AX tire pressure vs. tread roll, kumho V710 Options

[mskala]

Question is this for autocross purposes: If I'm running 24PSI hot and the
tread wear triangles are still not touched, should I go lower? My thought
is that either:
1) lower pressure will improve the contact patch and more grip will improve my time.
or
2) lower pressure will make slaloms slower by having more flex/lean in the car which will increase my time.

Using Kumho V710 215/40/16 on a 7.5" rim on my narrow -6. 21mm torsion bars
in front and 180 pound springs in the rear, because that's as high as I want to go
and still be driving this thing on the street regularly.

Appreciate any input.

Mark S.

[Randal]

Question is this for autocross purposes: If I'm running 24PSI hot and the
tread wear triangles are still not touched, should I go lower? My thought
is that either:
1) lower pressure will improve the contact patch and more grip will improve my time.
or
2) lower pressure will make slaloms slower by having more flex/lean in the car which will increase my time.

Using Kumho V710 215/40/16 on a 7.5" rim on my narrow -6. 21mm torsion bars
in front and 180 pound springs in the rear, because that's as high as I want to go
and still be driving this thing on the street regularly.

Appreciate any input.

Mark S.

Looking at the picture it would seem like your tires are hooking up well. Of course only tire temperatures will tell you if this is true or not. What readings are your getting?

Secondly, I'd consider increasing the spring rate in the rear or dialing in more rear bar, if you have one. Unless you like driving with all that body roll.

[mskala]

I've never measured the temps, but I know it seems to increase or at least not decrease grip
thru the whole group of runs (4 or 5 at a time).

I have as much camber as possible with camber plate in front and lack of shims in the
back. It's about -2.0. No rear bar, since I don't have an LSD. Although thinking of
acquiring a torque-bias type for the future.

[Randal]

I've never measured the temps, but I know it seems to increase or at least not decrease grip
thru the whole group of runs (4 or 5 at a time).

I have as much camber as possible with camber plate in front and lack of shims in the
back. It's about -2.0. No rear bar, since I don't have an LSD. Although thinking of
acquiring a torque-bias type for the future.

I personally don't like a rear bar, prefer instead to run heaver springs. TB differentials are the bomb. I can thank JP for that one....once you get one you'll never go back. (IMG:style_emoticons/default/biggrin.gif)

There are some cheap IR temp sensors at places like HF. If you buy one and have one of your buddies measure the outside, middle and inside of each tire, following an Autox run, you will know whether or not your suspension settings are right.

[DanT]

Your car is very soft in the rear.
As far as pressures...
On my orange RaceCar with the 2056 motor I ran V710 in 205x50x15 on 7" wheels..
I ran them 24psi hot.

[mskala]

Thanks, sound like I have nowhere to go if I want to keep the
soft, cushy (yeah right) feel for the street driving.

[yeahmag]

With those tires on 5.5" rims, same torsion bars, and a -4 I'm up to 225lb rear springs and moving up to 300lb springs as soon as the new motor is done. With adjustable shocks it's still very streatable with the 225lb rear springs.

[mskala]

With adjustable shocks it's still very streatable with the 225lb rear springs.

Hmm, I suspect the roads in your area are better than the roads in my area. (IMG:style_emoticons/default/idea.gif)

[yeahmag]

That may be true... I lived in Mass for 5 years. They were pretty rough.

[Heater Guy]

One way to get an idea of how much the tire is rolling is to mark the outside of the tires with tire marker, chalk or crayon. Run the car. If the chalk mark is rubbed off up the shoulder, add a little air. If the is not rubbed off at the full width of the tread, take some air out.

This is not rocket science, but better than nothing.

[ConeDodger]

Not really related to the original question but seeing as this thread evolved to questions of 'cushy' on the tushy...

I modified my suspension in steps... Not really because it was planned but because I learned as I went and less money was spent at one time. First, I put in stock bars front and rear. My car didn't come with them and the difference was purely dramatic. Next, I put in Koni Yellow adjustables all the way around. A note here, when these were set to full stiff the suspension was jarring enough to keep a family dentist's kids in tuition money. Then, I put in a Tarret bar and Elephant bushings... Here is where I got a really unexpected result. With the Elephant bushings, it didn't matter what my shocks were set at. The ride was very supple. The bushings suck up everything the road throws at it and allow the shocks to work. Lastly, I sent my car over to IPB Autosport in Sacramento. Ron Kain who really knows his way around a 914 and is a good friend of Brad Roberts, called Brad to consult on my suspension. When I came to pick it up, I found that they had put 200# coilovers in the rear. I was a bit apprehensive. What would happen to my ride? Still supple.

This last weekend, I drove a two day autocross at Reno-Fernley Raceway. My friend and neighbor, Brady Halbasch was going with me in his 350Z. Brady had never autocrossed before but is a former National shifter kart champion who trained Marco Andretti and Graham Rahal among others. We discussed which car I should take and Brady wanted to drive them both. "Oh my God! This is the closest thing to a go-kart I have ever driven that isn't an open-wheeled car!" I pointed out that it was also very smooth on city streets and he said "yes, your suspension is very active" "what is that?" I told him about the bushings and he said that they were very much like the roller bushings he used in racing...

Bottom line; if you use roller bushings (source - Original Customs) or Elephant Bushings (source - Elephant Racing) you don't have to 'suffer' through your suspension on the street.

(IMG:style_emoticons/default/agree.gif) with Randal on the TB differential. Raby keeps saying I will break mine in the 901 with the torque from this motor but I will soft launch until then and put one in a 915 when it does break. I would not be without one...

[Van914]

Elephant bushings - One of the best Mods I did to my car. I was surprised at the difference in the ride. Much smoother than other bushing I had in the car. I know they are expensive but I would do again.

My set up was:
21mm Torson bars
19 Welt Sway bar in front with adjustable links - no rear bar
180lb rear springs on adj. mounts
Elephant bushings front and rear


Van914