · Mar 21, 2017

I got a regular bmx chain on with no tensioner, I broke the chain to length and turned the cranks backward to get the chain seated. The only problem is that the chain is wayyyyyy too tight, it takes a little effort to turn backwards and is a little "stuttery". I'm hoping that one link out of the half link will give me appropriate tension without the tensioner.

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