Running with Larger Rear Wheels

[zboot]

I just picked up a wagon with different wheel sizes front to rear. Fronts are 235/65/15 and rears 235/75/15. I'm planning on getting a new set of all seasons for the wagon (Nokian WRG3s) which will all be the same size but I had some concerns about the current setup.

I know the viscous coupling essentially is "speed sensing" rather than "torque sensing" and works to equalize the axle speeds between the front and rear. In RWD, the different front to rear won't be a problem because only one set of wheels is being driven. In AWD, my rears will always be "slower" than the fronts (assuming good traction).

So thinking about it, giving more power to the slower (rear) wheels under that circumstance isn't really a problem because I've got good traction anyway. In low traction situations, if my rears the the ones slipping, I'd get even more power to the rear, so still ok. But if the fronts are slipping, then the wheel speeds start to equalize, I have less difference between the front and rear, so get closer to balanced power delivery front to rear - less goes to my rear (which has traction) and more to front. This is the normal behavior, except in normal conditions, my fronts would have more power. So worst case is likely when both wheels are slipping. Then I still have the rear biased power delivery.

So, after thinking this out, it seems the biggest impact my current setup has in AWD is that my power will be rear biased by the difference in front/rear speeds (7%). Given the weight balance, most of the time, it probably actually is a good thing. But I will have reduced front power delivery when the fronts are slipping and worst case behavior when both sets of fronts and rears are slipping. Make sense?

[eaglefreek]

Driving in AWD with the rear tires smaller than the front, will ruin the viscous coupler.

[amcfool1]

don't do it, playing with fire. An Eagle is VERY sensitive to tire sizes, all four NEED to be the same size. I have been driving an Eagle since 94, i don't just get the same SIZE tires. I replace all four at the same time with the same brand/size/type. Remember, back in the day, AMC employed a bunch of really smart engineers, and much more than most, they usually got it right, Stick with the Plan, Stan! get your car running, not re engineer it!  With different size tires, your transfer case, specifically the VC., is guaranteed toast!! good luck, gz

[zboot]

Driving in AWD with the rear tires smaller than the front, will ruin the viscous coupler.

My rears are larger than the fronts (235/65 front vs 235/75 rear).

don't do it, playing with fire. An Eagle is VERY sensitive to tire sizes, all four NEED to be the same size. I have been driving an Eagle since 94, i don't just get the same SIZE tires. I replace all four at the same time with the same brand/size/type. Remember, back in the day, AMC employed a bunch of really smart engineers, and much more than most, they usually got it right, Stick with the Plan, Stan! get your car running, not re engineer it!  With different size tires, your transfer case, specifically the VC., is guaranteed toast!! good luck, gz

Ok, so I'll stay out of AWD until my new tires arrive and are mounted. I know the viscous coupler is still good so perhaps the PO kept it in RWD most of the time.

[JayRamb]

I'd stay away from that all together. That Viscous is hooked to the tranny. I even stayed away from using spacers in the back drums to push out the tires to line up with the front for fear of axle issues or bearings and who knows what else could happen.. I left things as is as the engineers of AMC to keep it as it is with the rims, new tires all around, proper rotation and inflation to specs.

[zboot]

I'd stay away from that all together. That Viscous is hooked to the tranny. I even stayed away from using spacers in the back drums to push out the tires to line up with the front for fear of axle issues or bearings and who knows what else could happen.. I left things as is as the engineers of AMC to keep it as it is with the rims, new tires all around, proper rotation and inflation to specs.

After doing more reading on the design of the transfer cases used in the Eagle, I think I have a pretty good idea what could happen. In RWD, the viscous coupling isn't active to drive the front axle. So, there's no heating up of the fluid or anything that can cause problems. The tire size difference won't affect the drivetrain in that case. I'd have other issues, but the extra wear on the front tires doesn't matter to me since I'm trashing them when my new ones arrive.

The issue (based on my reading) with running with different sized wheels front to back is that in AWD, the viscous coupling is always active. The way the viscous coupling works, when there is a speed difference, it causes the fluid to heat up and kinda solidfy, while it transfers power to the slower axle (similar to the way an automatic transmission transfer power from the engine). As the speeds equalize, the fluid cools back down and things go back to normal. The problem that occurs when you have mismatched tires front to back is that your fluid heats up and never has a chance to cool down. If it gets hot and stays that way for too long, the properties of the fluid change. There are pretty much two major failure modes. The ok - you essentially end up with viscous coupling acting like an open diff and the bad, you end up with your viscous coupler gluing your front and rear together into a part time 4WD. Anyway, it's this second one that can destroy your drive train. The first just bites you in the snow and hopefully all that happens is you get stuck in pretty deep snow. As the open diff drops torque to both axles down to essentially the one with least available traction, you still end up with some semblance of AWD capability.

Long story short, so long as I keep things in RWD until I get my new tires, I'm not concerned. Plus, I've still got an NP229 transfer case I bought some years ago for my first wagon. I'm almost in need of an excuse to install it.