Any advantage to smaller rear wheel?

[eflyersteve]

I'm building an electric enduro (http://em3ev.com/store/index.php?route=product/product&path=47&product_id=209) and am about to purchase the rear hub kit. Likely a leafbike 1500 watt kit. I have the 26" front wheel (20mm thru axle to match double crown fork) and have been considering a 24" rear wheel. My reasoning was to keep the spoke length short in hopes of gaining a bit more strength and a bit more torque, but is it really necessary? As you guys know, this frame is heavy steel. Is there any downside to using this smaller wheel on the rear? I'll ask for a wider wheel as I plan to run 2.5" wide tires if that makes any difference. I think I'd like the look of a slightly smaller diameter on the back, but function trumps form in this instance.

Anyone else running a 24/26 combo on this frame?

Thanks!

 

[MadRhino]

There is a neat advantage lacing a hub motor in a smaller wheel, as long as it doesn't compromise handling because of rough terrain, or geometry. Most likely, the geometry of the bike is adequate. This means that, unless you can do a mod to correct the geometry, you would be better using a 24" wheel on the front as well, or both 26".

 

[Drunkskunk]

... My reasoning was to keep the spoke length short in hopes of gaining a bit more strength and a bit more torque...

No on the strength. the smaller wheel will be weaker, though a properly built 24" wheel with 14g spoke could be dropped from an airplane and survive. A smaller diameter wheel increases the spoke angle, already at an unfortunate angle due to the large diameter hub, thereby putting more stress on the nipples and increasing the chance of failure. Also, shortening the spoke also shortens the amount of stretch they can give. the whole point of a spoke instead of a solid bar is that it does stretch, allowing the wheel to deform and absorb impacts that would damage the rim, hub, and spokes if they could not.
Not really a problem for a well built wheel, even at 20", but it isn't stronger to have shorter spokes.

As for torque, yeah, there will be more. About 8%. And a loss of about 8% top speed.
If it was me, I'd give some of that up by fitting a 3" tire instead of the 2.5", and get a better ride quality, better braking, and more traction.

A lot of people do the small/big combo for various reasons. it works well, especially for torque, but it does screw the geometry of the bike up a bit. the bike won't preform as intended. That's not going to be as big of an issue on that single pivot style frame you're using as it might be on something like a Demo 8. Everything is a trade off.

Another alternative that would actually be stronger, is to fit a 19" motorcycle rim and tire. A 19" MC wheel is the equivalent of a 23" bicycle rim. a bit smaller than your 24", but you would run a little bigger tire. The downside here is that it's (a) heavier, and (b) an actual motorcycle part. On a vehicle that is basically an unlicensed, unregistered, and uninsured motorcycle with pedals. Convincing the police that you aren't up to no good will be that little bit harder. Although getting away from those police will be that little bit easier.

 

[neptronix]

Many motors' continuous power handling can be increased by running in a smaller wheel with higher volts.

For example, the MXUS '3000w' motor is rated at 3000w in a 20" bicycle wheel, but when put in a 26" wheel, we see that it is more like a 2500w continuous motor.

http://www.ebikes.ca/tools/simulator.html

Same can be said for the leaf.

If you play around with the ebikes.ca motor simulator enough, you'll see exactly what i mean.

 

[brumbrum]

Depending on the wind/turn count of your motor a smaller outer diameter tyre will also lead to less heat loss when going up big hills at full throttle and full amp pull. A smaller tyre means your motor is more efficient as it can get to its designed max rpm easier and quicker,, again less wasted heat, and a cooler running motor less likely to overheat.

 

[dogman dan]

Yes, there are huge advantages to a much smaller wheel, like 20", in torque, higher motor rpm leading to better efficiency.

However, those are negated when you go off road and need a larger wheel, and or compromise the geometry. The funkiest e bike I ever built had the geometry changed too much. It became more or less impossible to keep the front wheel on the ground. This was fun to ride in a dumbass way, but no way I'd keep it that way.

However, 24" is not that big a change, particularly if you change it back with a tire that is taller. The rest you may be able to get back with adjustments, or, you may actually prefer a slacker headset angle for your particular ride.

But you notice,, what I'm talking about now is not really a wheel diameter change, or very much of one. So the advantages will be equally small change. I'd stick with 26" if the tire you want to run will still fit.

 

[eflyersteve]

Thanks for all the replies. I'll probably just go with a 26". This will likely be moved to a standard bicycle frame later if I decide to go with a 19" f/r setup and a larger hub motor.

 

[Doctorbass]

I agree for that question about power. But confort decrease alot. i would say it depend on the road you ride on. but on rough trail, it will bounce alot more than if using a 24 or 26".

I would be curious to see how well a fat 20" wheel on the rear could be! or even a 16" fat.. this could be a compromise for confort and power gain.

Doc

 

[MadRhino]

You remember those old Honda Mini Trail ?
They had small wheels with fat tires. They were a good ride on the street, but a freaking bucking poney in a mountain trail.

Off road, unless it is flat and nice, you need larger diameter wheels and tough tires. This means that a hub build does require a slower winding motor, heavy enough to shed the heat. Small fast motors in a large wheel are never good, and frying very quickly in the mountain.

 

[eflyersteve]

Pretty much flat terrain - some small hills. Hard packed dirt country roads. Desired max speed in the 20-25mph range.

I would be willing to give a small diameter wheel-set a try on this frame eventually - something akin to a supermoto style setup. I can get buy with a wider rim on the shorter diameter stuff (the swingarm is tapered immediately after the dropouts so the larger the wheel, the thinner it needs to be).

 

[MadRhino]

Well, that is another advantage of smaller diameter wheel. A wider tire is better comfort, traction, puncture resistance...

That is why my street bike is on 24 X 3.0 with wide rims, letting me ride much lower PSI. I have tried 20, but the streets are too rough here, and 20 really doesn't like curbs or stairs. Yet, 20 in fat wheels, 4 or 5 in may be fine.

 

[markz]

As for torque, yeah, there will be more. About 8%. And a loss of about 8% top speed.
If it was me, I'd give some of that up by fitting a 3" tire instead of the 2.5", and get a better ride quality, better braking, and more traction.

A lot of people do the small/big combo for various reasons. it works well, especially for torque, but it does screw the geometry of the bike up a bit. the bike won't preform as intended. That's not going to be as big of an issue on that single pivot style frame you're using as it might be on something like a Demo 8. Everything is a trade off.

Another alternative that would actually be stronger, is to fit a 19" motorcycle rim and tire. A 19" MC wheel is the equivalent of a 23" bicycle rim. a bit smaller than your 24", but you would run a little bigger tire. The downside here is that it's (a) heavier, and (b) an actual motorcycle part.

I think this is the reason people are riding fat bikes on cement.

 

[WoodlandHills]

There is a reason that even the smallest motorcycle has a tire that would be considered "fat" by bicycle standards. :wink:

 

[dogman dan]

Yeah, we won't tolerate the weight of an MC tire and rim. However,, NOW we are talking about motors that negate that weight penalty by quite a bit.

Handling still matters though, so you may not want to go all the way to moped rims and tires. Keeping it bike like and light still has virtues.

 

[MadRhino]

Yep, every pound of bike is a loss of effective HP, and even more so when it is a pound of wheel.

 

[ecycler]

I'm building an electric enduro (http://em3ev.com/store/index.php?route=product/product&path=47&product_id=209) and am about to purchase the rear hub kit. Likely a leafbike 1500 watt kit. I have the 26" front wheel (20mm thru axle to match double crown fork) and have been considering a 24" rear wheel. My reasoning was to keep the spoke length short in hopes of gaining a bit more strength and a bit more torque, but is it really necessary? As you guys know, this frame is heavy steel. Is there any downside to using this smaller wheel on the rear? I'll ask for a wider wheel as I plan to run 2.5" wide tires if that makes any difference. I think I'd like the look of a slightly smaller diameter on the back, but function trumps form in this instance.

Anyone else running a 24/26 combo on this frame?

Thanks!

I have that combo on one of my bikes.

24 inch in the back and 26 inch DH Alex rim in the front. Marzocchi shocks front and rear. It looks fine and handles VERY well. Don't skimp on the shocks and you will be very pleased with these cheap frames. I recently ran the dragon and was out of sight by the wife following in a relatively fast car by the third turn every time!

What exactly do you want to know other than if there are others out there running this combo with street tires? I can probably dig up a picture if you are curious to see what it would look like.