Hey Guys,

I need some advice for Doris' car on Limited Slips.

We are putting the car down for the winter as it is getting some mods done over the next couple of months and I need to dismantle some of the car.

Mods are:

1. Going to TCDesign in Sept/Oct for new cage with door bars for safety. The bolt in cage has the door bars too low to help with a collision. At the same time I will have Tony tie in the shock towers front and rear and add chassis stiffening. The cage will also be NASA/PCA Racing legal without using a Petty bar, which makes it more comfortable inside the car.

2. Roger Sheridan is making me up his lightweight front and rear lids along with a new targa top that will allow more room for Tony's cage. At this point I plan on going back in with the dash, door panels, and some carpeting to make the car more "stealth".

3. I have ordered 21mm hollow T-bars for the car and I have plenty of springs in the garage to try. I may bump the spring rate in back up a tad to 275lbs/300lbs from the current 250lbs to help with turning and then with the softer front end I can get some Anti Roll bar adjustability back as I have the front bar full soft and it still pushes. The car is way too stiff in front.

4. We are going to bias ply slicks for the setup. We will use R35 / R45 compound Hoosiers for AX/track and eventually move to Avons for AX.

I'll also redo shock valving after I figure out the T-bar/rear spring rates. At that time I will have the spindles raised.

That brings me to a limited slip/trans combo. Doris hates the 901 and remembers the 915 we had in our 930 (I had swapped the 930 4 speed for a 915 5 speed which was much nicer to drive around town). She liked that trans. I have a 915 box and will spring for a WEVO kit and flipping the R&P. So now I need to decide on a limited slip. The hot AX ticket is a Torsen style but it offers no help under braking on the track. I really liked the feel of our ZF style Guard on our previous 914-6 as you can feel it settle the rear end under braking in high speed situations. It made the car easier to drive. Since Doris will be driving the heck out of the car at DE's, I'm thinking a 40/60 clutch style from Guard. 40% lock up under power and 60% lock up under braking. That should tame the car in braking Thill or BW and still make the car hookup out of slow corners.

What do you guys think? I'm trying to setup the car for her to enjoy and not make it a stiff race car like I would drive.

Thanks,

Bill P.

If your doing more track work then go with the limit slip, but IMHO limit slip will never work as well for autox. You just can't dial out the under steer like you can with a torque bias.

On the other hand I've been really happy with the torque bias differential on hill climbs which is as close as I can get to your track work. Maybe you should drive 222 at Sears Point and tell me what you think.

Paul guard admitted to me that in a mid engined car with low up the the could be faster on the track.

The reason we had that conversation is because a builder was telling me the same.

Bill:
You've reached a Y in the road for building an AX vs track car.

Hi Brant,

It appears your comment came out garbled. What did Paul Guard say?

Thanks,

Bill P.

Hi JP,

I would say it's more of a W then a Y and back to a W!....

Actually, the only difference in the hard parts is the diff. The car will be set softer with an AX bias, but I won't be cutting off the windshield, but may go Lexan. I also have to take in the main driver's inputs as they are important.

I'm getting quotes today on the 915 trans build with both the Guard and the Wavetrac unit in it.

Bill P.

So what is the party line on why TB diffs are not good on the track? Mine works everywhere I've driven it, in fact, it's works so good that you wouldn't even know it's there. The car just handles. Need some enlightenment.

A very helpful difference in the design of the clutch type is under braking. There is a ramp rate for locking up the rear wheels where they track straight together. At track speeds entering braking zones you can really feel it in the rear end. A lot of the wiggle you may feel goes away.

On a TBD, under braking it acts like an open diff, because it is designed to do just that, be open. You only feel it under acceleration.

Entering high speed turns such as THill's turn 1, 10 and 14 and BW's Star Mazda can be felt. That being said, many classes of cars, including Spec Boxster, do not allow Limited Slip's, so it is something that becomes a skill thing as you have to navigate the braking techniques as well as the acceleration / throttle techniques. Having had a clutch LSD before, it does make the car much easier to drive on the track. I feel it more on the decel than the acceleration.

Randal,

Maybe on a hill climb your car is so balanced already and since you are going up hill the car brakes and tracks very well due to the inherent gravity forces of going up hill and slows down easier and quicker?

All I know is that on a track it's pretty nice. But a TBD would still help on the track and with the slicks on Leeloo, she will be getting some good grip, so it may not be that big of a deal, especially for Doris, who is a much smoother braker than I am.

Bill P.

I'm hating this spell check

Paul guard admitted that on a low hp. Mid engined car. That the tb could actually be faster in lap time despite the feel of a clutch diff under braking. We went with a tb for 100percent track work.

Our previous tranny was a welded diff. We ran it on the 4 and the 6.

I've never had my car on a high speed braking situation like Thunder Hill or BW. Highest speed for me is braking down from 95mph going into the slow down chicane at Cascade Lakes and several other corners where you are having to brake hard to make 70-90 degree turns. If the TBD was going to do something funny my guess is that it would be there as the road is crowned, which would seem an ideal situation for bad behavior to show up. My actual experience is that the car tracked perfectly, which was a confidence builder.

It would be interesting to do some testing at TH and turn 10, which you picked out, which would be the perfect setting. Hauling the mail down hill and having to brake for a 90, yea, good test.

It is about grip......more so, which end grips and when.

The sad fact is that at AX, the front end needs to grip first and let go last. This is hard enough to do with a 914.....and keep it nose first . The 914 likes to "track" and using large slip angles is not one of it's finer points. The low polar moment is a 914's advantage & can be dealt with consistently......with a lot of practice ......if you need to go out there on the edge.

The TB diff is one of the mechanics of making that happen.

The cluch type does the opposite. This too can be overcome in some circumstances....if you're into "dirt tracking".