edub

I'm interested in suspension tuning, and much of the data necessary to calculate things like Installation Ratio, Motion Ratio or Wheel Frequency doesn't seem to be well documented. As these are not uncommon cars (718 GT4), I would have expected someone to have documented it somewhere already. Is there a better place to look for this information?

Frank 993 C4S

It would be easier to help you if you stated what you are trying to do. If you are looking for suspension geometry measurements, just take your wheels off and measure. Same for the springs. They usually have marks on them.

edub

I'm trying to change the spring rates to improve the track performance without compromising the street drive to much.

I put together this sheet with some numbers I have, but other numbers are a guess without getting more measurements

https://docs.google.com/spreadsheets...it?usp=sharing

I need the installation ratio and unsprung weight for example but it's more than taking off the wheel and measuring with a tape. Some things I lack the tools to do on my own, and I'd love if this information were more widely available because people can make more informed choices when it comes to changing things.

Frank 993 C4S

What specifically are you trying to make the car do on track that it doesn't do with the current springs?

edub

There is a corner where it bottoms out, and different springs may let me run less camber in the front or move the roll centers around.

There may be a benefit to heavier springs on the street, less scraping underbody strakes. They are replaceable and I don't really worry about them, but they are getting shorter as time goes on.

stownsen914

How are you planning to move the roll centers around?

edub

Ride height is all I can do I think

edub

I took the wheels off and measured some things today. It's quite difficult to get accurate measurements. Either something is in the way of the measuring tape, or you can't get a good reference point. Even using a laser to line up or extend a ray to a point where you can measure something, I don't have strong confidence that these results would be repeatable by another.

Motion ratios for 718 GT4

Front 0.8
Rear 0.776

stownsen914

Keep in mind that things like motion ratio and roll center are baked into a car's suspension design. To alter them, you'd need to change suspension pickup points, which requires fabrication in most cases and most don't do. Granted, roll centers move when you change ride height, but the purpose of changing ride height is usually focused around setting the height, not changing your roll centers.

Wheel frequency is tunable by changing to different shocks or adjusting shock settings.

When dealing with a commonly used performance/racing platform like yours, there is usually a lot of info. out there about what works well. Porsche does a pretty good job designing suspensions on their contemporary sport cars, but they can be optimized. You might consider starting by asking around about which parts and settings work for people.

911ted

This should help:
https://soulpp.com/product/porsche-9...age-2-package/

edub

My goal of having the suspension geometry and measurements is to be able to have a quantitative reference for driver notes on the chassis handling. With measurements as a reference, I can make inferences to how a new change may affect the car. Ie. A higher frequency rear wheel rate increases oversteer, or how a lighter brake kit will increase wheel frequency.

The thread wasn't really to ask what parts I need, because I don't know the method in which I'll measure to track progress to my chassis goal.

winders

Ok. You are talking about tuning a street car here. How accomplished of a driver are you and what is your experience? Do you know enough to recognize what you need to do when the car has understeer entering the corner and oversteer on exit?

edub

I'm just a HPDE guy that likes tinkering on cars. I don't think this question is really fair but I like thinking about it so as long as you will provide your answer, here are some things I'd consider, absent any data to look at.

For understeer on entry
-can we get more caster?
-how is our toe out?
-can we increase front track (spacers, wheel offsets, with the goal to reduce lateral load transfer)
-increase front tire width
-lower roll axis
-reduce front strut low speed rebound
-how much authority does lifting throttle have on understeer
-try trail braking?
-Too much front brake bias? (brake balance (pad compounds), improper wheel diameter entered to ABS, mid-high speed shock rebound reducing grip)
-less front swaybar

To reduce oversteer on exit I might consider
-increase front damper low speed rebound
-increase rear tire width
-lower rear roll center
-increase rear track
-consider toe in
-softer rear swaybar

Is this question thinking about a 'so tight its loose' scenario? high speed or low speed corner? what kind of limited slip differential is there (this scenario could just be caused by shallow ramp on a 2 way diff)

I recognize there are some conflicting things in my list (like reduce low speed rebound) but this feels like it could be answered a number of ways depending on whats going on, how the car is setup, or how it's driven.

Anyway the whole point of this thread is simply to get some numbers that I can't get myself easily (like say, center of gravity. I can't even estimate that as I cant lock out a strut nor can I lift the car). Thanks for the question it was fun to think about.

T&T Racing

Front and rear camber influences cornering tire contact patch and slip angle depending on the tire construction and tire wear rating. Front toe affects crispness of corner turn - in where slight toe-out makes for crisper turn-in. Rear negative toe-in makes the car more stable under braking.

Tire temperatures is a key metric to suspension tuning and setting cold tire temperatures. Read articles on tire temperature measurements and how to adjust suspension to maximize tire contact patch

winders

You don't need any of the information you are trying to find to tinker with your suspension setup. Porsche already has the car in the ball park. If you want to increase spring rates, find out what spring rates and sway bars the the shops that are supporting 718 GT4 street cars are using to make the car more track worthy. Find out if they are replacing dampers and what dampers they are using. Then all you need to do is adjust sway bars and turn some ***** on the dampers to dial the car in.