4 wheel drive tire size differences

[gfmucci]

I would assume that a 4 wheel drive vehicle requires all 4 tires to be of equal diameter. How much can the fronts deviate from the rears without creating a problem - and what kinds of problems can be created?

[clueless]

You are right in saying that the fronts are supposed to be the same diameter as the rear, this is true of most vehicles, not just 4x4's.

Changing the size of the tyres front / rear will have a similar effect as changing the gear ratio's in the axles.

Some people run a different ratio front to back (under / over drive) to get better performance when crawling, but I dont think that this would give you the same benefit!

What are you trying to achieve by doing this? Is it just for looks?

[gfmucci]

Not so much for looks as for learning what works and what doesn't, and what works better/best. Well, maybe for looks if I wanted slightly larger tires on the rear for the Twin Hammers.

So this leads to another question if different tire sizes are feasible: Would different front/rear sizes strain the drive shafts or is there a type of front/rear unlocked differential, like there are on the Twin Hammers fronts? In other words, is it ok for the fronts to run at different RPM's, as would occur with different tire sizes - the larger tires will run at slightly lower RPMs, right?

[lonleycreeper]

Originally Posted by gfmucci

I would assume that a 4 wheel drive vehicle requires all 4 tires to be of equal diameter. How much can the fronts deviate from the rears without creating a problem - and what kinds of problems can be created?

Alot actually...


It all depends on what you want.

Mud trucks like the one pictured have relatively tall gearing especially in comparison to the rear axle. The front axle is thus overdriven keeping the front planted, and pulling the truck straight.

Problems...if you don't match the gearing to the tire size, you will have drive line issues, breaking driveshafts, or gears depending on which is weaker, traction or drive line.

[Olle P]

To ease things up there are devices like differentials...
An open differential beween the front and rear axle (combined with some front axle overdrive) will of course make things a hole lot easier.

In my crawler I have overdrive on the front axle, but also effectively a smaller wheels radius since the front wheels have softer foams and carry more weight.

[gottorque]

Im not sure running softer foams makes the OD smaller since the rollout of the tire will always be the same regardless.

On an RC, running different sized tires front to back wont cause all that much more stress than running equal sized tires. Its the same as people running OD and UD gears...one axle will simply spin faster than the other.

[clueless]

for argument sake however, generally wouldnt it actually make the truck less capable offroad with the smaller wheels? Typically people at least in 1:1 sized rigs try to go for the largest tyre possible to get the best outcome off road.

[EeePee]

1:1 is irrelevant.

I ran different tire sizes on my Losi and it was fine. Larger front for an overdrive effect.

[Olle P]

Originally Posted by clueless

... wouldnt it actually make the truck less capable offroad with the smaller wheels? ...

Win some, lose some...
Bigger wheels give the advantages of more clearance below the axles and less ground pressure. For RC ground pressure is pretty much irrelevant and the axle clearance is more a matter of finding a good line.
The disadvantages of big wheels are less steering (due to the wheels touching the links) and raised COG, both relevant to RC.

That's why I use Rovers instead of Sedonas. The Rovers are still larger diameter than the Losi Rock Claws though.