Is LSD worth 2-points...

[Gary Burch]

Is an LSD worth the 2-point penalty in an early 911? Or would a better use of the points be for wider rear tires or strut bar or whatever.

[ttweed]

Is an LSD worth the 2-point penalty in an early 911? Or would a better use of the points be for wider rear tires or strut bar or whatever.

Gary-
My $.02 is that you don't need an LSD on a low-powered car like yours. Are you having wheelspin problems on corner exit?

Tires are the #1 factor in lap times, so I would spend the points (and $$$) there instead. If you are using a 2-point tire, go to Hoosiers or V710s. If you already use a 4-point tire, go wider.

TT

[Don Middleton]

Tom,

I've wondered the same thing. I encounter wheel spin when exiting fast corners. I assume that the wheel spin is due to my suspension being too soft to keep the inside rear wheel down OR my Targa body flexing enough to lift the inside rear wheel, or both.

So, I've wondered if LSD or TBD would get the power down to the outside, weight bearing wheel and stop the wheel spin.

From reading Bob G. and Paul Y. in the related thread, LSD/TBD sound promising.

Thoughts?

[ttweed]

I've wondered the same thing. I encounter wheel spin when exiting fast corners. I assume that the wheel spin is due to my suspension being too soft to keep the inside rear wheel down OR my Targa body flexing enough to lift the inside rear wheel, or both.

Don-

Actually, your softer suspension and twist-o-flex chassis help keep the inside wheel in contact with the ground in hard cornering. A softer, more compliant suspension is actually better for tire grip, as long as you aren't bottoming out the chassis or hitting the bump stops, and your suspension geometry isn't changing camber and toe settings too radically throughout its range of travel.

A big, stiff, rear anti-roll bar and a chassis with high torsional rigidity will lift the rear wheel sooner, as the weight shift to the outside wheels in hard cornering happens quicker, and the ARB's job is to transfer some of the load on the outside tire to the inside tire, lifting it in the process. Also, stiff springs will cause the chassis to bounce up over bumps, lifting the wheel momentarily off the ground. All of these effects exacerbate the need for an LSD, so along with higher power, I would say that stiffening the suspension will increase the need for a LSD sooner, as cornering speeds increase.

So why do race cars run such hellaciously stiff suspensions, if more suspension compliance gives better tire grip, you might ask? The main reason has to do with the effects of lowering the Center of Gravity of the car. A lower CG is good because it reduces weight transfer during cornering. When a car corners, centripetal force acts on its CG. The higher the CG, the bigger the lever arm transferring load to the outside tires. The beneficial effect of lowering the ride height overcomes every other aspect of suspension tuning.

In order to ride as low as possible without bottoming out the chassis over bumps (and under the influence of high aero downforce in some cases), a compromise is made to stiffen the suspension. There are a couple of added bonuses to this compromise as well--one is that body roll is limited, which keeps the CG lower and adds to driver comfort, and the second is that if the suspension geometry is less than ideal, changes to roll centers as well as toe and camber curves are limited with less suspension travel, helping keep the tire in an attitude which allows it to produce the most grip.

In your case, I think the higher torque of your 3.0 engine (as compared to Gary's 2.2) may be able to overcome your inside tire grip and cause it to spin. Adding an LSD will definitely help, but it is not an inexpensive upgrade. A wider pair of wheels and tires may improve your situation as well, by increasing the rear track. So might decreasing the rear ARB or disconnecting it entirely, if this doesn't result in unacceptable understeer. Simply being smoother and more progressive with throttle application on corner exit may help keep the tire from exceeding its limit of grip and spinning.

If you do go with an LSD, the discussion by Bob and Paul in the other thread is very relevant and helpful. I would only add that one of the other differences in the gear-driven TBD and the clutch-type (ZF) LSD is maintenance and adjustment. The plates in the clutch-type wear out, and must be adjusted and replaced at intervals, depending on use and wear. The bonus of this is that they can be changed on servicing to alter the percentage of lockup on both decel and accel, depending on the number of plates and the ramp angles. A high percentage of lock on decel will definitely induce understeer on corner entry when trailbraking, and if you autox a lot, this can be a big problem, since if you can't get the car to turn in slow corners, you are dead in the water. For this reason, and the lower maintenance, I chose a TBD for my 911.

YMMV,
TT

[Don Middleton]

Thanks, Tom, but now that you mention the suspension stiffness issue, I can recall that I've encountered the inside wheel spin on the faster, higher-g corners when the suspension was "stock", as well as through a couple of "upgrades".

Fortunately, there aren't many of these corners at the AXs. Still, it sounds like LSD/TBD holds some promise.

I guess we're back to Gary's question - is it worth 2 points? I think you would suggest that more rubber would be a better trade-off of points-for-performance gain.

So, here's rubber related question on the points - is it better to spend the 2 points on going from 225s to 245s in the same tire, or stay with 225s and go from RA-1s to Hoosiers?

I would guess you would say that, as long as money is not an issue, same size in Hoosiers would win out. But, if you are trying to conserve AX dollars, just stay with RA-1s in the next larger size. Right?

[kurquhart]

Hoosiers would win

It seems like you already know the answer.

[Don Middleton]

Thanks, Kris, I think I've heard this from Paul Young, among others, now including you. I just thought I would see if Tom agreed...

[993Panzer]

If you go with Hoosiers you'll never go back.

[Gary Burch]

My $.02 is that you don't need an LSD on a low-powered car like yours. Are you having wheelspin problems on corner exit?

I am not sure if i am having wheelspin problems or not. Since it has been so long since I didn't have LSD. I do know that I am drifting/sliding/pitching and catching around most corners and the car feels very stable even with all this going on. This must be the way I have overcome the inherent understeer problem I had for so long,Jae Lee's magic may have helped too.

If I switched out the LSD would I lose the feel of the car the way it is now?

[ttweed]

I just thought I would see if Tom agreed...

There is no doubt that the Hoosier A6 will do more for your autox times than any other 2 point improvement, BUT, if your wheelspin problem is really costing you a lot of time, by not getting the power down early, it is possible that going to a 9" rim with the slower, but wider, 245 RA1 tire may make a bigger difference, if the added track and rubber in the rear eliminates your wheelspin on corner exit. You would have to experiment, I think, to really nail down what the laptime differences might be.

TT

[ttweed]

Since it has been so long since I didn't have LSD....

....If I switched out the LSD would I lose the feel of the car the way it is now?

I am confused now, Gary--do you currently have an LSD and are considering taking it out, or do you not have one and are considering putting one in?

TT

[Gary Burch]

Tom
I have been running a factory 901 LSD for the past 3/4 years. That's why I was happy the points went from 6 to 8 so I could get the RA1's. Now I am wondering if I should take out the LSD and use the 2 points on something else.

[ttweed]

I would leave it in, Gary. You have obviously learned to "drive around" its shortcomings at autocrosses, and it is a big plus at time trial/DE events for high speed stability under braking.

TT

[gulf911]

I am not sure if i am having wheelspin problems or not. Since it has been so long since I didn't have LSD.

Obviously a child of the 60's and 70's....

[bibbetson]

Traction is key, however you get it. Without it there is no way to be fast. At the last AX, I ran a set of worn out RA1's for the first session. No matter how hard I tried (I have a TBD) I couldn't get under 76.5 seconds. I bolted on a set of new shaved RA1's and dropped 2+ seconds without breaking a sweat. If you are losing grip while exiting the corners, find a way to fix it and watch your times drop in a hurry. As everyone is saying here, there are lots of ways to cure the problem.