mid drive project for pikes peak

[Lenk42602]

fellas,

I am exploring doing a stokemonkey/hanebrink type mid drive using a mac 500 watt geared hub motor.

Still in conceptual phase, and coming from a purely FROCK background, I am stumbling a bit on the correlations between gear reduction and final drive ratios, considering:

1.) the MAC is geared
2.) double front chainring setup at the crank and its affect on final drive ratio
3.) 9 speeed rear cassette and its affect on the final drive ratio
4.) rear whell diameter and its affect on the final drive ratio.

I am sifting through a bunch of past posts, and still trying to grasp a better sense of the way this stuff all shakes out. The goal here is to determine if this is a better route to take than a DD in a small diameter rear wheel.

Parameters/considerations:

1. need to keep the mac @ ~ 400 rpm to avoid heat
2. associated grades w/ Pike's peak
3. minimum amount of battery capacity - ($$$$ broke from my last project, now complete)
4. optimum "wind" scenarios
5. i plan on pedaling my ASS off, still fairly good power generation as I ride a good bit

Anyways, if anyone can point me to some good discussions/links/input/etc that can help me gain a better grasp on this stuff, I would appreciate it.

By the way, here is the platform - I am basically planning on separating the swingarm from the main triangle, relocating it behind the seat post, and welding it rigid (no rear suspension) with sufficient space for the hub motor to fit. Nice elevated chain stays, and the whole thing is steel....




Rather than 11 months, this looks like it might take just a few weeks to get the frame together

Thanks,,

len

 

[Rassy]

Nice project and good goal len. I am very happy with my mid-drive hub motor on the trike. Of course, mine is strictly for pleasure and exercise, not racing etc.

Even though you asked the questions, I would guess you already know most of the answers. I found the most critical item was to make sure the motor RPM and my pedaling cadence were compatible. In other words, if the motor can spin your crank faster than you can pedal you won't be happy. So both you and the motor need to be able to perform together in the most efficient RPM, which is something only you can determine since it varies for each of us, and also varies for different motors.

Once you figure the above out, it just depends on all the ordinary gearing options that any bicycle has. I.e., top speed is determined by the ratio between the large chain ring and small freewheel cog, and the size tire, etc.

Good luck on building your hill climber.

 

[Lenk42602]

thanks Rassy,

I spent a good deal of time readin about your set up, and it has been a great primer.

just wanted to mention that I edited above - this bike is going to be a hard tail, I am just re-purposing the existing swing arm by welding it to the main triangle .

len

In other words, if the motor can spin your crank faster than you can pedal you won't be happy. So both you and the motor need to be able to perform together in the most efficient RPM, which is something only you can determine since it varies for each of us, and also varies for different motors.

Once you figure the above out, it just depends on all the ordinary gearing options that any bicycle has. I.e., top speed is determined by the ratio between the large chain ring and small freewheel cog, and the size tire, etc.

This last sentence is, I guess what I need a little bit of help with - academically....

Len

 

[Rassy]

If I understand your question properly, it's just a matter of arithmetic going from your crank to your tire. You need to change inches to miles (63,360 inches per mile), diameter to circumference (diameter multiplied be pi (3.14) equals circumference, and seconds to minutes.

The formula for MPH is:

Cadence RPM Times 60 (min./hr.) times Chainring tooth count divided by Freewheel tooth count times tire diameter times 3.14 (pi) divided by 63,360 (in./mile).

I'll just go through one simple example, in case this isn't what you really need.

Cadence of 80 RPM
Chainring with 40 teeth
Freewheel cog with 20 teeth
Tire outside diameter of 26 inches

Solution for Miles Per Hour:

80 Times 60 times 40 divided by 20 times 26 times 3.14 divided by 63,360.

Which equals 12.37 MPH (rounded)

Hope I didn't make any mistakes.

 

[LI-ghtcycle]

Very exciting!

I would say first off, 400 RPM is very doable, however getting your pedal cadence and the motor cadence is going to be the tricky part.

If it were me, I would gear for WOT to be about 130% of your max speed, and if you give yourself some leeway using the throttle to get the gear ratio just perfect to the specific riding condition, you will be shifting a lot.

Remember the KISS principal and have as few reductions, freewheels, etc. as possible (fewest failure points).

Since you are already starting with pretty well proven geared hub motor, I think you have made great motor choice, I am curious, where does the MAC fall in the performance to the BMC that it is similar to? High torque, high speed or other style winding? IIRC, the BMC comes in like 3 models (V1 V2 & V3?) and they all have different torque/speed characteristics.

If you go with a Stoke Monkey style BB, make dang sure you can pedal with the motor, or if you slip off the pedals at high speed you might get a pretty serious injury!

And if you're going with freewheeling cranks, don't skimp on the freewheel for the cranks! 40 MPH and a crank suddenly going "live" with the motor power could be a very frightening experience! :lol:

If it were me, I would separate the motor and pedal drives and use a NuVinci IGH since it's able to shift on the fly (especially the cheap $150 Dev Kits!) or even automatically with a sensor/shift map and push button.

By using the NuVinci, you have a hub that will always freewheel even if a motor were to lock/seize and separate pedal freewheel that is only engaged when the motor is turning, the hub freewheels as you are going down the road on it's own, so the free wheel on the pedal chain isn't going to even be used unless you are using the motor and "coasting" with the pedals.

IMHO, going with freewheeling cranks, and running high power through the bicycle drive train (bicycle chain, derailleurs) at race speeds is asking for trouble. The Stoke Monkey doesn't use a freewheel because it's for low speed high torque for a person who expects to pedal all the time with a heavy cargo bike that isn't usually going over 15-20 MPH at most, and often much slower up hills with a heavy or "precious" cargo on the back.

You might talk with Eco Speed about what type of freewheeling cranks they use, they have been putting loads of power through the cranks with good success, but I don't think they would recommend their systems to maintain peak performance (race conditions) as they are mainly a commuter set-up, and I would imagine the builds they use that ARE higher powered are the ones that are on the recumbents (using the motor as jack shaft) are still only really designed to maintain closer to 20 MPH constant speed as here:

Since you're already planning on putting the motor in the triangle, and going rigid, I would eliminate the freewheeling crank altogether, something like this:

Actually, since you're going rigid (or heck, keep the suspension), and we know that longer wheel bases help out at higher speed, I would actually do something more like this:

This is using an extended swing arm that could be built out of steel or just extend the one you have, and I'm only talking 6 - 8 inches, keep the same geometry, and now you don't have to worry about remounting the rear triangle on a custom mount! 8)

Also, the pedal chain line is to the outside of the motor chain, this way you have no clearance issues, no worries about needing any more chain tension issues that it had as a pedal bike, just longer pedal chain now, and running it with a NuVinci in the back, you have a heck of a gear ratio range and as many chain rings up front to be able to pedal along with it at any speed, even 50!

 

[Lenk42602]

fellas,

thanks! tons of helpful input from you both!

Rassy, your quick primer anwsered my gearing calculations. Additionally, it has helped me to see 1.) the crankset gearing to hub gearing, then 2.) the hub gearing to rear cassette gearing as isolated systems, both with the 400 rpm/ 25-30 mph goals indicated for climbing the mountain. then I can just look at the motor characteristics and make any changes internally as needed.

Light cycle - thanks also! I am not intending to go nutz with this bike - in the spirit of LFP's builds - it is going to be pupose built - designed to climb 6-12% grades without heating up @ 25 mph. As a result It will be rigid, simple, and will finish. It will not have freewheeling cranks. I am going to be using an off the shelf set of mountain bike wheels with 8/9 speed freehub spacing. No internally geared rear bicycle wheel hubs (nuvinci,etc), as I am going to keep this relatively low buck and simple. The fixed cog on the hub motor will be the drive chain to the rear wheel and will be shifted across the cassette via the rear derailleur.

Hanebrink's fat tire design appears to use a fixed cog off the hub motor to drive the rear wheel through the rear cassette, and a second freewheel along the axle of the hub motor is connected to the crankset for pedalling input. That looks like the only real semi exotic part required to be fabbed up in all of this....

Using the narrowness - of - purpose mentality (pikes peak race), I am going to probably run a double front chainring with a front derailleur in order to mantain effective pedal input over the hub motor's~ 400 rpm running speed at various grades, and not much more, and utilize a rear cassette that helps do the same, but, again, not much more... If i can get away with a single front chain ring I will do that instead...

So pedal chain from cranks to hub motor, drive chain from hub motor to rear wheel....

Rigid frame.


Thanks guys, your willingness to help is just awesome, keep it coming - you are slowly watching a blue gill turn into a piranha

len

 

[Rassy]

Sounds good. With your last post, I understand how you plan to hook up the mid-drive hub motor. It's what I call the jack shaft version, where there is one chain from the crank to the motor and another chain from the motor to the casette on the rear wheel. It does give you more choices in selecting chainwheel and cog sizes.

If you can get by with a single chainwheel on the crank you eliminate the need for both the deraileur and a tensioner. Might still need a tensioner, but at least not one that has enough action to allow changing gears.

And yes, the tricky part will be getting your two cog wheels on the motor. Several ways to do that, but it does seem you might not need to have a freewheel there, which might help simplify things. If you have no freewheel at the cranks or at the motor, it just means you have to pedal whenever the motor is running, which is how the Stoke Monkey works. In any case, you will probably want to run the motor and pedal during your entire hill climb anyway, except during true down hill coasting.

I am looking forward to seeing how all this works out. I know my trike could easily make the climb, but not at the speed needed to be competitive, and I have no interest in trying to make a faster version.

 

[Warren]

Len,

According to Wikipedia, the Pikes Peak race averages 7% grade. At 25 mph, that will require about 1200 watts at the wheel. Will that 500 watt motor stay cool at 2.5 times its rated load?

Warren

 

[Lenk42602]

that's ultimately the biggest question mark i have regarding ths set up.

I have never run one of these motors before, only read about them an the volt/amp configs es members are currently using. even those have to be taken with a grain of salt, as climbing, continuosly requires a relentless, continuous draw on the motor.

challenge for a guy like me is to first understand how the drivetrain would be optimized for the motor, and then take a long hard look at that hub motor's capacity.

if it is a no go after that, then I am going to probably heading back to rear whee, dd hub motor land......

len

 

[Whiplash]

The part you are missing is he wants to be able to shift. THAT is what will make this kind of bike successful. Nice Lenk, I have been planning this very thing for a few weeks! You neat me to it dammit! Lol!


EDIT: Sorry, I didn't see the other responses since im in my cell. This was directed at the guy thinking of direct drive earlier..

 

[Lenk42602]

The part you are missing is he wants to be able to shift. THAT is what will make this kind of bike successful. Nice Lenk, I have been planning this very thing for a few weeks! You neat me to it dammit! Lol!

whip,

i have a long way to go, and have very little experience with this side of the forum - show me what you got! would begreat to collabrate and earn! Is that why you asked Paul if the side covers were interchangeable?



Len

 

[Rassy]

Whiplash said:

The part you are missing is he wants to be able to shift. THAT is what will make this kind of bike successful.

Right. If the temperature is monitored and you have to run in a slower gear on the real steep strethes but can keep going and finish you have won, regardless of whether you are first or not. In fact, you may not need to monitor the temperature if you keep the cadence (directly linked to motor RPM) up and the amperage under a pre-determined value, just by selecting the appropriate gear.

 

[Whiplash]

Well being that I am trying to make the race, I'll help as much as I can but hey, I can't give up everything up my sleeve! Lol!

 

[Lenk42602]

Well being that I am trying to make the race, I'll help as much as I can but hey, I can't give up everything up my sleeve! Lol!

just remember, its not me against you - its us against them.

besides, I plan on running with as little power as possible. i'll be sweating to compenate for my relative ignorance in all of this.

P.S., If you don't help I'm giong to jam a stick in your front spokes, puncture one of your lipo packs, and fill your inner tubes up with water before the race!



Len

 

[Whiplash]

LOL! Now that's the racing spirit! So you are planning on going from the cranks to the motor, then to the cassette? Are you planning a freewheel crank? I would, just to keep from accidentally breaking an ankle! What voltage are you planning? I would consider as low as possible with that 8T motor, its going too be a bit fast I think. I was going to go with the 12T myself just to simplify the drive. I am thinking you tmight need a two stage reduction to get the rpm down to crank speed

 

[LI-ghtcycle]

Whiplash said:

The part you are missing is he wants to be able to shift. THAT is what will make this kind of bike successful.

Right. If the temperature is monitored and you have to run in a slower gear on the real steep strethes but can keep going and finish you have won, regardless of whether you are first or not. In fact, you may not need to monitor the temperature if you keep the cadence (directly linked to motor RPM) up and the amperage under a pre-determined value, just by selecting the appropriate gear.

I believe having the pedal cadence in agreement with the motor speed is a huge bonus, you can tell when it's a gear that allows you to spin the crank too easy, or too hard, and if you have it tuned with motor, you can instantly know if the motor is going to be struggling and creating heat, or spinning away at an efficient RPM.

I like the simplicity of this build concept!

I didn't realize that was how the Hanbrink worked, pretty smart giving a second freewheel on the top of the multi freewheel so that you can let the motor use the derailleur.

That is identical in concept to how I use a NuVinci. 8)

 

[Kurt]

Len,

According to Wikipedia, the Pikes Peak race averages 7% grade. At 25 mph, that will require about 1200 watts at the wheel. Will that 500 watt motor stay cool at 2.5 times its rated load?

Warren

Not the exact same motor but very similar . I have been running the 500w bafang bpm motor at 15s 60v with a 45A controller. Its a fast code 9 wind laced into a 20" wheel on a recumbent trike. I have been pushing max of 2500w into it climbing through the lacal national park . Its not pikes peak but its a steep climb for a good 15 miles or so the first 10 miles have some very steep sections. I find if I set the cruise control at a pace where I am delivering around 1500w to the motor and contributing my own 200w the 1700w gets things tick along nice up the big climbs with very little heat.

I feel if you get the gearing working in your favour 1200w at the wheel isn't out of the question.

Kurt

 

[Tom Tom]

I like this approach as I have been thinking of almost the same setup.

I'm probably going with a 6x10 9c(thanks Methods!) mounted in the triangle and driving a freewheeling crankset so as to
take advantage of the bikes gearing.

And yes the goal is to ride up Pikes Peak as I live in the area. I'm wanting to keep pace with the o pti's and/or finish in the
middle of their pack. I'd like to average 20mph+ so as to finish in under 1.5 hours. Just gotta get the build going!

Pic of older specialized that will more than likely become the victim.

 

[Warren]

Kurt,

" I find if I set the cruise control at a pace where I am delivering around 1500w to the motor and contributing my own 200w the 1700w gets things tick along nice up the big climbs with very little heat."

What is your average speed up that section? Are you running the 20+ mph that Len and Tom Tom need?

Warren

 

[Lenk42602]

LOL! Now that's the racing spirit! So you are planning on going from the cranks to the motor, then to the cassette? Are you planning a freewheel crank?

Whip, no free wheel crank - the mid drive hub will have some sort of carrier that holds a freewheel, and a fixed cog. Freewheel connected to front crank, fixed cog drive chain connected to rear wheel freehub cassette.

Don't need a frewheeling crank. What i do need, however is someone to build me a carrier that threads into the threads in the hub side cover, and can carry both a regular, right hand thread freewheel + fixed cog. If I can use a right hand thread fixed cog + two lock rings before and after, I can keep the threading all the same- (right handed) and still circumvent "precession" (unscrewing of the fixed cog by the hub motor).

I am working on the tooth counts for the front chainring on the crank, but I am pretty sure both fixed and frewheeling cogs on the hub motor are going to be 14, and rear cassette gearing will be 11-34.

once I get a better sense of what I am dealing with on paper, i will post it up. All I know is that riding in to work today on my big (120 pound) hub motor bike, I was able to climb a 6-7% grade @ 23 mph pulling ~ 1000 watts (1/3 throttle), and with my pedal input, sustained, I was able to drop that down to 800 watts. This is running a 46t/11t combo, and I know I could have sustained that cadence/power output for a few miles.

if all of this ends up being more work than I need to get into, considering the additional $$$$ for MORE batteries, there is a very real possibility that my 5303- motored, 24s/20ah bike bike is going to Pikes Peak instead!

Uness someone has excess batts they want to put up for the cause!

Len

 

[Whiplash]

I have found about the same watt figures for that type of grade with rider input that is pushing but not uncomfortable with my conhismotor'ed bike. Truly, I think the Opti bike guys are either not pedaling at all, or their bikes are weaker output wise than they say! It should be really fairly easy to beat their time I think, but I am still a noob here with only about 2 years under my belt. I would be really surprised if your big bike could not do it... Just watch the motor temps in the steep sections and you should be fine. You might have to kill yourself on the big climbs though to keep it from over working, but I have read here that the C-Lite motors can take about 1000 watts continuous even in stock configuration no?

 

[Lenk42602]

Stock wiring is supposed to be good up to 3kw, more concerned with wires melting than the hub motor temp.

the only thing about my other bike is that is is more akin to a Stealth than a electric bicycle - I really would like to beat them with someting as light as their own machines - 60-70 pounds in weight, and do so with a bike 1/10 the price....

show them how special their bikes really are, and keep more $ in my pocket


Len

 

[Whiplash]

I hear ya there!

 

[Warren]

Len,

"I am pretty sure both fixed and frewheeling cogs on the hub motor are going to be 14"

That won't work. Smallest standard freewheel is 16 tooth. There used to be a 15 tooth made, where the cog was outboard of the freewheeling mechanism, but they are no longer made. There are 14 tooth freewheels made in the smaller metric BMX thread size.

Warren

 

[Kurt]

Kurt,

" I find if I set the cruise control at a pace where I am delivering around 1500w to the motor and contributing my own 200w the 1700w gets things tick along nice up the big climbs with very little heat."

What is your average speed up that section? Are you running the 20+ mph that Len and Tom Tom need?

Warren

Around 30 -35 kph so yes . its my day to day bike so understandable I am not trying to kill the motor on this kind of ride. If it was a once off at PP then its a different story I would risk a little more throttle.

I feel the best bet is to just test run some climbs at the speed- watts you want to go and pull over every 2 miles and monitor temps. This way you get some idea where the saturation point is. I do this same climb without bothering even checking any more because I know the motor is fine. You will soon get a feel for what's doable and what isn't.

Kurt