burglar

I have a sneaking suspicion the next wave of hi-po street tires is on its way. The Dunlop Z2 and RE-11a have already been released in Japan. The Kumho XS and AD08 are long in the tooth, and the RS-3 isn't far behind.

Also, BFGoorich has inked a multi-year deal as the "official street tire of the SCCA" so I wouldn't be shocked if they announced a killer tire within the next few months either.

I think I'm going to hold off on tire buying until the last minute.

Earlydays

Interesting...I've heard lots of good things about the AD08's, though not the earlier AD07's, along with my own experience.
I've driven RE11's and my view is that the AD08 has much better initial bite/turn-in.
In my sizes they are also much lighter than the RE11's.

edfishjr

Yes, that's the word. Lots of new tires coming soon. I hadn't heard that about BFG. They got nothing now, so they must have plans. Man, that takes guts to sign such a deal without a proven, competitive product!

edfishjr

I started out running a heavy car (3850 lbs) and the Yoko was supposed to be good for heavy. They did well. Switched to a lighter (3200 lbs) car and put RS-3s on it. I think they maybe have slightly better lateral grip when fully warm than the Yoko. My issue was, as a mostly single driver, they changed so much from 1st to 2nd to 3rd run.... I had trouble adjusting. With a co-driver, your second run is the 3rd for the tires. So, the tires would be consistent and warm for the primary driver's 2nd & 3rd run. Also, they were worn out by early summer, so my early use was in cool Spring weather. I switched back to Yokos, as much for better sizing (the 295 Yoko is over 1" wider than the 285 RS-3) as any other reason. I thought they were much better cold and don't change so much as they get warm, though they do get better, of course.

This was on a 9.5" wheel, so both the 285 and 295 were oversized... could be the RS-3 didn't tolerate being squeezed as much as the Yoko.

kjchristopher

bingo. Most street tire people put too much tire on the wheel.

burglar

So the Addco bar showed up. Just the bar and two bushings were in the package. Don't exactly get frills for the price. Ah well, at least it was made in the USA. Looks about 3mm bigger than the original I guess.

Problem - I can't quite get the sucker in there. I kicked the old one out. I'm assuming that you are supped to over insert one end to make clearance on the other end, but even if I do that I cant get it in the a-arm on the opposite side. It's almost like the bar is too wide, but it measures almost exactly the same as the olf one. Any bright ideas?

burglar

Update:
I ended up taking the entire passenger side a-arm off to get the bar in. On the bright side, I'm now comfortable working on the suspension of the car. Wow, is it dead-nuts simple.

Also, I contacted Addco and they have a custom department, which means they can bend whatever size stock style swaybars you want. Turns out they are overbooked until spring, but might be a nice and possibly affordable option down the road if anyone wants to pursue it.

So now it's time to start thinking about torsion bars and shocks. This is an excellent resource. Through a lot of searches, there seems to be two schools of thought on torsion bar sizing. More common is the keep the balance close to stock school. Some of the track guys seem to like to go well stiffer in the rear, while being more modest in the front. Any guesses on what to try?

And for shocks. I found out that fortunately my car was ordered with the Bilstein strut option. A lot of searches later, and looks like Bilstein will rebuild and revalve my shocks for not a lot of cash. However, there are mixed reviews when it comes to how they pick their valving. Another option would be to pay a little extra to have a reputable Porsche shop match the damping to my setup, but that valving would remain "top secret." Next dollar level up option would be to grab a set of new adjustable shocks pre matched to my setup, and tweak.

I'm not a particularly gifted tweaker. If you give me too many options, I'm bound to really screw at least one up. Therefore, I'm leaning on option B - getting professionally matched non-adjustable shocks for my needs, and leave tweaking to only maybe an adjustable rear swaybar, alignment, and tire pressures. Someone convince me otherwise.

edfishjr

I would recommend adjustable rebound. Even if compression is right, auto-x sites are so different. What you can do on a smooth site you can't if bumpy, etc

Also read Dennis Grant on proper natural frequencies. Many STR cars are set up very stiff, maybe too stiff for anything but very smooth lots.

Also decide if you will track the car. What is high speed stable is usually too stable for best auto-x.

edfishjr

http://farnorthracing.com/autocross_secrets.html

Above is link to Dennis Grants online book if you don't know about it already.

burglar

So weird, two references to Dennis's website in the same day - one on my local autox board. Looks like the site got a revamp since last time I was there, looks great! I could get lost on there for days.

Anyone want to throw their $.02 in on torsion bar sizes? I can't rule out a track day with the car, and I'll still want to be able to drive it on the highway without white knuckling it.

burglar

Related, has anyone filled out the calculator on that site? There sure are a whole heck of a lot of values I don't know in there.

ToSi

re: spring rates & starting points.. here are my thoughts - Porsche publishes wheel rates & ride frequencies for many of its newer models. The sporting variants seem to use ride frequencies around ~2hz, with the rear being sometimes higher but not always.

I had to dig out my copy of RCVD by Milliken to sort out the torsion bar rates but re-read your posts & see someone already did the legwork -> http://tech.rennlist.com/911/pdf/torsionbars.PDF

Solving for wheel rate from the data provided in the link above suggests 21mm front / 31mm rear bars result in ~ 1.9hz F&R while 22/33mm works out to 2.1 / 2.2 hz. No idea what the fast kids run, not sure many of them know either though.

Not too hard to model swaybar rate if you measure the various dimensions - much harder to predict weight transfer as front & rear cg heights are 'difficult' to measure.

Dennis's calculator assumes the same motion ratio for springs + shocks so won't work for a torsion bar car. Depending who you talk to, the 0.65 damping ratio is an oversimplification anyway so best to see what Bilstein or one of the race shops recommends.

Wanna bet who spins more while sorting out their car next year?

burglar

Are you going to show up to more events?

I'm building a bit of a notebook of bits and pieces, but another one is here:
http://instant-g.com/Data/911CoilConv.html
Supposedly was directly measured wheel rates per torsion bars. Thanks for the tips, I was sort of leaning at starting with 23/30, but was suspicious that might be not enough rear.

I'm kind of surprised there doesn't seem to be more open-book 911 autocross specific setup threads. Then again, you don't see a lot of the cars out there.

EDIT: The 964, 993, and I'm assuming on down the line got progressively more advanced rear suspensions, including some passive rear steer to help tame the lift-throttle oversteer. I'm thinking a modern car rear-biased ride frequency setup might make an older car scary loose?

edfishjr

The usual reason stated for having slightly higher natural frequency in the rear has nothing to do with "handling." It is so that when the car encounters a bump, the rear catches up to the front (which hits first) to produce a "dead bounce", i.e. both ends comes down at the same time. Of course, this is only true for a certain range of speeds/bump sizes. When you crawl over a speed bump a 10 mph the rear is never going to catch up to the front!

What would be really cool would be if someone with a twin-turbo would take a speed bump about 175 and tells us which end comes down first. Enquiring minds want to know.

burglar

0_o

What's frightening is that the more I learn, the less I seem to know. I just want to get my old *** car around some cones in a parking lot quickly.