Welded Diff - Setup Suggestions on a JH?

Matt (zooracer) brought up a welded diff in FWD and asked for suggestions. I need some for my JH and thought I'd ask here. I've not driven the car, ever, so I don't know much about it - I can only give you specs.

-2240lbs weight, and we'll have to ballast to make it
-Rear: Live axle, four link with panhard rod, sway bar, drum brakes. 250lb springs, Koni shocks.
-Front: double A arms, 650lb springs, Koni shocks, disc brakes.
-Motor: 2L, 4 valve, probably 140-150 rwhp.
-Tires - Toyos for now

Suspension and alignment has not been setup. I am making an assumption it will push some due to the welded diff not wanting to turn.

Any suggestions on setup? Do we want the rear loose so we can rotate it quickly? Front suspenion - any suggestions on reducing push? Closest car I can think to it as far as layout would be a 240z, but no IRS, and the JH is lighter.

It is looking like the first track it will run at will be CMP. Wish it weren't true, but the TR8 blew up, Miatas have work to be done, the 260Z needed some help, so time goes out the window quickly!

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Ron Earp
NC Region
Ford Lightning
RF GT40 Replica
Jensen-Healey ITS
1/2 a 260Z ITS

[This message has been edited by rlearp (edited April 19, 2005).]

The ultimate situation for a RWD car is generally a touch of understeer off-throttle, and a smidgeon of oversteer when you are on the gas. A welded diff will likely push if you are off-throttle but I'm not sure that it's a safe assumption that it will do so ALL the time, once you have some slip angles to work with in a corner.

Not knowing any better, I'd start by asking the GenII RX7 people what kind of alignment they are running.

K

You basically have a Pinto convertable, although the rear suspension is more like a Mustang. There should be several folks on here that can help with those.
From my experiance with Pinto's you might need a little more front spring, depending on how much front sway bar you have.
Rotate, dude, it's rear wheel drive! The throttle controls that if the car is anywhere near balanced.
Off throttle the car will deaccelerate and plant the front tires for turn in. Neutral throttle will be understeer. On the throttle is fun time. Just remember you need to modulate the oversteer with the throttle. Too much fun (dirt tracking) and you scrub off too much speed. You should not spend much time at neutral throttle, you are racing after all.

Ron:

I'd ask one of the guys from Appalachian Tire, they've pretty much seen it all and ought to be able to give you a starting point for both alignment and pressures. They're very helpful, especially if you catch them at a non-crunch time.

Al Seim
Action Digital Race Data Systems
www.actdigital.com

Ron,

I will be at the CMP Friday test day before Memorial Day with a chassis/data acquisition client, helping him with his FP Miata. Look for the dark blue #89 FP Miata.

I have a lot of experience with welded diffs (spools) with Late Model stock cars, so I can probably help you sort it out. Although the tracks might be different, the priniciple is still the same: get the car to turn and get on the power as soon as possible, right? http://ITForum.ImprovedTouring.com/biggrin.gif The trick is to overcome the inherent understeer in a locked/spool rear-end under braking/turning at corner entry. This can be done several different ways (bars, shocks, springs) at each end of the car depending on the chassis layout, components available to work with, and driver style (trail-braking, left foot braking, etc.). On corner exit what you have to watch out for is a throttle-on push that suddenly turns into snap oversteer as the front end gains traction as you track out.

If you (or anyone else) wants help with CMP, I also provide race driver coaching/consulting services!

MC

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Mark Coffin
#14 GP BSI Racing/Action Digital/Airborn Coatings/Krispy Kreme VW Scirocco


[This message has been edited by racer14itc (edited April 22, 2005).]

Have to add a little of my experiences here.

I had to modify my driving technique a little to accomodate the locked (welded) differential. A RWD with an open diff will usually have reasonable turn-in, where as the locked rear will maintain traction equally in the rear, and "resist" the front turn-in (same concept as what happens when trying to push the car in the paddock, and turn it at the same time) frequently referred to as "corner-entry-understeer".

The technique is to slightly lift off (breathe, by other descriptions) the throttle to increase front weight transfer (increase front grip and decrease rear traction) momentarily, then on to the throttle to continue to rotate the rear through the corner.

Not doing the above will increase the amount of push, due to the traction of the rear tires. Sometimes this will be large amounts of understeer (seen in AS). I watched an AS "newbee" slide off the corner during a school, lap after lap, until we realized he didn't understand the dynamics of the locked rear. Advice corrected the problem forever.

Various Limited Slip diffs don't require this due to their ability to allow different rotational speeds of the rear wheels entering the corner, eliminating the corner-entry-understeer.

I have used this with both a Pinto BS (GT3), ITB Pinto, and various ITB Mustangs.

Good racing.

Bill

[This message has been edited by bill f (edited April 27, 2005).]

[This message has been edited by bill f (edited April 27, 2005).]

Bill,

That advice might help me with our Z too. The diff is packed up damn tight and until it heats up it behaves as locked. So, the first few laps I could maybe use this technique to help out. Once those laps are over the diff is fabulous. Looks like this will work for the JH too, it makes sense and I look forward to trying it. It might be at CMP so we'll look you fellows up, thanks,

Ron