Clean Sheet Front End

[ccracin]

Mark,

First, I really do appreciate your sharing your experiences. It sounds as though you make a living at this stuff. What is your company? The only reason I was looking at the MII was the availabilty of them to me from my circle track days. As I mention the AFX spindle has really peaked my interest. I need to talk to ATS to get some dimensions for layout. I am also considering some fabricated spindles from various CT suppliers. I absolutely agree with your comments on scrub radius.

As for the roll center migration, it was very important for consistancy on the circle track. I'm gathering from the lines in between the ones you wrote that it may not be the biggest battle to win in this situation. I'm going to get some more information on the AFX spindle before I do any more iterations.

With regard to your final points, 1. IMO there are a couple answers here. It has alot to do with your tires and the sidewall. The stiffer they are the less - camber needed. In turning it depends on your theories on caster and caster gain. Comments? On the CT 6deg of caster with an aggresive gain curve do to anti dive was very common.

2. Many people that I raced with ran what I called 3 wheel cars, they too forgot about the inside tire. This may not mean much to you, but I had my car setup that after a 25 lap race on a .5 mile oval the left front tire temp. was within 2-5 deg.F of the right front. This is why roll center migration was very important then. Most teams had 50+degF of split. It took 4 years to get there, but I did it.

I know bump steer, ackerman, caster gain,......... fun stuff. We have to be looney!

Thanks,

Chad

[TLWiltman]

What do you think or am I missing something?

Off the top of my head, I THINK your upper control arm may be too short (you'd need to move the inner mounting points inboard, then lengthen the arm). (From "Chassis Engineering" :Herb Adams p.47). "In general, the upper control arm length will be between 50% and 80% of the lower control arm length... With computer analysis, it is possible to make the upper control arm length exactly right so there is little change in the roll center location...". Your roll center height may be a touch on the high side as well. (p.51) ...But a higher roll center causes jacking effects and erratic suspension movements... Most sucsessful cars have the roll center height between 1.00 inch below ground to 3.00 inches above ground..." (Most of us will probably end up at the 3" above ground end of things)

It sounds as though you make a living at this stuff. What is your company?

He's the head honcho HERE, a cool guy, and a complete NUT to boot.
Good luck
Tad

[ccracin]

Mark,

I attached some numbers, this is still using the MII spindles. Scrub is 3.47" at the moment. What do you think?

Chad

[novanutcase]

Damn..........Everytime I read one of Marks posts I learn just that much more!

[Mean 69]

I would not be terrified of 3.5" of scrub radius, it'd be better to have a bit less, but this in itself isn't horrible. What doesn't look so good to me from these numbers is the roll center lateral migration. I did note before that I felt like having it absolutely minimized is neat, but there is probably more to the bigger picture, which might have implied "let it do what it wants." Sorry if that was the case, I "still" like to keep it quiet in bump/roll, so seeing it move 10" + at 1"/2 degrees would concern me. A lot. Looks like you can play with the upper arm angle a bit, it'll raise the static roll center case (hint-hint, wink-wink...), but it might make things happier overall. Again, BIG disclaimer, this is my "opinion," your mileage may vary, and if it doesn't do what you expect it to, don't blame me!!!!

Just another note on roll center height. There is, without any doubt a physical importance to keeping it low, as Tad pointed out: jacking. But here again, be careful about getting too deep into the "rules" that the text material states, or rather don't be scared about it. I say this because a lot of the time, the cars they are referring to are open wheel cars that are really light, and have incredibly low gravity centers. We should be so lucky with our cars, but we're not. My personal rule is to get concerned if the static roll center height is greater than 4" for the front, if everything else looks super happy, then I am basically cool with it. If not, then I work to address this (i.e. consider lowering FRCH) at the same time as slaying the other dragon(s).

All that said, I'd bet that your sheet represents an improvement on a LARGE number of existing aftermarket suspension setups currently on the market, I can't confirm that because unless you have all of the numbers it's impossible to make specific claims, but just understanding the rules a bit, well... You can look at a setup after a while, and make darned good educated guess at whether it has issues or not. Lots of them do. Lots.

One more point, you noted the circle track type spindles. They can be used with tremendous success, and all of the ones that I have seen (use the steel ones, not aluminum for the street, please) are really tough. Biggest downfall that can be a deal breaker is the hubs, they generally run a 5x5 bolt pattern with 5/8" studs, and scrub radius is not of primary importance to them seemingly. It's really, really hard to find street wheels that you can run with those bolts. Bummer too, the stuff is cheap!

Have fun!
M

[ccracin]

Mark,

Again, thanks for taking the time. I only did one more iteration and posted it to give you a better idea where I was. As I said I am not going to use the MII spindles in many more iterations until I can give the AFX spindles a go. The above setup is with a 19" lower and 10.5" upper and a FVSA of 124". This is considerably better than when I had the 8.5" upper.

All this being said, I understand exactly what you are saying. I have known for a long time that the best setup for a particular vehicle and situation is always a compromise. To be perfectly honest the ratio of good-guys type event vs. track days is going to be 100:1. So looking good is very high on the list and I think anything that can be done along these lines is going to be a vast improvement over putting dropped spindles and lowering srings on the stock front end.

As soon as I get some info on the AFX deals I am going to do some more refining. I'm going to try a 17 lower and 9+- on top to help with packaging issues.

With regard to the RC height, I think I want to stay above ground. This being a truck, even significantly lowered it will still have a CG around 20-22 inches.

I was going to model a hub that uses the impala bearings and would mount on the impala nose circle track spindle with a 5 on 4.75" bolt pattern. I have a friend with some nice cnc turning centers. Some chunks of 4340 material and some machine time should solve some of the issues with the circle track spindles. We'll see.

On a side note, what do you rate your 3 link setup for power wise? I ran 3 links exclusively on my race cars. They are tremendous. I haven't done the calculations on pull in the 3rd bar yet, but this truck is going to have 700-1000 hp depending on alot of factors. I know a truck on street tires will never put this much down, but we are still looking at significant forces. I can package alot under a pick-up bed. I'm either going with a 3 link or triangulated 4 bar. Comments. I can make either of these work.

Thanks again Mark.

Chad

[Teetoe_Jones]

Chad- Check your Email, I sent you a print of our spindle.

Tyler

[Marcus SC&C]

Chad,Mark`s put you on the right path. The AFX spindles will help you a lot too. The extra spindle height and drop vs. the MII spindles will help the lateral RC migration too. The fact that they take a bolt on steering arm is also nice once you get to doing the steering. The 7* spindle inclination will make it easier to get a longer upper vs. lower arm length ratio as well.
Your latest arm lengths are just about on for the AFX spindles. Set it up so that the upper ball joints are about 1" higher than the inner pickup points at ride height,with the AFX tall spindles and the arm lengths you mentioned (17"/9") and you should be able to get a VERY STABLE RC (less than 1/2" lateral migration at 2* roll) about 3.5"-4" in height,a FVSA around 110-120" and camber gain around .75*/in from ride height. You`ll have to mess with the vertical pickup points to get it that good but you`ll be in the ballpark.
We`ve designed several clean sheet suspensions around the AFX spindles and though different from yours they were close enough that those numbers should get you in the ball park.
In the rear I`d stick with a good 3 link. The C4L suspension is easier to make work well on a circle track car where it`s binding and roll steer characteristics can be used to your advantage than it is on a ProTouring vehicle that has to turn well right as well as left and without any banking. Mark SC&C