mismatched gearing

[northguy]

What would the ride feel like if the gearing on the front wheels were different than the back (if the rear diff was say 3.9 and the front 3.7)? Would one notice this only when 4wd were engaged, or would there be a constant battle going on? How would the vehicle handle in 4wd?

[idosubaru]

if part of the drive shaft was removed, it would just drive like it was a FWD vehicle (so long as the center diff was locked or it was in 4WD or a fuse was inserted in the FWD holster of a 4EAT, or someone tweaked something like the TCU to lock the center diff on an auto tranny). the rear output shaft would spin, but it wouldn't do anything because it's not connected to the rear diff. in that case you could get away with it.

 

depends what transmission you're talking about. the 4EAT TCU would probably flip out, cycling the proportioning of the transfer clutch on and off as you drove. it would feel like the car is braking when you let off the gas but are not hitting the brakes.

 

if you have a manual trans with locking center diff (like on manual trans XT6's), it would definetly experience some binding and hopping, which normally occurs on pavement while turning, but you'd experience it at all times. it would feel like a constant drag, constant braking.

 

while driving straight you might hear your tires chirp as the binding gets too severe and one tire breaks loose to relieve the pressure.

[NorthWet]

If not a FT4WD, and 4WD not engaged, you should not notice any difference.

 

You would NOT want to run this in 4WD, except maybe on totally loose surface. And even then, you would probably get either a strong understeer or oversteer, depending on which end had the higher (lower numerical) gearing.

[bushbasher]

with open center ft4wd you wont notice any difference, because the diff will make up for the difference. Actually you will feel a different final gear ratio, it will be the average between the 2 ends, as a result of the center diff. Lock the center diff and you will get binding. That said, on gravel I wouldnt worry about the 2 different ratios, I drove my car around the property for quite awhile with 30" tires on the front and stockers on the rear (big speed difference) and while there was binding and strange handling, nothing broke, and to get it back out of 4wd I had to shift right after spinning a tire.

[northguy]

That's interesting as when I put my 83 Brat into 4wd on snowy roads, it is almost impossible to steer a straight line with it. While I can go down the same road in 2wd no problem, as soon as I put it into 4wd, it breaks loose and drifts all over the place. It drives like I did years ago when I used to drink.

[Snowman]

Wow, that's really strange. You might try jacking up the rear end, turning the driveshaft by hand and counting the revolutions of the shaft per revolution of the wheels to verify the ratio. I suppose it's possible that the diff was swapped at some point. Is this a new problem or has it been going on for a while? The only other thing I could think of that would be causing such a condition is that something in the rear driveline/CV's is binding up. Make sure that everything turns freely. Aside from that, I really don't know. I'd be happy to come out and go for a ride in it this weekend (not much that I have to get done, amazingly) and see what I think.

[northguy]

If you're going to be around tomorrow, I'll swing by and show you what I mean. I've had the same problem since I bought it.

[idosubaru]

sounds like different ratios.

 

when you're in 4WD the center diff is locked, making your front and rear axles turn at the exact same rate. but the differentials will be drive the tires at different rates as indicated by the gear ratior of the diff. so one tire will keep breaking loose to make up the difference. you won't experience binding because your tires are allowed to slip on the slippery surface, but you won't experience traction either because either your fronts or backs will keep braking loose due to the binding caused by the different gear ratios. which ever tire maintains traction will want to push or pull the car in that direction.

[ccrinc]

Mismatched gear ratios can make the rear wheels lock up...completely.

 

Emily

http://www.ccrengines.com

[NorthWet]

Funny coincidence today...

 

I was driving home in the rain, approaching the Tacoma Narrows Bridge (you might remember its predecessor from the old black and white film of a bridge twisting apart in the wind, dumping a car into the water as it collapsed?), when I decide, OK, I'll pushbutton into 4WD for the trip across, given the crosswind and metal grates.

 

GGRREEEEEECCCHHH!!!! Slow speed got even slower suddenly, accompanied by horrid drivetrain noise and binding. Hit the button again, and after several seconds it came out of 4WD with a loud CLUNK!

 

OOOPS! I forgot that a couple of days earlier I had swapped out the front, balding tires for another pair of wheels with more tread. That difference between the tire types and wear between front and rear was almost enough to do me in on a flooded highway.

[northguy]

Took it for a ride with Kelly (Snowman) today to confirm the squirrelness of the Brat, and he agrees, something is WHACKED. So, I guess I'll be looking to do a conversion to a 5spd D/R and while I'm at it, I may as well put in a LSD (3.9). I can probably find the tranny and driveline locally, but will need a headhunter from the land of Pull-a-parts to drum up one of the coveted and highly elusive 3.9 LSD's. Ed, are you interested?

[oddcomp]

i had a 76 k5 blazer on 44"s i bought a long time back previous owner was a idiot and had mis mathed gears in the front and rear axles

i had to pull someone out of my yard "don't ask" and it blew the chain thru the side of the big old cast iron np 205? i think it was

this was also a big big mismatch not a small one

 

if your racing or mudding its usually a benefit to have the front geared one step higher than the rear so it ends up pulling the back around in stead of being pushed around better steering ect but only if your on loose surfaces sand mud snow gravel

[richierich]

Well let me add a little insight to this. This was my old BRAT. When I bought it would not go into 3rd gear intermittenly. Sometimes it would if I decelerated and the put it in gentle. Pulled the transmission out and my father took it apart, found 3-4 shift fork was worn and needed replaced. Called the dealership and tried to order the part. It was backordered in Japan. So we put in a different 4speed 4WD Transmission. (later we were going to get the parts and reinstall the transmission) The differential was never touched, no sign that it was ever had been replaced. Unless there are different gear ratios in the 4 speed the ratios can not be different. It was my understanding that 82 4speed 4WD had a differient shifting points but the same final drive of 3.9 If it doesn't then, then it might have a 82 tranny in it, BUT with my father's 82 BRAT it would register 4000 rpm at 65mph, and with Northguy's BRAT and my current 85 BRAT I got 3500 rpm at 65mph showing that it is the newer transmission.

 

 

I can send you the 4WD portion of the original transmission since we never put it back together because we found out you had to buy an update kit not just 3-4 shift for which was $200 bucks. And the tranny we installed shifted fine, so no need for replacement. The problem has to be interally in the transmission.

 

BUT if you can find a 5speed 4WD transmission than I think you will come out a head. In addition if we can find you a 3.7 LSD then we can convert it and send it up to you as a 3.9 LSD.

[northguy]

Richie, that's the route I'm looking at going - converting to a 5spd d/r tranny and an lsd. Since I'm not sure where the problem is, I am probably money ahead to just swap out the entire driveline and throw in a lsd while I'm at it. If you should run across an lsd that can be converted, or one of those elusive 3.9's, snag it for me. I'm good for the cash at any time. Parts aren't always easy to come by here, but I can probably find a tranny and driveshaft locally.

[idosubaru]

you could have your driveshaft lengthened or shortened by a driveline shop or a good machine shop to fit the new transmission. get the desired length from a group member.

 

could you also keep your eye out for the newer 4.11 LSD's and swap internals as well into a 3.90? i think i've heard of that being done also. or maybe i'm thinking the 4.11 can be converted to LSD with the 3.70/3.90 internals?