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  #11  
Old 03-26-2018, 09:49 AM
csdilligaf csdilligaf is offline
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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.
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  #12  
Old 03-28-2018, 10:46 AM
csdilligaf csdilligaf is offline
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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
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  #13  
Old 03-30-2018, 03:04 PM
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David Pozzi David Pozzi is offline
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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.
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  #14  
Old 03-31-2018, 08:58 AM
csdilligaf csdilligaf is offline
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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?
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  #15  
Old 03-31-2018, 03:54 PM
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David Pozzi David Pozzi is offline
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Quote:
Originally Posted by csdilligaf View Post
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.
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Last edited by David Pozzi; 04-11-2018 at 02:54 PM.
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  #16  
Old 03-31-2018, 04:10 PM
csdilligaf csdilligaf is offline
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OK, thanks for the input. I will play around with numbers some more.
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  #17  
Old 04-11-2018, 03:07 PM
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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.
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  #18  
Old 04-11-2018, 07:05 PM
csdilligaf csdilligaf is offline
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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.
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  #19  
Old 04-13-2018, 04:23 PM
csdilligaf csdilligaf is offline
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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.
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  #20  
Old 04-16-2018, 07:36 PM
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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.
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