Looking for input on chain-sprocket combo for 8kw drivetrain

[JoltJockey]

Hi there!

I'm planning on building an experimental e-Dirt Bike . I'm mainly trying to figure out how I will transfer power to the 27/½" rear wheel of a Mountainbike.


POWERPLANT:
is going to be: (2x Flipsky 7070 110KV 4200W - motors powered by a 16S 7P battery pack (24,5Ah | ~64V | ~175A max. Cont.) through a Flipsky-75300 200A dual FSESC.

FIRST IDEA:
was a "mounting-platform" for motors and gearing on rear-rack. -> GT2 15mm pulleys, belts and idlers (1:3 into 1:2 - reduction) transferred to a driveshaft with a 6-speed bicycle cassette 14-26T going to a dedicated single 60T sprocket (~80 km/h speed at max rpm = goal) mounted in place of the rear disc or on the right side next to a narrow 3-Speed Cassette connected to the Crankset for peddaling with a seperate chain.

Some rough calculations on the forces the chain might survive: E-Bike Chain -> rated for 600W assistance and maybe 400W of pedalling for combined 1000W -> i could see the chain running at this torque and with an RPM "only" twice as fast as a human would achieve 2000W total. Still not enough.. let alone shifting the cassette into low-gear.

2nd IDEA:
was to ditch the cassette and instead have one ~24T sprocket at the end of each motors belt & pulley - reduction + two chains going to 60T sprockets on each side of the hub thinking two good chains might be able to survive like 400 miles of riding.
But from what I gathered this chain system would be too fragile still.

QUESTION:
Now I'm researching Moped, Dirtbike, Trial and similiar compact sprocket and chain solutions that were designed for the range of torque and power I'd like this project to put on the road. A single speed solution + ideally a way of decoupling from the peddaling action.

I'm thankful for any suggestions on solutions and maybe your own experiments and knowledge from similiar projects!

Ride safe!

 

[amberwolf]

There's a lot of projects in the non-hubmotor drives section over the years, that will probably help show you want can work and what probably won't.

FWIW, 8400w of power is likely much more than the typical parts on a mountainbike are designed to handle, so I would recommend expecting significant failures from the torque of this setup, and be prepared for this to happen on your rides, possibly without warning, especially offroad where such power is less likely to be regulated against on a bicycle.

Motorcycle frames and wheels are likely to survive it, and would be a better choice.

What specific conditions / usage do you have that require this much power?

I ask because even with my SB Cruiser heavy cargo trike, I can haul a piano or hundreds of pounds of cargo of various types with much less than half of that, with two common hubmotors and generic controllers, on city streets, at up to 20MPH, with no significant pedalling force provided by my aging aching joints.


If it's "dirt bike" type performance, you should probably look at the Sur-Ron and similar bikes for inspiration.

 

[DogDipstick]

No one knows how to use a " hunting ratio" anymore.

Design knowledge lost to time. Like it never existed.... nor is important.... consideration when designing any chain-line, powertrain. Never ignored in the last 50 years of motorcycle design.. car design.. bus truck plane.. hell .. everything that has a ratio.. When designed correctly.... has a hunting ratio.

The reason for numbers like "4.10:1" or "3.27:1" or ... or....

" 17f / 41r ".

.........forgotten. lol.

 

[JoltJockey]

There's a lot of projects in the non-hubmotor drives section over the years, that will probably help show you want can work and what probably won't.

FWIW, 8400w of power is likely much more than the typical parts on a mountainbike are designed to handle, so I would recommend expecting significant failures from the torque of this setup, and be prepared for this to happen on your rides, possibly without warning, especially offroad where such power is less likely to be regulated against on a bicycle.

Motorcycle frames and wheels are likely to survive it, and would be a better choice.

What specific conditions / usage do you have that require this much power?

I ask because even with my SB Cruiser heavy cargo trike, I can haul a piano or hundreds of pounds of cargo of various types with much less than half of that, with two common hubmotors and generic controllers, on city streets, at up to 20MPH, with no significant pedalling force provided by my aging aching joints.


If it's "dirt bike" type performance, you should probably look at the Sur-Ron and similar bikes for inspiration.

Expectations:
While I do want the setup to be usable and not break down every 50miles (aswell as be reasonably safe) I will mainly ride it for fun and just really enjoy the feeling of a punchy acceleration and having some extra power on hand.

Why this much Power? + Riding Conditions:
[My main reference is the E-Skateboard I built a year ago. Powered by two no-name 6354 Motors + VESC limited to ~60A for a total of ~ 3000W made the Setup fun and strong enough to get up light inclines. Even though I used full throttle only occasionally, the system especially the motors heated up to ~60°C on summer days which made me feel asking for more would be pushing it.]

Given that the MTB will be heavier and a much more stable platform I'd like to give it at least 2x the power and not have to worry about the operating temp of the components + especially with a single speed setup. I want to make sure the bike can achieve a good top speed on flat asphalt - cycling paths but will also be responsive on medium inclines on gravel/ dirt paths up small mountains and out of a dead stop - with and without pedalling.

The Base - Mountainbike:
I'm still putting together the shopping list for the MTB. Either an aluminum or steel frame as often recommended on the forum. Full suspension with a big Downhill fork in the front. I won't be riding anything close to as demanding as what actual enduro or downhill riders put themselves and their gear through.
I just want a ride that feels confident off-road and a beefy suspension that takes some load off the frame when riding over roots, rocks potholes etc. given the added weight and higher speeds. (I'm 5'8 155lbs / 170cm 70kg). "e-Dirt Bike" might be a overstatement in terms of the capabilities I'm looking for.

 

[scianiac]

You and this bike sounds pretty similar to me and the bikes I build. What I think you should clarify is the type of terrain and speeds you are planning on. A bike you ride on fire roads at high speed has some different design requirements from a bike designed to actually ride technical single track at slower speeds and while it can do both it should be optimized to for what you'll be doing most in all ways. Gear ratios, suspension, weight, etc are all important things to consider.

While I too love the idea of using big outrunners to make a high power to weight ratio bike and had planned on that for a long time eventually I decided it just wasn't worth the time and effort considering the end result would be worse in a lot of ways than the alternative and with only some very slight advantages. The alternative for this style of bike is an Lightning Rods kit, which will be a little heavier than the outrunners but will have plenty of power, really you can't compare the power because those outrunners really can't be run at those power levels very long unless you have tons of forced air cooling and then you're on to some whole issue of ingesting water and mud and etc. But the LR kit has the motor, mounting, reduction, driveline all sorted out pretty well for a bike of this style. It's not exactly a one and done kit, more like gets you a good start to a complete bike. Yes one day I would like to build a drive totally from scratch but I'm very glad I went this route and have had a ton of fun building, riding, and upgrading the bike with custom parts.

If you do want to go totally custom I would use a similar setup to the LR kit, that is to say, motor in front of BB, 1 belt or chain reduction (ideally belt) to a freewheeling BB and then back to the rear with a single speed. With lots of reduction from the BB chainring to the rear you can put quite a lot of power through a regular bike chain (I'm putting 5kw and I don't worry about the chain at all but I was popping links off my chain on my 800w TSDZ2, it's very much a matter of condtions what a bike chain will take) and if you need more you can just change the chain to a 219 and have no worries about power. Now this design is partially optimized for off road use, that is so say you can't have too much moving weight on the rear swingarm too far from the pivot which is the issue about putting the motors above the wheel on a rack as you describe but this depends on your intented use like I said at the start, if we aren't talking real techincal terrain that might be fine.