dmilzoff,

The MPSC's will be the difference of night and day compared to what you are currently running. Use the manufacturers recommended pressures, run it, see what it does and then make small changes. Starting with someone else's tire pressures is not always a good idea unless the cars are set up, parts wise, the same.

In addition, if you are serious about the handling performance, when you go to the MPSC's, have the car corner balanced.

 

jro,

What you described, that you like, is corrections for understeer. If it is perfect for you, then leave it alone. If not, try some of the other suggestions for understeer correction. Just do it one perimeter at a time and in slow increaments.

 

I have a question. Handling wise, which rim is really better? The 18's or the 19"s. I realize the 19's have a limited choice for DOT legal R compounds, however when they become available, how will the 19's fare?

Which leads to another question. If I was to buy magnesium BBS wheels in the 18 inch size, they would be about seven to eight pounds lighter per wheel than my current Fikse FM10's. How noticable, handling wise, would this reduced weight be?

If anyone has knowledge as to websites (like the two above) that offer information about what this thread deals with, please share them with us. Thanks.

 

I think the only time you might notice the difference of lighter wheels would be in rough washboard surfaces. The consensus has been that 18" is superior for performance vs 19". I have not autocrossed the turbo but in low speed corners it seems to plow straight ahead with heavy throttle. I'd don't know if you want more oversteer in autocross, I do know my best times were the runs that felt slow with no sliding.

 

Lighter is better, and 19" wheels impose shortened
sidewalls, which I *believe* is sub-optimal. On this
topic, I have not read anything, or spoken to any
tire experts (though I would like to dearly). I take my
position from observed evidence:
In no vehicular racing in any venue or format, whether
by fixed spec or by free choice for competitive edge, do I
see anyone running the ultra-low profile tire, big wheel
option. Even in such grass-roots street tire formats as
autocross (which can be nevertheless very competitive),
in classes where cars are restricted to street tires, I do not
see this. It seems likely that the extra weight of more wheel
less tire, and/or the lack of compliance and cushioning of
less sidewall are performance deficits.
My opinion, pending tire expert testimony,
Joe

 

Mike, by increasing the tire pressure, you are reducing the tires' tendency to roll when experiencing lateral forces, thereby, preserving more of the contact patch. That is why in general, you increase the tire pressure to gain more grip when cornering.

Actually, while these are commonly accepted generalizations, the truth is, you have to EITHER increase or decrease to get more front grip... in other words, while most times, due to the commonly lower U.S. tire pressures specs, you increase tire pressure to find more grip, it is not unheard of for certain setups to find more grip when lowering pressure.

The proper way to deduce tire pressure solution is to 1st, increase the pressure to see if you reduce understeer... and if nothing changes or it get's worse, then try reducing the pressure. Hopefully, one of them will help with neutralizing the chasis. In the event that neither one does anything positive (or makes the situation worse), then chances are, you achieved close to the optimal tire pressure for that given tire's grip and chasis setup, and you need to start looking at adjusting the chasis setup for damper/rebound, sways, compound, etc.

Stephen
'01 Turbo Ti Upsolute
1stephen1.com

 

I think the whole tire pressure question is by far most easily dealt with by using a pyrometer rather than all of this guessing. They are not expensive at all and are invaluable in determining not only tire pressure but camber as well.

Just my .02 worth...

 

chad,

thank you for starting this topic. Joe, zippy, and Sephen, thank you also for contributing. Looking forward to hearing Carey chime in as well. At the recent LA meet he had my head swirling with ideas.

I, too, know way more about how performance upgrades work, or thought I did until some recent posts and having a bone stock turbo, I am still learning the car. And I am pushing it more now with more comfort.

What I notice most is understeer (correct me if my terminology is wrong) -- I turn the wheel and the car wants to go straight. Now that I've had the car a while, I can get the car to kick out coming out of the corners, but this seems to be less of an issue (read: I do this intentionally, because a little slide is fun ).

My plan for the car has been to improve the handling AND augment the car's looks. So with compromise to the handling, for street use, I will be using 19" wheels and stock twists for the track (when I'm ready). Now, my simple understanding of suspension was, "If you want better handling, get a kit." Is this necessary? My goals are modest: I want a very drivable car, no bone crusher, but still to have an improvement over the stock setup. Does anyone have suggestions? I have heard pss9, jic magic, jrz. Is there a kit that will meet my needs or should I stay stock and adjust some of the above mentioned methods such as camber and tire pressure.

thanks for any comments on this.

Regard,

Geoff

 

Geoff,

My thoughts on your question re suspension components is that the kit setup is by far the easiest and probably most cost effective. I went with the Ruf PSS-9 kit as I was doing the Ruf Rturbo motor conversion. It is very good...stiff enough to keep the car from wallowing when pushed but still soft enough for comfort and still retaining enough suspension compliance to work well on the less than perfect back roads I play on. I think this latter issue is often missed when we are doing performance mods...the parts that work well on generally smooth race tracks do not work well on real world back roads. They are simply too stiff and the whole car becomes unsprung weight in effect.

 

Geoff,

From your description of events, you learned to, drive a different line or use weight transfer to your advantage. Another way to say you improved your driving skills, which is not to be underestimated. Probably the best place to start.

If, after improving your skills, you still find understeer, and you have dialed in your tire pressures, I would spread the front track just a little using spacers to see what happens. If I had adjustable sway bars, I would also change them a little also. I would not make both changes at the same time. Make a change and then check to see what happened.

I have not used them, but the PSS9's seem to be a good choice given your wants. You would have the ability to quickly make front and rear changes to your liking. I believe the JRZ's, given your wants would be an overkill.

 

Hey, no one has brought up caster? I don't believe the turbo suspension has the ability to make adjustments here. The GT2/3 does. I have installed the GT2/3 parts on my turbo that allow for this adjustment.

Any ideas as to what changing the caster does and how it relates to over/under steer?

 

Caster is only a front-end tuning option. Adding caster can
be useful. When you turn the steering wheel, your front wheels
don't turn on an exactly vertical axis. There is a small angle of
inclination of this axis. It typically is angled toward the back of
the car by a few degrees. Because of this, turning the wheel
will actually slightly tilt the chassis. This takes a little effort so
caster provides an on-center feel, with the car wanting to return
to center. Too much caster is bad because it increases steering
effort, and also makes you turn the wheel more to get the same
amount of actual turning.
The turning performance benefit of caster is that it induces
extra negative camber in proportion to how much you turn the
wheel in, without having any negative camber in a straight line.
To understand this, imagine exagerating the caster, tilting the
turning axis farther and farther till it is parallel to the ground.
Then turning in woud impart nothing but more negative camber.

Just to go a step further, don't forget your "Ackerman Steering
Ratio"! Note that when you turn into a corner, the inside front
tire needs to traverse a smaller radius turn than the outer tire.
If you want to maximize the inside tire's turn-in contribution,
then that wheel should turn in more that the outer wheel!
Ackerman is the name for the steering geometry's amount of
induced toe-out during turn-in. This provides a degree of extra
turn-in for the inside wheel. The amount of ackerman you need
is dependent on the radius of the turn. For a freeway curve
you probably can't set/measure the small amount you need,
but for a very tight autocross corner on a sticky track, you
might benefit from a silly amount. However, the contribution of
the inside tire even when optimized, decreases in proportion
to the load on it, which ideally drops to zero in the perfect corner
with all the grip one could use, so it becomes moot.
Yours in the enjoyment of backwater handling topics,
Joe

 

Joe...help me out here. How is it optimal to have the inside front wheel have zero load on it in the perfect corner? I lost you there. I thought that if one had the inside wheel off the ground (zero load) that was indicative of too much roll stiffness on that end of the car. And that the reduction in overall grip from reduced load on the inside tire was greater than the increased grip from transfered weight increasing load on the outside tire.

What am I missing?

Thanks

 

Joe,

I just learned something. I though caster related to the distance between the front and rear wheels.

My understanding is to correct understeer you increase positive caster. To correct oversteer you reduce positive caster. Would that be correct?

 

Chad,

In so far as one can induce greater neg. camber (as Joe stated) in the front end when the wheels are turned with greater castor...if a lack of neg. camber is the reason for the understeer then yes. If one already had plenty of neg. camber then no. A quick trip to the track with a pyrometer will clear that up.