Porsche 917/10 & Performance Trends Suspension analyzer

[csdilligaf]

I am building a Porsche 917/10 replica and using Performance Trends suspension analyzer to work out the suspension. I will be deviating from the original as getting the blueprints might be impossible and there may be better designs now days.
Project Overview
2000 lb track car
95" wheel base that is 4.5" longer than stock
Flat 6 mid engine layout. 400 hp
weight dist. 45% front 55% rear
full tube chassis, double A-Arms front and rear.
I've done builds to this extent before but this time I have purchased Performance trends suspension analyzer to see if I can do it better.
How could I post the Performance Trends file so others can look and have input?
Is there any interest in following my progress?
Here is the real deal.

[csdilligaf]

Here is the CAD version of the front left

[syborg tt]

You had me at Porsche

and yes I have subscribed

[csdilligaf]

The rear was more like this picture with longer trailing links going forward but I will go with a H shaped lower and an A-arm for the upper. That will work better with a deep backspacing. The Porsche die hards will hate me but I'm building a car for me not them.

[SSLance]

Ron Sutton posted shots like this from Performance Trends in my build thread.

He may be able to explain how it was done.

[csdilligaf]

Well I have been playing with the Suspension analyzer and think I can attach the file. If you have Suspension analyzer please try to open it. I was able to click "File" "Save as" "Advanced" and pick a location outside of Suspension Analyzer and save it. Then I was able to open Suspension Analyzer click "File" "Open from floppy drive" and locate the file and open it again. I guess they just never bothered to change the language. I have not had a floppy drive for 20 years. Also I have no idea of the exported file's type but was able to put it in a zip folder to post it here.
So, please if you can open this do so and see if it works. Keep in mind that I am new to the program and this is just a rough start. There are lots of info to be corrected still but I am trying to nail down the basics of the front suspension.
I have read that for a car like this I may want to account for 1" lift and dive and 2 degrees roll for the calculations. Does this sound about right?

[csdilligaf]

I've been working on the suspension in CAD and had to make changes so the upper arm does not come in contact with the wheel at full lock doing an 18 ft radius turn. I am thinking about no anti dive or squat. I figured out how to print screen into paint and post it. Still lots of spring rate, roll bar and other things to be worked on obviously but what do you think of these numbers with 1/2" dive, 2 degree roll and a tight turn? Am I getting closer?

[David Pozzi]

I haven't looked at the file but none to a little anti dive in front will work. A low CG car like that with high rear weight won't have a lot of dive transfer anyway. The rear needs anti-dive anti-squat. With IRS you won't have much anti squat effect due to drive train housing torque like a stick axle car, but it needs as much anti-squat as you can get. I expected to see 60% rear weight, would like to hear your comments on what you think a good rear weight percentage is.

On your front lower A arm, I would not use rod ends on the inboard ends. I'd make them spherical bearings in a housing welded to the arm ends and jig weld them so they are identical. There is going to be a lot of front to rear loading on them from braking and side loads on rod ends will wear them out or break them in time. Do all your adjusting on the upper A arm.

To get at your suspension files, open your Perf Trends folder, look for 3D data. It should be in there. Something like C:program Files/Performance Trends/Suspension Analyzer/3-Data/ then pick Examples, or My-Tests, or make a new folder. It's not good to store files elsewhere, the program can lose track of them according to a warning they included in earlier versions I've had. You can make a new folder for your personal projects, but keep it inside the 3D-Data folder.

I just got the Brian Redman book and he stated the 917 Prototype was aerodynamically unstable. The fix was to raise the tail to increase rear downforce. Perhaps the wing used in the photo and the small lip spoiler helped enough on the car in the photo but be aware of the issue.

[csdilligaf]

Thanks for the input. I am still getting used to Suspension Analyzer and getting better at it. I have been working on a new KPI/Castor combo with a 5 degree kingpin and 7 degree castor. That's the great thing about using software is I don't have to make each change and try it out. I am trying to get my castor gains correct for each front tire and think I am still a little off. With 1" dive, 1 degree roll wheels at a 15 degree turn I have -1.93 degree on the outside tire and .94 degree on the inside tire. Where do you think I should be?
As for the Aero issue, yes the 917 Lemans cars in 1969 had a little wobble at 240mph on the Mulsanne straight. They changed the tail shape in 1970. The Can-am cars never got to that speed and were shaped like the first picture of a 917/10. As for the weight balance it helps to have me(200lbs) so far forward, the pedals are actually on the center line of the front wheels along with the battery, radiator and two sidepod fuel cells in the front half of the chassis.
Here is the latest changes.

[David Pozzi]

I’ll take a look at your file when I get time. I recommend consulting with Ron Sutton to get some help. I did see on the previous file, the roll center moves outboard with 2” dive & 2 degrees roll. It would be better to have ithe RC stay closer to chassis centerline or move towards the inboard tire. I don’t know how to accomplish it though. Your dynamic camber looks very good. I’d read Ron Sutton’s sticky thread in this forum for what he likes.

[csdilligaf]

Yes, I have been reading Ron's thread on front suspension, about 1/4 way into it so far, I want to really soak it in so have been re-reading over and over. I have a post in it on the last page and Ron has gave me a little input so I am working on making changes but struggling a little with it. I would have thought 2" of dive and roll would be more than I would get from a purpose built track car that is mid engine and with a flat 6 the center of gravity will very low. Other than fiberglass there is nothing above the top of the rear tire other than the roll bar and my helmet. I am going to try to calc the CG but have not figured that out yet. I assume Performance trends can assist with that? What do you think I should account for with dive and roll? I would be happy to end up anywhere near the 917/10 or 908's handling ability. I don't even know if there is much now days that can keep up with a 350 hp 908 on most tracks. But I am having fun with diving this far in trying to get the best handling I can.

[csdilligaf]

Well I am getting closer to what I think I need. Reading Ron Suttons Sticky thread on front suspensions has been a great help. I had to move it too rear steer to get the ackerman angle where it should be. I would have preferred front steer but the steering tie rods would hit the rotor or the other option was to move it towards the rear but still in front of the wheel centerline. But with manual steering it would increase the force required to turn the wheels. Moved to rear steer and all is good. A lot of great cars had a rear steer set up with the rack positioned high above your feet. The Porsche 956/962 was manual steering with rear steer and it still holds the fastest lap at the Nurburgring.
Here is what I have for numbers now with
1" dive 1 degree roll with wheels a 21.1 degree's outside 25.1 degrees inside
-4.28 camber outside tire 3.61 degrees inside tire
66% ackerman
25.5% antidive
4.49" roll center
Now I just need to put the numbers into CAD and make sure there are no rim to control arm issue's. If no problem there I will model the uprights and machine them and see how it goes.
Opinions appreciated.

Chip

[David Pozzi]

I wonder how much higher the steering effort is with a high caster design? Your front weight should be very light so maybe not an issue.

I think a lot of Ron's steering geometry advice here is based on Pro-Touring use, Autocross and track. For Track only, the turns are not as sharp and I think Ackerman is not as important to get exactly right. Going to rear steer is helpful in allowing you to move the tie rods away from the rotors.

Unfortunately Performance Trends does not predict CG that I have seen. I wish it predicted front end dive for a given G deceleration, that would be nice.
Your front RC looks high at 4.49" But make sure your RC does not go below ground level in dive.

[csdilligaf]

I know the high castor may make the steering effort a bit harder. Also the less distance between the kingpin axis and the tie rod ends will increase the effort. There are several mid engine kit cars that use the C5 suspension and geometry with manual steering. I'm hoping to find a happy medium or just may have to go to power assisted steering. Don't really want to do that as I am all for the raw feel of man and machine like the glory days of CanAm.
As for the roll center I thought a higher roll center and low center of gravity meant a car will be less likely to roll in a turn? Shorter lever to use as leverage?
If Performance trends 2001 Corvette example is correct and you turn the outside front tire to 25 degree's like in Ron's example of Ackerman calculation the Corvette has 12% ackerman. Do you get the same?

[David Pozzi]

I know the high castor may make the steering effort a bit harder. Also the less distance between the kingpin axis and the tie rod ends will increase the effort. There are several mid engine kit cars that use the C5 suspension and geometry with manual steering. I'm hoping to find a happy medium or just may have to go to power assisted steering. Don't really want to do that as I am all for the raw feel of man and machine like the glory days of CanAm.
As for the roll center I thought a higher roll center and low center of gravity meant a car will be less likely to roll in a turn? Shorter lever to use as leverage?
If Performance trends 2001 Corvette example is correct and you turn the outside front tire to 25 degree's like in Ron's example of Ackerman calculation the Corvette has 12% ackerman. Do you get the same?

I've heard to not trust that Corvette file. It's not 100% accurate but I have no proof either way. I do know Corvette's have poor Ackerman correction. You can feel it in a parking lot, my wife has a C5 Corvette. We set some toe-out to compensate for it.
I would try for a low scrub radius, maybe .5". On a heavier car like a Corvette or Camaro 1" isn't bad at all.

A higher RC stiffens the suspension in roll but also causes jacking effects and the geometric stiffness forces bypass the suspension so the springs and sway bars do not affect it to some percentage. I have heard to shoot for a 3" high front roll center. Your front dive in inches is going to depend on spring rate at the wheels and percent anti-dive. I don't have the formula to calculate it though. I think spring frequency relates well to dive travel and that may help.

[csdilligaf]

OK, thanks for the input. I will play around with numbers some more.

[David Pozzi]

If the 4.49" RC is rear, then it's OK. If it's the front, then it could be a bit lower like 3.5" but the RC for cornering in dive and roll is more important than static. I just downloaded Rev3 and the front RC height looks OK.

[csdilligaf]

I spent the weekend at the GoodGuys Del Mar Show and watched the Auto Cross. Sure gets you fired up to work on car projects. There was a real deal 1966 Shelby Cobra there that was the fastest time to beat. One owner car that is still tracked, now that's awesome. The car had seriously been reworked to be on top like that. Pretty much the entire suspension and geometry had been replaced. He give some hint of the work done in the pic attached below, and he is the older gentleman behind
So now with my update. I have parts in hand and fitting them all. I have 2007 C6 Z06 wheels, C7 spindle hubs front and rear, Wilwood 6 and 4 piston Radial mount calipers and 2015 Camaro SS 14" rotors. I had to machine the ID of the rotors to fit the C7 hubs. Next is to design the uprights in CAD to fit the hubs and calipers then machine them up from solid stock. I am waffeling on to use ball joints or Uni-ball. I am leaning towards the uni-ball with 5/8" id to fit heat treated 4130 hardware on the uprights. Similar to the Porsche 917 uprights attached below
I think the Scrub radius I have is around 3/8" in the latest Suspension Analyzer and will try to keep it there.

[csdilligaf]

Well the more I read this forum the more I learn. You guys all put in such helpful information. I see now that it is a no brainer to design and build my front suspension with ball joints since you can change out the stud length to change the geometry. Those 917 uprights with uniball sure are sexy but ball joints are more practical.

[David Pozzi]

I like a greaseable joint if possible, they last longer and greasing helps push out dirt and moisture. On a track only car, it probably won't matter. Check with Howe before assuming there is a great selection of stud lengths. I ordered a Corvette late model longer pin and they cancelled the other. They only stock std and .5" longer pins even though they list others on their website.

They probably have greater actual selection for balljoints used by circle track racers.