Tire/wheel size

[csocs_wolf]

This is all very complicated and confusing. Question for my son's off-road kart. I currently have 14x5.71-6 on front and 16x8-7 on rear. I was given some 18x9.5-8’s and want to put on the rear to “beef it up” and add traction with a new larger engine. Is the extra 2" rake going to kill handling or make dangerous? I’m not racing, but my 9 yr old might as well be. Thanks.

Note: Will be putting new tires on the rims once decided.

 

[itsid]

well chances are the engine - rather the drive train gear ratio - is not able to keep up with the larger wheels
as easily so you might destroy your clutch/drive belt in very short order using the larger wheels.

the way to compensate is divide the axle sprockets tooth count by the OLD wheel diameter and
then multiply that with the new wheel diameter ( new_toothcount = 18*old_toothcount/14)
and get the closest toothcount sprocket (in your case one that has 28-30% more teeth)

then the overall ratio (engine to road) doesn't change at all, the wheel torque will stay the same and so will top speed

'sid

 

[csocs_wolf]

well chances are the engine - rather the drive train gear ratio - is not able to keep up with the larger wheels
as easily so you might destroy your clutch/drive belt in very short order using the larger wheels.

the way to compensate is divide the axle sprockets tooth count by the OLD wheel diameter and
then multiply that with the new wheel diameter ( new_toothcount = 18*old_toothcount/14)
and get the closest toothcount sprocket (in your case one that has 28-30% more teeth)

then the overall ratio (engine to road) doesn't change at all, the wheel torque will stay the same and so will top speed

'sid

Thanks sid. It had a 49cc GY6 clone w/ internal 3 speed. It will now have a 212 Predator with stage 3 kit and 30 series torque converter. While I am plenty sure this will more than cover the big tires, I figured I'd have to play with gearing anyway. Going from 3 to 18 HP will require adjustment. Just wanted to make the rake decision before moving forward. Meaning, will adding inches on the rear and displacing weight forward be stable? Will displacing weight forward negate the traction benefits of the fatter tires in back? I'd like to raise the front 2" too, but tire and rim options aren't easy to find.

 

[itsid]

don't underestimate the internal gear reduction of gy6-style engines

especially the smaller ones are usually geared internally rather low (7:1, 10:1 or such)
and when swapping to an industrial engine all that ratio will be gone!
the final chain ratio (on gy6es usually in the realms of 2:1) is NOT too telling.
but when you change the engine anyways, you will need to set up and calculate a new gear ratio anyways,
if you do that for 14" or 18" wheels is just a matter of sprocket size.

'sid

 

[csocs_wolf]

don't underestimate the internal gear reduction of gy6-style engines

especially the smaller ones are usually geared internally rather low (7:1, 10:1 or such)
and when swapping to an industrial engine all that ratio will be gone!
the final chain ratio (on gy6es usually in the realms of 2:1) is NOT too telling.
but when you change the engine anyways, you will need to set up and calculate a new gear ratio anyways,
if you do that for 14" or 18" wheels is just a matter of sprocket size.

'sid

Great info. Unfortunately I can't keep the old one running. It's seen better days. I've been rejetting and tuning for the last 3 months. I get about 30 mins from it running like a champ and then can't start it for 2 days. Prompted all this work to begin with. I went with the easier "throw in a new engine" opposed to rebuilding the old one. Maybe a bad call. I don't know. But I was also lured in by all the Predator mods.