Suspension tuning RF84
We competed in our first event last weekend. We're getting the bugs out the car thanks to the help and advice we've gotten here and from friends. So now we're getting serious. The car turned-in was fantastic, but our impression was the car was loose. We're coming from an Honda S2000 so we're very familiar with loose, however with the added weight in the back, the VD comes around a lot quicker. Tips and tricks for tuning the RF84 would be appreciated.
Thanks in advance.
George
Make sure a complete and proper alignment has been done. Paying particular attention to the rear bump steer. When I used one of these chassis for a sports racer I recall running fairly light springs in the rear.
Make sure you have decent tires, and are operating at reasonable hot tire pressures. Otherwise you are wasting your time......
Assuming good warm tires on a warm clean grippy surface keep in mind that even a Honda S2000 is a "pig" in regard to transient response and reaction time compared to a formula car. In general I think most of us prefer a little too much oversteer to any understeer that can rear its ugly push in tight turns. Whee!
Regarding alignment I think the common starting point is very close to normal RR settings.
Dick
Is this car being used for autocross or road racing? If road racing, you do not want a "loose" set-up (oversteer).
I assumed autocross given the forum and previous threads.Originally Posted by Rick Ross
How are you determining optimum tire pressures? If you don't have driver body weight in the car when you are doing your set-up, your wasting your time.
Good point, Dick.....I totally missed that this post was in the "autocross" forum. Please disregard my previous post.I assumed autocross given the forum and previous threads.
When the back end of a S2000 moves you feel it much sooner because you're sitting in the rear of the car. In the FF you're sitting in the front and probably to feeling the back move as soon and not correcting as early.
In an autocross situation where you are on the power early in fairly slow speed corners, and with a Formula Ford with an open differential, you can over-power the rear driving wheel and get both wheelspin and a loose condition. I would start by doing the normal setup as advised, run no rear ARB, try lowering the rear of the car, use front ARB and springs to hold the car level, and play with rear springs and shocks to tune the back of the car. After I got the rear of the car as good as I could, I would balance with the front of the car.
Brian
Setup
It took me until the final couple of years that I owned my RF84 before I could get it to even approach oversteer - it always pushed a great deal. First off search the forums for spring rates and recommendations - I went with what Richard Pare suggested and that allowed me to lower the car. Depending on how smooth the parking lots are that you are running this may or may not be feasible. You want to be able to induce oversteer if you are going to be able to get through the really tight corners fast - so work towards getting the car fairly neutral and then use your brake bias, trail braking and the throttle to induce oversteer.
Measure your motion ratio and shoot for springs equal to your corner weights - if you find Richard's posts it highlights how to do this. I then lowered the car 1/2 inch front and rear maintaining the rake. I moved the lower A-Arms mounts to below the plate in the rear. I added ackermann to the front steering by moving the pickup point on the upright out towards the wheel by about 1/2 inch. Front rollbar was about 3/4 of an inch from the end of the rear bar was at 45 degrees when measured with magnetic inclinometer.
Once I did this the car handled very predictably but could be induced into oversteer when needed to get through the tight turns. Good luck.
Wow. Lots of great info. At our last event of the year we reduced the rear bar stiffness again before our last run and the car went from eager to rotate to near neutral. Each of us took off 1.3 sec on our last run! Later during fun runs, I was able to take a fairly tight solemn full out. I sure do like Formula Fords!
Mark
1990 Van Diemen, the Racing Machine, CM AutoX, 2016 Frontier
You can try to make a street car into an autocrosser or you can do a lot less work and make a race car into a great autocrosser
Thank you all for the great information.
Joefisherff, your post was very interesting but I couldn't find any posts by Richard detailing ff set-up and an email to the address on file bounced. Do you have any idea where I could find addition info to add to your post. I had heard about lowering the rear A arms, but as a pure novice, more detail would be greatly appreciated.
Thanks again folks for all the input.
I'll also be lurking on this thread, for sure.
My RF88 is almost ready for my first pass at aligning, corner weighting and shock/bar settings. I don't even know what springs are on the car, to be honest, as I'm just sticking with what the PO installed when he got the Konis back from being built. We'll see how it goes...the tires are pretty fresh, so I should have enough grip to learn where I'm at, anyhow. It is going to be the what-to-do-about-it part that I'm sure will be the challenge.
Hope I have the car good enough to make it worth towing down to SD, I'd love to be there with the rest of the new west coast CM crew.
Ran the first event in the new car yesterday. I discovered that one of the front Koni 2812's. left side, had a bad seal, allowing oil leakage and air into the oil...not good. So, the car was pretty...umm...interesting (on right hand turns, in particular). OK, it was just pretty bad, to be honest. Runs strong, shifts well and the brakes work, so there are some bright spots!
Took the front shocks off the car today and brought them up to PSI at Sears Point. For now, I'm just going to have them freshen the one bad shock and dyno/compare with the one that is not messed up. Eventually, I may rebuild/revalve all four shocks, but I don't know enough of what the car needs to go down that path, at present.
I really have to figure out what I've got, I guess...and then start thinking about this whole stiff vs soft set up question that seems to exist in CM. I did determine the front springs are 500#. I haven't pulled apart a rear coilover and there are no marks on the outside of the coil, so I don't know what they are...they do look lighter than the fronts.
Joe
I agree with Joe ,the spring rates should be around 250lbs.in the front and 300lbs to 350lbs in the rear.Rear toe in should be about an 1/8th " with an 1/8th" toe out at the front.just basic setup to start.
Lee
Some more help
Caster if I recall was 4 degrees, for autocross toe out probably a good idea for stability under braking, on the road course I ran 1/16 toe in the front and 1/32 toe in the rear. From a search I found this thread which should help in determining your motion ratio. You basically remove your springs, shocks and wheel - as you move the hub through its arc you measure the suspension travel at the shock mount. One inch of movement at hub = X at the rocker or bellcrank. Plot it and then use the formula that Richard provides. This will give you your effective wheel rate. Use this to determine what spring rate gets you close to your corner weight.
http://www.apexspeed.com/forums/show...t=motion+ratio
Thanks. I'm going to measure the front today, when I'm reinstalling the front shocks, and see what I have. Rob posted on the other thread that the motion ratio on the RF88 may be in the .5-.6 range in front, in which case I'm needing at least 550# front springs to get wheel rate near corner weight...but, that is before the contribution in roll of the ARB. In any case, I may be swapping out the existing "500lb" springs, as we tested them and one is 474# and one is 516#! I guess that kind of dispersion is common for Faulkner springs? I don't know, I've never used their springs and have never seen that much difference in spring rate from a "matched" set.
I also will be pulling the rear coilovers out today and determine what I have currently. I did measure the ratio of the rear UCA/rocker and found that it is 12" from pivot to upright and 9" from pivot to shock mount...so a 4:3 ratio gives a MR of .75, correct? So, to get the rear wheel rate in the 275# range, I should have the same 500# spring rate I currently have on the front, if my math is correct.
Wheel rate is motion ratio squared.
times spring rateWheel rate is motion ratio squared.
The formula hasn't been the problem, it has been the numbers to input!
I measured the front and, as Rob suggested on the other thread, the MR appears to be in the .5-.6 range for the RF88. I got a little over .5" travel at the shock mount of the bell crank for the first 1" of upward wheel travel.
Next up is to get accurate corner weights, then decide whether to start at 100% of static corner weight or some fraction of that and go with a softer set up....
http://www.apexspeed.com/forums/show...corner+weights
http://www.apexspeed.com/forums/show...corner+weights
Ok this Wood fella seems to be getting the drop on me as I work out other problems. LOL
So back to the RF84, one of the posts suggested lowering the rear lower A arms to below the mounting plate in the rear. I've also heard from Pease in San Diego that this transformed his RF85, but I've been told these are two different cars. Can someone shed light on this issue of lowering the A arm.
Mike, Robert's and my car seem to be running neck and neck right now. It will be great to have another car to compare to. Can't wait for the tour and Pro Solo. Are you doing both this year? Any other Cmods coming down?
You California guys are really making me wish I hadn't moved to Raleigh from Silicon Valley in 1986 . . . doesn't matter that I probably wouldn't have gotten a CM car if I had stayed.
Regarding David Pease's rear A arm lowering can he provide pictures? I don't know enough about suspension geometry to guess at what he might have done that would be an improvement other than setting the overall rear ride height so that the arms are "level" if the ride height was originally too high. Pictures of Nick Meyer's 85 VD (2nd and 3rd at Nats in 2011) show the attachments at the normal location and the arms level.
Dick
85 VD
CM 85
George, Mark Mervich up here was first to get into CM and he has Ben Martinez (former DSP National Champ) co-driving. They should be fast...VD RF90 with all the good stuff. I believe they also intend to do the Tour, but not idea about the Pro.
I've only run my car once, with a broken shock and mystery alignment, so I have work to do prior to thinking doing bigger events, but Tours are part of my longer term plan, of course. Not sure about Pros...how hard do you guys think they are on a car with a Hewland, 7.25" clutch and FF CV's?
I and others ran tons of Pros without problems back in the day. The was one failure that I saw at the start with a car that had a lightened or aluminum diff.
I broke a halfshaft once in a local event due to a pothole in the launch area which caused sudden acceleration of one wheel followed by GRIP. Snapped a recently magnafuxed axle instantly and cleanly.
Your results may vary.
Dick
Is this Sirota's old VD?
Nope, not Mark's car. Not sure if he's sold his, yet.
My car was last run seriously in Solo by Eric Jones out of Texas. Funny thing, I know Eric (actually co-drove with him in his RX8 at Blytheville a couple of years ago), but had no idea he was a prior owner of my car when I bought it....
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