Random question about diffs and running taller tires in the rear

[sumguy75]

Ok, first off, I am not planning to do this! I said that first because I don't want to get replies warning me I'll break stuff if I do.

This is just a question because I want to do a custom build on a 2wd Bandit buggy and want to run belted Badlands MT tires in the rear and Badland buggy tires up front. That made me wonder why a 4wd buggy might have issues with it based on what I've read so far about different tire sizes.

When I've seen people mention running rear tires taller than the front, there is always a mention on changing the diff gears to accommodate. I am curious what this would do to prevent issues with the different tire sizes. When running same size tires with a low center diff fluid weight, the front tires tend to balloon more. That makes them a larger diameter than the rear. Wouldn't this cause damage over time if it was an issue running different sizes?

Again, not planning to do this, just trying to understand what makes it different.

 

[SrC]

Good question.
With 4WD, as the tires balloon, the tire's Rollout (diameter increases) So technically if the front tires balloon more, the drive ratio changes at that axel. It is momentary. But does affect driveability for sure. With 2WD, this does not come into play. In fact with 2WD, front tires are usually different. Chosen to enhance the undriven axel at the front. To enhance steering.
The only stupid question is the one you don't ask.
My spin.

 

[#ARRMArcfanboy4life]

I really wanted to know this. Thanks. I have seen Kevin Talbot use different gearing for front and back (the same effect as using different diameter tires) to make the front end of the car pull while the back end pushes.

 

[SrC]

I really wanted to know this. Thanks. I have seen Kevin Talbot use different gearing for front and back (the same effect as using different diameter tires) to make the front end of the car pull while the back end pushes.

KT does many silly things. Is he wrong? That is subjective I imagine. The drivetrain and driveability will suffer IMHO. The RC Mfrs. know best IMHO. RC drivetrains are designed around Full scale vehicles. How the MFR designs them out the box. In reality, you can do whatever you want with RC cars. Trial and error will determine what works for you.
With RC Crawlers that drive super slow, Over/Under driving the axels has advantages for climbing the rocks. By design. This is strictly a Crawler "Thing" however.

 

[Diem Turner]

Ok, first off, I am not planning to do this! I said that first because I don't want to get replies warning me I'll break stuff if I do.

This is just a question because I want to do a custom build on a 2wd Bandit buggy and want to run belted Badlands MT tires in the rear and Badland buggy tires up front. That made me wonder why a 4wd buggy might have issues with it based on what I've read so far about different tire sizes.

When I've seen people mention running rear tires taller than the front, there is always a mention on changing the diff gears to accommodate. I am curious what this would do to prevent issues with the different tire sizes. When running same size tires with a low center diff fluid weight, the front tires tend to balloon more. That makes them a larger diameter than the rear. Wouldn't this cause damage over time if it was an issue running different sizes?

Again, not planning to do this, just trying to understand what makes it different.

On a 4WD car, if you have larger diameter tires on the rear, the rear will drive faster than the front. Imagine the diff input gear on the front and rear. They will both complete one rotation at the same rate. The larger diameter tires will travel a longer distance than the smaller fronts due to larger circumference (front and rear wheels will both complete one full rotation with the same amount of turns of the input gear...I hope this makes sense). To compensate this, it is necessary to run a different gearing on the rear diff to slow down the tires proportional to the amount they are larger (or the front to speed the tires up; same principle applies). By how much you'd need to calculate out based on the size of the tires. Unfortunately there aren't nearly as many options for diff input/ring gears as there are for pinion/spur gears so you'll be limited to what tire combos you'll be able to mix and match.

 

[sumguy75]

The rear will drive faster than the front.

This made me understand it better than anything.