Full suspension mountain bike Friction Drive


10 MW
Nov 8, 2009
Eastern suburbs Melbourne Australia
After great results with friction drives on hard tail bikes, I was looking for a new challenge. Using the experiences from my other friction drive designs, I decided I would try and build an under bottom bracket friction drive for a serious large suspension travel mountain bike.

Design criteria was as follows.
Total weight of drive system including battery under 2kg
Super strong to handle serious mountain bike riding.
Handle rain and mud
Stealth appearance
quite operation.
And most importantly, zero impact on how the bike rides with the power assist switched off.

Bike up for the conversion is my trusty Trek Fuel EX9


This is a really nice bike and rides great as a standard mountain bike. With some assist to get back up the hills it will have the potential to be epic.

Build and testing is well on its way and looking very positive. If the humble friction drive is of interest to you stay tuned as the results to date are looking very promising.
The first thing that was quickly evident was that there was no way of attaching the drive directly to the swing arm so i needed to come up with a system that would make the drive stay a constant distance form the tire.

This bike has a ISCG-05 chain keeper mount on the bottom bracket. The design is based on using the ISCG-05 mount as the drive attachment point.


The smooth round surfaces were perfect to create a pivoting mount that could move with the suspension. A rotating bracket with Teflon bushes was subsequently designed to pivot around the bottom bracket.


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Next job was to work out the geometry to have the swing arm drive the pivot bracket so that the drive would maintain a constant distance from the tire.



With the drive now following the tire, the motor bracket can now pivot and and climb the tire under load and decouple from the tire when power is dropped.


Mechanical parts count is surprisingly low to achieve the desired result. Being a hand made prototype, the main parts are made from 5mm aluminium plate. Not sure if I will go to stainless steel or perhaps even carbon fibre.

Total weight of the drive as pictured is a wopping 700 grams (1.5 lb)


Nice and light but that will be negated the first mud patch you go through and the system gets loaded up due to it's placement and as far as I can tell become ineffective at the same time? Certainly won't work here in Oregon for the upcoming wet season fast approaching. Should work ok in dry climates though.
I'm interested to see how this turns out...

Even if wet conditions are not optimal, a mountain bike is the type of frame that often can make a useful street commuter. Full suspension, heavier duty rather than being as light as possible.
Bigwheel said:
Nice and light but that will be negated the first mud patch you go through and the system gets loaded up due to it's placement and as far as I can tell become ineffective at the same time? Certainly won't work here in Oregon for the upcoming wet season fast approaching. Should work ok in dry climates though.

Dont bet on it :) Although I haven't tested the system in deep sticky mud (which i never ride in any way) I have tested in wet conditions with plenty of muddy patches. No problems with slip or mud load up so far.
Next part of this build was the installation of the speed controller and throttle interface. The bike is setup with a Vesc 4.2 mounted inside a sealed machined aluminium case. It has been programmed in FOC mode and setup for 6S LiPo operation. Battery amps are set at 15A with Phase amps set at at 30A. For throttle control I am using an Arduino interface and a single non latching button that provides 100% throttle with a quick ramp time for instant assist when needed.

With the 2:1 ratio between phase amps and battery amps, power at low speed tapers back and keeps the assist gentle through slow technical stuff while still providing the full 15 amps at higher speeds.

The Arduino interface is mounted inside the Vesc aluminium case.

The controller is attached to the water bottle mount with the water bottle cage that holds the battery mounted on top of that.
Tire selection has been critical with this build. A friction drive ideally needs a smooth tire which obviously is not great for a mountain bike that is used for serious offroad duties.

The best tire I could find for this application was the Schwalbe Hurricane. This tire works suprizing well off road even in damp conditions. In combination with a nice aggressive front tire, the bike has kept its good off road manors and is close to silent with the drive engaged.


Of course grip tape was required on the motor to work in wet conditions. I use 120 grit belt sander material on the motor which gives me plenty of traction to the tire even in muddy conditions. Not sure how long the tire will last but I am finding due to the low power and typically low speeds used, accelerated wear has not yet been noticed. I am sure if the drive was being used for high speed commuting, wear would be much more noticeable.

Regardless, the tire is cheap at around $30. Even if I got 15 rides out of the tire I would be happy. $2 a ride is not much to pay :)
Assuming optimal results, how long before you could sell units and any chance of a turn-key system (less battery, of course)?
Power is from a single 6.6ahr Multistar LiPo mounted in a tool bottle. Capacity is 150 Whr and as a part time assist, I am getting around 20km on single track or about 10km full time assist (which I never do)

Not hard to build. Just cut the bottom off a BBB tool bottle, fit the battery which is a perfect fit, Heavy duty tape the bottom back on. Then fully heat shrink the bottle. Perfect :)


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So the bike is fully functional and I am currently doing a heap of off road test rides. Today's ride was hard to fault with the drive operating flawlessly. I have plenty of video, just need to edit it. Tomorrow I get serious at one of our local mountain bike parks. This park includes a world class down hill track that is good fun but scares the hell out of me. Plan is to use this as a serious shakedown. GoPro will be fitted to capture all the action.

I think the next improvement will be to get the drive up tighter to the swing arm to improve ground clearance. Also a bash guard might be a good investment. In the meantime, I just need to be a little careful not to knock the drive on a rock or fallen trees. However, the drive is built really strong and should be able to take a knock or two.
This is the friction drive from you that I would like to see made into a kit,
at least for those of us that have F.S. bikes with the ISCG-05 mount.
I could easily take the kit and my bike to a Machinist and have him make the custom shaped mounting plates for my bike !

You could offer the Kit with thick paper/cardboard/Mylar or such pattern of the
BB mount, then we can make our own shaped mount for the arm that pushiness the BB Mount/Motor in the tire, according to our individual bike.

My F.S. bike ... https://www.santacruzbicycles.com/en-US/archive/bike/bantam

Kepler said:
No plans to try and turn this into a kit. Too many variables with full suspension bikes.
Those plates on your suspension design of your exact bike that push down on the rotating mounting plate around the BB , can be made to fit different frames ( to be done by the end user ) by using a mounting clamp that clamps around the seat tube , just like how a front derailleur clamps to the seat tube , or even a larger clamp that clamps around the lower area of the down tube, a larger clamp in this case, could be a two piece clamp made of delrin or even hard wood ? Just enough holding force to allow the bracket to push the rotating bracket .

On my bike it would be very easy since I have on my down tube on the underneath side two holes/bolts in which to bolt on a bottle cage. I could mount a plate under the bottle cage that would be the anchor point with it being spring loaded , with a lever on the handle bars to tension/detention the spring thereby pushing the motor onto the tire . ? .
I still cant really see this kitable. Hopefully there is enough detail here for anyone wanting to build a similar setup :)

Gave the bike a good shake down yesterday. Bike went really well but always room for improvement. Got around 20km the single battery with plenty of part time assist.

Perhaps sell the VESC and a throttle ? ( thumb throttles are too sensitive to bumps and hurt the thumb , and half throttle hurt the hand from constant griping, I am finding a full throttle with a cramp buster is what is needed for e-bike and my motorcycle.
A ergo full throttle, available in right or left side would be better on the hands.

Is the motor different/larger more wattage and amperage than the motors you use on your Road Friction Drive ?

Kepler said:
I still cant really see this kitable. Hopefully there is enough detail here for anyone wanting to build a similar setup :)
Oh I would LOVE to convert one of my mtb's to this, but, I don't think it would last a week. First boulder chute would possibly destroy the motor and mount.Definitely works on dry trail centres as John has proved in the video though.Horses for courses etc... The only system that works for technical stuff seems to be a crank based system, and we all know what they cost! The Vivax system would work as well ,but I believe it's incredibly inefficient in it's power delivery and a wee bit pricey.
Nevertheless, if any man can make this viable ....it's John.
Has anyone tried to use a similar motor setup but with a cog near the cranks providing the assist? Is that feasible,or is it opening a whole can of worms regarding gearing etc. I know it's not a friction drive,but I can't help thinking that mtbing has a whole different set of requirements that a friction drive can't address.
Probably just showing my ignorance here in all things technical.....
I tend to avoid rock gardens if I can however sometimes you get caught by surprise. I was thinking of a guard something mike this. It would mount to the back of the ISCG-05 mount so plenty of strength there. I think something like this would protect the drive 90% of the time. Always some compromises but I think the compromises are out weighed by some significant advantages.

BTW, the bling and carbon fibre are just concepts at the moment. I would love to build it from carbon fibre in the future but not sure it is really practical for a drive hanging under the bike. Stainless Steel probably makes more sense.


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That is what I am waiting for, at least an affordable one, John seems to have the most affordable RC drives for DIY,
Tangent Dave has done this, but he is using very expensive RC motors that are in the hundreds of dollars, and a large amount of custom machined parts.
Mike at LightingRod's was going to work on one, however he got stuck on some part of it, I think he is busy providing the same Drive he has been selling for a few years now so he is too busy perhaps to develop / design a RC mid drive. That sure is a shame, I was waiting for his for nearly a year , only to find out it is still not available.

There is talk of a GNG Pro X1 but we do not know the price of it yet and if it will have a throttle , and what the BCD is on the Crank , etc.

Kepler is the one person I think that can make a RC mid-drive , at an affordable price, he is using the cheaper motors from Hobby King that cost around $ 100 or even less.

Now in late 2018 it is possible to take a cheep Hobby King motor and a VESC 4 and do a very affordable mid drive, ... possible because Sram just came out with a Eagle EX 12 speed drive train. the whole drive train minus the BB is just under
$ 400 ! even less if you do not buy the crankset, which you do not need because with a DIY RC mid-drive you are going to need a freewheeling crankset.

The Sram Eagle EX 12 speed drive train is made for the torque of electric bikes, and has a large cog of 50 tooth, or is it even 52 tooth.

Combine that with a front crankset that has a 53 tooth chainring and a low KV RC motor and there might not need to be a reduction cog between the RC motor and the Chainring .

I sure would like John to work on this, that would be kit-able with the mount made by the end user for his/hers own bike design.

That would be the Holy Grail of Light Weight , budget priced Mid-Drives .

urga said:
Has anyon;e tried to use a similar motor setup but with a cog near the cranks providing the assist? Is that feasible,or is it opening a whole can of worms regarding gearing etc. I know it's not a friction drive,but I can't help thinking that mtbing has a whole different set of requirements that a friction drive can't address.
Probably just showing my ignorance here in all things technical.....
John, I know you stated that you couldn't mount the drive via the chainstays. Can I ask why ? If the drive could be clamped to the stays it would mitigate having to have the motor move with the suspension arc. What you have done is clever but would not work on some other suspension systems.But then you did state that this was for your bike :roll: so ignore that.
Scooterman, I personally don't like the look of most of the midrives as they are too much of everything. Too much power,too heavy and bulky. Although the one on the Knolly on Tangent's website is close to what I thought could maybe done using John's mount for a smaller motor with a small cog to mesh with the chain before the crank. It would still be almost as stealthy as the FD which would be amazing. I also think the modest power requirements and output of the FD system are ideal and wouldn't dilute the riding experience too much.
You mention a low KV motor.How low can you go while keeping the motor size to around the 63mm dia? I am right in thinking that ,if the Vesc can handle the current,you could run a lower Voltage with a higher current to get a similar overall power output but by lowering the V's the rpm would be lower as well? I also personqally wouldn't have a problem with the cranks always turning when the assist is on(just like riding a fixie) I think that would help simplify things ,and like you mentioned ,with a Eagle 1x12 setup there wouldn't be much you couldn't get up!
We just need to work on John to adapt his system to maybe make this possible :wink: What we need is the love child of John's FD system married to https://tangentmotors.com/ascent/ John ?.......John ?...........Who left the door open?
Just bumping my gums again here.Looking at John's picture of the Trek,would it not be possible to leave the motor sited where it is, complete with it's shaft and a small 9T cog . This would engage just after the crank and shouldn't be too affected by chainline. It would only work on a 1x setup. Normal cadence on a road bike is around 90rpm at the crank,so maybe a bit less on a mtb. Let's say 70 rpm .If you had a std 1x setup with a 34T chainring the motor rpm at the 9T cog would be 264rpm, with no load ,Yes? So even the 270KV motor I have fitted just now would possibly do ? Feel free to smack my wrists.... I'm not sure how the motor would behave in this situation as I don't have any experience in this department.
Is it not also possible to install magnets in the cranks and use hall sensors to further enhance control ala Andre ? I think if the drive came via the chain it would mean a very direct and static fit regardless of suspension types and std rubber out back with minimal excessive wear on parts(except the chain)
I'm off to bed :oops:
urga ,
Many questions and thoughts there I will address what I know and it is little compared to people who work with RC motors.

1) Very few people can ride with a cadence of 90 , same with 80 rpm cadence, those kind of high rpm's are mostly very fit Roadies on 17 lb bikes.
I have a average of around 69 rpm and that is on the road with speeds of 20 mph +/- a little. so it is best to target for around 60 rpm when designing a drive train ratio.

2) Just going off of memory now but those small H.K. motors that are around 63 mm dia,can be bought in KV's of around 150 or so, I have seen even down to 100 and under KV. But really just get a slightly larger motor that would be more suited to a bicycle drive train application .
A slightly larger motor would be just a few dollars more , just a few grams more , but can be had in lower KV's . Someone here on E.S. said they found a RC motor with around 50 KV but I think that one would be a few hundred dollars so it is in the category that you say you do not want to get into , it should be possible to get one with around 150 KV.

3) You guessed right about less volts and more amps. That is why RC motors are run on 6s / 7s / 8s battery packs. And lipo's are made with 50 up to 120 amp ratings, although when buying from HK, and other sites it is said to cut that number to a realistic 25-35% of what they say in regards to constant amperage draw on lipo's .
Same goes for the RC motors wattage rating, I do not know the percentage there but I have heard it is much lower than the numbers indicate, so count on getting at least a 2,500 watt rated motor .

4) I had thought about not using a freewheeling crankset once, but realized that unlike fixe's that ride around at lower speeds, when we put a motor on a bike there is allot of speed and torque on that crank, it can come up and twist or brake your ankle.
I wonder about putting a small, very small clutch on the motor so that when you let off the throttle the motor will not turn the sprocket to the cranks , but then with the expense of that it is perhaps just better to just get a freewheeling crankset. Work still needs to done to produce much lighter weight versions of a freewheeling more modern crankset. Like the Shimano Hollowtec II , or the Race Face, FSA , Praxis Works , etc. more modern cranks.