Okay all you 4X4 guru's here's the question for the day? My friend has a Yamaha Big Bear and he cut his rear tire down on his last ride. He bought a set of GBC Dirt Devils to replace the rear tires. He still has the stock dunlops on the front. We were looking at the tires and noticed that the GBC were smaller in diameter than the stock dunlops by about an 1" will this mess up his full time four wheel drive ratio? Or will he be okay?

 

Yes that would affect the ratio. Normally on trucks you will tear up the transfer case by doing such.

 

Cummins is exactly right. If riding purely in mud, or very slippery conditions, where the two differentials can turn at the same time, regardless of tire speed-due to slippage of the larger tire-you should be OK. However, when you get on any type of hard pack or terrain where both front and back will be turning at the same speed -no slippage- you are causing the gear ratios to try and turn two different sized tires, causing a 'bind', and consequently messing up your t-case. (the smaller tire has to go faster to travel the same speed as the larger tire, yet if the gear ratios are the same in both differentials, as they should be in the big bear, you get the "binding" effect.) If he doesn't want to buy new tires, he can either air up the smaller ones to give a taller profile, or lower the pressure in the tall ones to make them smaller, the same size as the shorter ones. Of course, if he really wanted to keep the stock tire up front, not mess with changing air pressures to compensate, and run in his current condition, he could change the gears in the front diff. to match the proper ratio of tire size, and allow the t-case to turn both equally, yet that isn't very practical! [img]i/expressions/face-icon-small-happy.gif[/img]

Anyway, good luck!

Mike

 

What they said. Keep the same ratio always on a 4x4. If the front and rear are the same height, keep them the same height. If the front is an inch or two lower, then replace it with that size tire.

When changing to larger tires, if you go up an inch or two on the rear, go up the same on the front.

 

Just remember if you let air out of a tire to make up the difference that will only work a slower speeds, centrifugal force will make the tire taller at high speeds. So you would be better off filling the smaller ones to make up the difference... Bill

 

Okay here is what we did. Assuming the transfer case is the same for the front and rear then it should have the same gear ratio, correct? So if they have the same gear ratio then as long as the tires are turning the same distance then we should be okay, correct?
Now we took the quad and rolled it one revolution and then measured the distance for both the front and rear. There is only 1/4" difference between the old fronts and the new rears. They are almost the same and I don't think that the 1/4" is going to make that big of a difference or is it?

 

Well I was going to do all the math for a mile. But didn't want to use up all my thinking for the hole day.LOLYour talking about 1/4" per revolution(appx.80 inches), doesn't sound like much. Add that up for 1
mile and you will see that something is going to give sooner or later especially if you drive on hard pack like Cowboy said. If it's because of the money and you don't need four wheel drive take out the front drive shaft, just a thought maybe. It might last a long time like that but I myself wouldn't run 2 different tire sizes. The parts you might tear up I'm sure cost more than two tires.