Whether it's understeer or oversteer, it's caused by a lack of traction on the respective end of the vehicle. You can increase or decrease traction by shifting weight which can be done by forward transfer during braking or rearward traction during acceleration. You can induce oversteer during decelaration by turning in under braking or by adding throttle under acceleration on turn exit. For example, if the car is well balanced, you can induce oversteer by adding enough throttle during a turn to cause the rear tires to start spining thus losing traction causing the rear end of the car to come around. If the car is well balanced and you have good throttle modulation, this is going to be a very gradual process. If you add too much power too suddenly or your engine comes on with sudden spike of power, this tendency to oversteer will be much more sudden. If you look at this video, look at the coming through turn 11 at time 3:25, 7:15, and 11:10. You will see how this plays out in this right hand turn. The car turns in crisply under relatively heavy braking and transitions to a nice neutral turn through which the car is powering through at partial power. As throttle is increased through the second half of the turn, the rear wheels will start to break traction which causes the rear end to start coming around causing oversteer which is then neutralized by opposite steering input. Hope that explains it...

 

That explains it.

I set up my track cars a bit different with slight tendency to understeer all the time. I overcome the understeer on turn-in by trail braking, a lot, sometimes all the way to the apex. From mid-corner to corner exit if I still need to rotate the car I do a slight throttle lift then back on throttle and I will repeat it as much as necessary to get the rotation needed. I grew up racing go-karts and this whole trail braking then lift-throttle rotation is quite common because of the rear weight bias of the karts.

I wouldn't know how to drive a track car that was balanced where on throttle it oversteers like yours is. Lol.

 

I think effectively we are both talking about the same thing. I think the car is probably set up with a very small amount of understeer which is mitigated with trailbraking.

 

Probably, and yes, just enough understeer so I can count on the rear of the car staying planted especially when on throttle. The front will almost always start to slip away before the rear does. Unless I induce the rear to let go by lifting the throttle.

 

The trouble I have with this is determining how much trailbrake I can do and how much throttle lift I can do before ending up in the grass. I have inner ear and balance problems.... it's hard for me to feel lateral accelerations especially in left hand turns. I have to rely on peripheral clues like tire noise and steering feel.
I did find instrument flying easy to learn, as I had no confidence in my "feelings" and complete confidence in my instruments!

 

Do you have access to a skidpad that can be wet down? Or even a wide open parking lot that you can fool around in when it is raining?

 

That is F*&%$# bada55. Wow incredible tech going into a 20 yo car. I'm sure you'll be putting in some incredible laps. Love to see it at UMC sometime. Nice work to you and your team of talent.

 

Thanks brother. We'll be at UMC for sure next year. It's my favorite track. This build has exceeded my expectations on all fronts for sure....

 

Quick question, who made your headers and roughly what would a pair cost? Realize they are overkill for a stock internals motor, but probably the only real choice after a ported stock unit.

Other side question, I know your car is track only, but what are your thoughts on a 5.5" clutch on the street? Maybe not in a heavy stock 996tt, but something lighter.

I really enjoy your build, your methods, detail and dedication to experimenting and developing on this chassis is a major inspiration. And most of all, open discussion to help bust some of the common misconceptions.

 

The headers were made by a company that is no longer in business if I'm not mistaken. With that said, we plan on using our welder to make these ourselves out of SS321. The price will be between $3-3.5K. I think we have everything in CAD already so I just need to get our welder in the loop so he can order the 321 tubing. We will probably get this done this winter. Porting the stock headers does relatively little in my opinion. The restriction in the OEM headers is the 37mm ID tubing much more so than the flange weld you can partially grind away. The headers I'm running have a 48mm ID which effectively increases the cross section area by almost 70%. I believe these would benefit an application using only turbo K24 or larger pushing 650+whp.

With regards to the 5.5" clutch, I do not believe it would be useable on a street car. The reason is the fact that the clutch pedal travel is roughly 1/3 of that of the stock clutch. While the engagement is buttery smooth with the carbon plates, the engagement window is very narrow. The clutch is set up to engage at the very top of the pedal to allow for a fast engagement / disengagement. A clutch stop must be used to limit travel. The other problem is the fact that the clutch has an extremely small moment of inertia (MOI) due to its light weight and small diameter. The whole 5.5" clutch weights 13lbs. Compare that to the 30lbs for the 964RS LWFW clutch or over 50lbs for the OEM DMFW clutch. The Tilton 5.5" was the OEM clutch in the 997GT3R and RSR so it's really meant for a sequential gear box. Chris managed to make it work beautifully in my application by deleting the high/low lift of the variocam which drastically improved the engines torque right off idle making engagement rather "friendly" for a clutch with such low MOI. To get the car rolling, I simply slowly release the clutch and lightly add power while the car gets rolling without any drama, no chatter, no noise. The Motec M150 ECU really makes this possible. I seriously doubt this clutch would be useable on a stock ECU application to be used on the street. Let me rephrase, no way. It's still very tricky on an incline and I don't attempt to drive the car up on the trailer, I pull it up with the electric winch I have in the trailer to avoid excessive slippage and wear. Tilton does make a larger 7.25" version of this clutch with a heavier flywheel as an option which would be a great alternative option for street/track. The 7.25" carbon triple holds 1300 ft.lbs while the 5.5" holds 750 ft.lbs. Naturally, the 7.25" would have a greater MOI which negates most of the benefits of the 5.5" clutch. Everything is a trade off.

With that said, the 5.5 Tilton really changes the one has to shift the gearbox. You have to do it fast because the revs drop so fast. Data shows the engine revs at 16000rpm/sec and decelerates at half that rate. Due to the low MOI, shifting it fast is very easy on the synchros. The way I shift with the Tilton is very different than the way I can shift the 4.0 RS LWFW clutch in my 997GT3. It's a drastic difference.

 

Thanks for all the clutch info! I come from sports bikes so the idea of light flywheels and dog boxes in cars always makes Me giddy. kinda blows my mind the variocam tunning is what really improves your off the line work, but it makes sense.

I'm guessing you have a good reason for not moving to a dog box?

For the headers, what would be the best way to stay in the loop for that?

Thanks again!

 

PM me your info and I'll let you know when we have some headers made. Cheers...

 

That instrument cluster screen is totally awesome. It's hard to believe how planted that car is coming out of the turns getting on the power hard. Very well done.

 

Here is a geeky suspension post. After having run the car and doing some basic initial suspension set up, we discovered that we are experiencing a decent amount of front chassis lift under hard acceleration, which on this car is harder than on most with +900whp pushing a 2800lbs. The car already has some very trick suspension components including the RSR anti squat rear thrust arms. The problem would have to be addressed in the front. After running the math we realized the we need to go with front tender springs that have a lower block load. This is essentially saying that we need a tender spring with a lower rate. Block load and spring rate are directly related with block load being a multiple of spring rate and spring travel. Block load is the force exerted by the tender spring when it's fully compressed. With that said, since a tender spring is fully compressed at static ride height, block load is the important number, more so than rate, as it tell you will how much force the spring is pushing up on the chassis. Since we have to run 2.25" (57mm) I.D. main springs, we naturally used 2.25" I.D. tenders. The tender we used is the softest 2.25" tender made by Eibach with a block load of 265 lbs. As such, the two front tenders exert a lifting force of 530 lbs on the chassis in the front. We needed to reduce this number in order to reduce lift. Since 2.25" tenders are not made in a very wide selection we looked at H&R 60mm I.D. tenders which are offered in a huge range. After looking closely at the rates, block loads, and spring travel, we selected a tender that would meet our needs and one which would reduce the lifting force exerted on the front axle by just over 220 lbs compared to the previously used 2.25" tenders. We also slightly reduced the front shock canister nitrogen pressure which in turn reduces the shock lifting force by another 20 lbs for a combined total of near 240 lbs. The combination of these two items should in turn significantly reduce front lift under hard acceleration. We incidentally looked at PET and discovered that Porsche Motorsport also went to substantially weaker H&R front tenders on several of the late model RSRs. Undoubtedly this was done for the same reason.

The only thing we needed now was an adapter that would mate the 57mm main spring with the 60mm tender. You can get 57/57 and 60/60 and 60/65 spring couplers but nobody makes a 57/60. Johnny quickly drew up the adapters in CAD and Chris machined a few sets on the HAAS. Woila, problem solved. These adapters fit perfectly and lock the two springs together without any slop. Suspension tuning is a bit of a black art and it's the details that make a difference and bring it all together. These are things that are rarely ever talked about, addressed, or frankly understood.

 

Nice having a "team" that can identify a problem, understand the limitations of off-the-shelf parts, conceptualize a solution, get it drawn up in CAD, and spit it out on a Haas machine all essentially 'in-house'!

Have you had a chance to proof the solution on the track yet?