JeffDC

If any of you suspension gurus are bored at work this Friday (as am I, obviously!), here is a fun 993 puzzle for you to ponder:

I just finished up my 3rd track day on my first set of R comp tires (NT-01s - not shaved but now with 400 street and 400 track miles) and have somewhat mixed emotions. The car feels great in slow corners and braking zones (lots of grip, progressive loss of grip, all good) but the front end feels twitchy on fast straights and the car seems to take a long time to take a set in fast corners (Summit Point T3, T4 and T10, VIR kink btwn T2 and T3, uphill esses and hog pen, kink on the front straight).

It's frustrating to exist a slow corner faster than I used to only to feel that I need to modulate my speed on the next straight...

Turn-in is far quicker than it used to be with the street tires and I am working to smooth/modulate my steering inputs accordingly. Still, it seems that I am having to wait for the car to catch-up after turning in.

The car is setup to RS height and alignment specs, which seem to marry up to Nitto's suggestions for the tires. Suspension is H&R Supercups, 26mm bar front (not adjustable), 24mm bar rear (adjustable - set to soft), original bushings, tie rods and links, as far as I can tell.

I suspect that the less compliant tires (the R comps compared to my old street tires) are transferring more of the cornering loads up to the suspension and that's what I'm feeling. Is there a way for me to settle the suspension down a bit without replacing the coil overs? I tried reducing hot tire temps from 38/38 to 36/37 and that helped a bit.

My car is used mostly for DE's but I do drive back and forth to the track and take it for spins on nice weekends so it needs to remain streetable.

Any input would be appreciated.

Happy Friday!

jscott82

Twitchy on the straight? That's not good... the only thing that comes to mind is Kinenmatic toe or worn bushings/tie rods. I would concentrate on that first then see what happens in the turns...

Once you get the car stable, if you still feel the car is taking too long to set (but feels good once it does) you need to stiffen the valveing on the front. If you don't like how the car feels once it sets, then you can look at springs / roll bars.

jscott82

still thinking about this... almost sounds like you don't have enough rake in your setup.... RS setting is pretty good and should have a fair amount.

DanQ

do u have RS uprights? If not, maybe bump steer affect

JeffDC

Thanks very much . Kinematic toe was checked/set at alignment but I will have it rechecked. Front bushings are original and have been on my list. Sounds like that may be where I start.

Is rake the same as camber?

I think that my uprights are original but I have not had the bump steer issue.

certz

When I first got my car the K-toe was all out of whack. Turning in to high speed corners it felt the like the car would not transition - kind of like it was floating - and even after the car set it still felt like it could snap spin at any second. If you are getting a feeling anything like that, then I would bet on a k-toe problem.

I have never experienced straight-line twitchy-ness though. Prior to having the bushings replaced I did have the steering wheel wiggle, but I would not call that twitchy - it was not like the car was trying to move offline erratically.

P.S. Love the color, more pics

trophy

Front end twitchy-ness is usually an indication of toe out on the front end. Is great for turn-in but makes the front end track back and forth. If you have toe out and worn front bushings it will only amplify the situation.

cgfen

Agreed, that's where I'd look first.
Rake is not the same as camber.
Rake is ride height difference front- to- rear.
I suspect the poster meant CASTER, not RAKE
Incorrect caster could also cause this issue, but not likely.

Caster description here

http://www.rapid-racer.com/suspension-tuning.php#Caster Angle

Good luck sorting things out.

Craig

jscott82

Actually I meant rake... Especially in the uphill esses (VIR), if the car is level it gets a lot of air underneath and is pretty unsettling.

Agree with toe out coupled with worn bushings... If the alignment is pretty aggressive it will have some toe-out, couple that with bad bushings and you guys are right.... twitchy city.

Bill Verburg

Sounds like sloppy front bushes and/or toe is off

what springs are on the car?
stock valving in the shocks?

The Big front bar does not help turn in at all, usually you have to work like crazy to dial understeer out of these cars softer front bar and wider front wheels are the usual way to go

do you feel any twitch in the seat?

at RS height you should have RS wheel carriers and mono-ball steering links

for reference here are some suspension settings


the settings for RSR/Cup/GT2/mine are too aggressive for driving to and from the track and getting any kind of decent life from the tires but are great for on track experience

certz

As usual great data Bill. Just out of curiosity (and sorry to the OP), but why the taller than front rear height?

Bill Verburg

It determines the car's rake, I like the nose right down on the pavement to keep air from passing under the car and helps aero efficiency, having the rear wing up doesn't hurt

Stealth 993

If the car feel twitchy on the straights it's usually toe, or over pressure on the tires.

Does it also feel loose under heavy braking?

jdistefa

An easy way to check rake is to put a digital level on the outer door sill. 1 deg rake is a good starting point.

Agree with all the above re. front a-arm bushings, toe, kinematic, bump steer, tire pressures.

If you have stock front carriers and tie rods at RS ride height then you have a bump steer issue, period The only question is how much it's contributing to your current symptoms.

Hard to interpret your bar choice/settings without knowing your spring rates.

Martin S.

"he car is setup to RS height and alignment specs, which seem to marry up to Nitto's suggestions for the tires. Suspension is H&R Supercups, 26mm bar front (not adjustable), 24mm bar rear (adjustable - set to soft), original bushings, tie rods and links, as far as I can tell.

I went the cheapo route when I put on my PSS9s, I installed TRG 25mm front and 22mm rear (I believe) bars. They were cheap...compared to the 23mm, 20mm 993 RS bars. The collars on the TRGs are not welded, secured with Allen bolts. I had one collar come lose on the rear, the bar shifted under stress and broke a drop link and bent one of the toe link, now that was a spendy fix.

As a result of the monster 25mm front bar, the car was always lifting an inside wheel on the tighter corners....I had to run it at full soft front, ran the rear in the middle, or full soft in the rear. Because of the interaction of the rear TRGs bars and RS drop links, I could not reach the full stiff rear adjustment even if I had wanted to. I was also breaking a drop link, generally the rear, once or 2X a year. I switched to Tarrett Engineering drop links and the drop link breakage problem went away, still raising the inside wheel.

Finally, I reached the breaking point (no pun intended) after I bent a toe link, these damn TRG sways had to go. I ordered set of 993 RS bars and continued on with the Tarrett Engineering adjustable drop links. Voila, wheel lifting problem solved, and now I have full adjustability on the front and rear sway bars. I am no fan of monster sway bars....

Agree with all of the above, you need to check front toe as well as the front bushings...unsteady under breaking, bushings most likely. Check with FD Motorsports; they sell the "Walrod" hardened bushings.

In closing, I am so fortunate to have access to several first class alignment shops for these sorts of issues:
1. Johnson's Alignment, Torrance, CA Steve Alarcon
2. Hergesheimer's Motorsports Lake Forest, CA Mark Hergesheimer
3. SGS Motorsports, San Luis Obispo, CA (My primary shop) Guy Ober and Briton
4. MKS Performance Inc., Camarillo, CA (Mike Schatz)