Thread: more ??'s about rearends...

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Hello there,

Sorry I know this subject has been beat to death, but I’m still confused about posi & rear end ratios. I understand the concept of both wheels spinning for the posi as opposed to just one for the open drive. However, I’ve heard two different things about them which concerns me. Particularly about cornering, I’ve heard posi will lock up and spin you out if you corner and I’ve heard it does better because it allows one wheel to travel slower than the other giving the car more “grip”.

I am putting together a T Bucket, and am working on the rear end now. So far I’ve got the 9” Ford rear end housing and have been told all sorts of advice and not been able to make sense of it all yet. I’m running a 350 SBC (bored .30 over) backed by a Powerglide transmission. I don’t have the back wheels but my rims are 15x15 which I *think* will need 31” tall tires.

The car is not for racing, just driving around occasionally. I was told that with such wide tires, I would flop the car over if I didn’t run a posi. Then I was told by someone else a posi would lock up on me and could spin me out if I were cornering. I’m sure it all comes down to how you drive, but I want to make sure.

One last thing, about the ratio, I was trying to do the formula suggested but I have no idea what RPM my engine would be at while doing 60MPH. How do you find out without running the engine? (I’m feeling really dumb right now!)

If anyone could clear that up, I’d so appreciate it! Thanks!
-Jackie

In street rodding the term 'POSI' is usually generic for limited slip. What you want is a limited slip differential for streetable trouble free performance.

A formula for carspeed based upon engine rpm, tire diameter and final drive (rear end ratio) is;

carspeed=0.002975*rpm*OD/FDR where OD is tire diameter in inches and the FDR is the rear end ratio. rpm is engine speed in rev/min. This equation assumes a transmission final drive of 1:1. You can easily put in the overdrive factor as well if you have one.

Regards, Kitz

A limited slip rear end will not spin you out going around a corner, a spool could have a tendancy to do that if you hung the rear end out a bit but even with a spool it has a tendency to put a push in the front end more than loose on the rear.

For a 60 mph cruise at 2,300 RPM on a 31" tall tire, your rear end gear should be a 3.50:1 ratio.

In a car as light as a T bucket, you shouldn't run a posi rearend. A normal, non posi rearend is designed to let one wheel turn faster or slower than the wheel on the opposite side, because when you go around a corner, the wheel on the outside of the turn actually has to travel farther than the wheel on the inside of the turn. This means that the wheel on one side of the car must be allowed to turn faster or slower than the wheel on the other side. This same built in mechanical ability will allow one wheel to spin more than the other when under extreme acceleration.
The positraction rearend was an attempt by Detroit to limit the amount of unequal spinning during hard acceleration---to do this they put friction clutches on the "slippage" mechanism to limit the amount that one rear wheel could turn without the other one.
This worked fine under hard, straight acceleration, and in a heavier car, the clutches would still "slip" enough to turn one wheel faster or slower than the other while going around a corner, as required.
On something as light as a T-bucket, there isn't enough load on the rear tires to activate the clutch mechanisms when turning a corner, which means that the rearend keeps both inner and outer wheel spinning at the same speed.
This makes the rear of the car skip and slide when going around a corner, as the rear tires now have to slide on the pavement to allow one to turn at a different speed than the other. THIS GETS REAL SQUIRELLY, REAL FAST!!!!
If you have gobs of torque being transmitted to the rear wheels on a light car like a T-budket, you will still have to be really carefully on the loud pedal anyways, but a stock 350 engine running through a powerglide isn't going to be that scary anyways. I recommend a gear ratio of 3.55:1, definitly not a 4.11:1 for what you are doing.

Lets do math. A 31 " diameter tire is 31 x 3.1416=97.3896" circumferance. A mile is 5,280 feet. or x 12=63360 inches. So. in order to travel one mile. the tire has to turn 63360/97.3896=650.6 times.
At 60 miles per hour, your car is travelling 1 mile per minute.
That means that your tire has to rotate 650.6 times in one minute to reach the speed of 60 miles an hour.
If you are using a 3.55:1 rearend ratio, then you multiply the ratio times the number of tire rotations in one minute, so 3.55 x 650.6=2309.6---that is the speed at which your rearend input pinion and driveshaft would be turning.
If your car does not have an overdrive transmission, then we can assume that the transmission ratio in high gear is 1:1---this is hard and true for manual transmissions, with a very small percentage of slippage for "non lock-up" transmissions like a powerglide.
This means that at 60 miles an hour, your engine will be turning 2309.6 RPM. with 31" diameter tires and a 3.55:1 rearend ratio.
If you were to use a 4.11:1 rearend ratio, then the engine would be turning at 4.11 x 650.6=2674 RPM.

Something to think about Brian, but I ran a posi in my old roadster, weighed 2150 pounds. No problems cornering with it.

I had a '67 Camaro with posi; no problem cornering. Also the majority of T-buckets (and performance street rods in general) out there run posi's (limited slip). That's primarily because without that you can forget about ever getting it to hook up well with the street.

As far as the clutch friction goes Bryan is fundamentally correct, although I doubt if he or anyone else really knows the technical design specifications well enough to pinpoint the minimum weight car for a specific application.

As for me, I would not build or own a car or truck without a 'posi'. Especially not a performing street rod ...............

Good Luck, Kitz

Thanks so much for all your replies. Thanks Brian for the math and explanation, it makes a lot of sense to me now, and thanks Kitz and Dave for your input and experience.

I'm going to have to digest all this information and start looking around swap meets.

Thanks again, I'm VERY grateful for the responses.

-Jackie