Roll Center design theory

[Salt Racer]

...what are some simple tests that the basic guy can perform in order to tune and get a good definition of how well your car handles?....Are there any others that would help?

Like others have said, go to the track. You'll never find the limit on street, and only at the limit you can really tell how your car really handles.

Looking at pics would help as well. There is a current issue of PHR floating around in office, and I think it's the pic of your car on the page of bumpsteer article. Look at the front tire - lots of positive camber and fair amount of sidewall deflection. Deflection could be either due to insufficient pressure and/or the construction of the tire itself, or the rims are too narrow for your tires. Excessive positive camber suggests you need more static neg. camber and/or revised geometry. Increased caster will help as well.




Marcus,
Please post the numbers you're getting. You can also get Dennis' pickup point numbers somewhere in his website, so try them on your software (Dennis uses freebie Perf Trend, and I use WinGeo3). I should also note that Dennis has SCP UCAs. Length may be different from OE arms, and UBJ offset is different for sure. He initially had like +14* caster.

[harshman]

I haven’t seen the article yet (subscription ran out) but I can tell you that at the time I didn’t know my butt from a hole in the ground. At the time my car was tested my air pressure was around 10 psi in front and back and the only “mods” that I had (and still have yet to amend) were 650 lb springs and a sway bar. I did learn a very good lesson through all this though. The biggest factor (bone stock ’94 Z28) that spanked me was where the rubber meats the road. I have now switched to the Nitto DR sticky tires and will soon be changing the suspension around a bit (thanks to you guys). Again I am very grateful for all the information you have divulged to me.

[dennis68]

I think track times (same track and same conditions) and feel are the 2 best tuning tools out there. Only you know if feels better, loose, tight or whatever. Track times will confirm how you feel the car is responding.

There is a story circulating about Dales kid Kerry not being able to qual for a past event and blaming the set-up, little "E" jumped in it as is and turned damn near track record in a car he had never driven.
Stewart picked up a used sprint and took it out to a local track (never been to), took podium finish first time out against veteran drivers (familiar with the track).

Point being only you can determine how the car is responding to your abilities. While it may seem slow to you a more experienced driver may be able to turn quicker times. Maybe see if after you feel it is OK if a veteran driver is willing to take it out for a hotlap and offer some input.

[wally8]

I'll give my two cents.

Denny is right, the track is the final frontier. The tests you've done should give you an idea of being in the ball park.

It is all about your feel in the car, though. I wouldn't recommend going on someone else's feel. If the other driver is really good you won't like the car most likely. Loose to a fast guy is no where near the same as loose to an amateur.

Just make it feel the way you like. Learn what loose and tight really are and think about how it feels in all the different parts of the corner (entry, middle, exit) and how your transitions feel (since this is road racing). If you can think in terms of those 3 areas for every corner you can make the proper adjustments.

My Tony Stewart story is watching him race (several times) in a WoO Sprint and WDRL Late Model. He showed up at the track, jumped in a house prepared car and ran 2nd against the top guys in the nation in that class and would have won were it not for a severe bit of contact by the leader (Bloomquist) to keep him back. He put on an equally impressive show in the Sprint car. He was just as pi$$ed about being smacked in a dirt late model as he gets in Nextel Cup.

Oh yeah, we've been passed very quickly by Kenny Schrader in a dirt modified too :-) I could say we raced against him but that wouldn't quite be accurate......

(BTW, "we" in this case would be our driver. I'm more crew chief than driver. I've resigned myself to the fact that I am somewhat sucky in that respect.)

Wally

[wally8]

Marcus,

Let's hear what you've got. I don't have spindles in front of me so my info is somewhat suspect. I'd like to hear what your measurements are for a B-body spindle with or without tall BJ's if you have them.

Anything would be great.

Wally

[Marcus SC&C]

I posted some stuff here but after going over some of the older posts I decided to edit it out and reread it all when I get time. I entered most of what was discussed and I can`t seem to get anywhere near the quoted numbers with just a bolt on spindle and ride height change. I`m sure I just missed something. I`ve been doing a lot of A body geometry work lately,maybe I`m getting burned out on em. Marcus SC&C

[dennis68]

Nooo, only 9*. :icon996:

The SCP arms are 8.5" I beleive. I have made so many changes to plans lately I can't remember where I started anymore...

Link to tech print-outs analysis

[Salt Racer]

Hmmm, my WinGeo file shows +14* caster and so did your Perf Trend analysis... Better recheck your measurements!

Harshman,
Yes, tires are one of the single biggest improvements you can make but obviously there is more to it as you're aware.

Check out this video clip. This is what you can expect from old cars when things come together. You've seen Super Chevy's slalom sessions in person, so you'll probably have a perception to tell how fast this car's going through the cones.
http://www.artmorrison.com/albums/GT55testing/seg3.wmv

Also, bunch of us will be doing OT event at Buttonwillow on April 15th. Come on out if you can.

[harshman]

Check out this video clip. This is what you can expect from old cars when things come together. You've seen Super Chevy's slalom sessions in person, so you'll probably have a perception to tell how fast this car's going through the cones.

Yup, that pretty much sums it up!

[Mean 69]

You know, I really wonder how terribly important lots of positive caster is. I run a smooch over 6 degrees on the Camaro, which is what Global designed into the UCA. I really wonder if it needs that much. I always thought it was cool to have the steering wheel snap back to center after a turn, but the drawback is the wheel lift associated with lots of postive caster in a tight turn. I didn't study what the design target was on D's bad boy ride, but I can tell you, 9 degrees sounds like a ton. I am guessing that you guys are not trying to get anywhere near that. Anyway, the power steering masks the bad stuff with lots of positive caster, but it would be interesting to see how sensitive a car would be to this, for instance, 4 degrees versus 6 degrees. For those of you that run lots of caster, go turn the wheel lock to lock in your garage and see how much the front end moves vertically, more than you'd think. Hmm. Given, you don't normally use that much steering input on a typical course, but, well, you get my point.

Harshman, nice looking car in the magazine, good to see that you drive it. Awesome. I am not so sure I could agree with Tony's comment on Karl Chicca's car during the Pony Express. It certainly "could" have been a bump/toe issue, but also could have *merely* been a scrub issue, or something else. A buck-fifty is moving out, than you very much, doesn't take much road irregularity to show nasties on a setup. Anyway, Tony's comments were well put, this is a fundamental aspect of car setup (bump steer). Oh, and by the way, you have to be careful how many of those "praying" smilies you put towards Katz, he'll get a big head.

Mark

[Salt Racer]

...Oh, and by the way, you have to be careful how many of those "praying" smilies you put towards Katz, he'll get a big head...

My head is physically big (7-3/8" Simpson helmet is tight), so I need to match the mental size. ;-)

My decision on design static caster angle is based on intended use, King pin angle, scrub radius, the rest of suspension geometry, weight bias, etc.

Like Norm pointed out, the effect can be used as advantage especially on nose-heavy cars. But as with many other things, there is a point of diminishing return. I don't exactly know what this point is, but 89* caster obviously won't do any good (if that's physically possible).

Greater scrub radius (for a given tire size) will result in greater rise/fall effect caused by steering axis inclination. Earlier in this thread, Wally stated that roundy-round racers use different rim offset on left front and right front to tune handling. Because of this and other ill effects of scrub radius, I tend to stay conservative on caster when there is much more than 2" scrub radius.

IIRC, Mercedes uses over +10* caster.

[dennis68]

You know what they say....garbage in, garbage out. I must be off on my ball joint to centerline measurements; I'll bet my centerline to mount number is off a little too.

We'll see when I get the new spindles on and get real "precise" with measurements how far off I was. The 9* was on a freshly calibrated HunterP611 so I think it's pretty accurate.

[Norm Peterson]

Lots of +caster does lift the front end, but it's the inside tire that's doing all of the lifting (and gaining load in the process). Meanwhile, the outside tire is being lifted, and loses load. Since this effect opposes the lateral weight transfer due to lateral acceleration at the front end and improves its overall lateral grip, it's a "loosening" effect. A secondary effect involving rear LWT that occurs due to chassis stiffness, roll stiffnesses, and tire vertical spring rates is also in the "loosening" direction. Not necessarily a bad thing, but it could perhaps result in different hardware choices relative to an otherwise similar car running only a couple degrees of +caster.

I'd guess that this effect occurs even more immediately than LWT through the roll centers, since you first need a steer angle before lateral acceleration can be developed, and that sounds like more responsive "turn-in" to me. As you unwind the wheel on corner exit, this effect diminishes, for a tendency toward "push".

Norm

[Marcus SC&C]

Thanks for the link to the specs. I`ll plug them in as soon as I get a chance and compare them to my numbers. It may be a little while. I`ve got so many projects (customer and R&D) going right now I`m going (gone?) nuts. BTW one is a C-5/C-6 based suspension package for A body. Does anyone know for sure if there are any dimensional differences between `64-`67 and `68-`72? Our test car is a `65. I`ve been told the earlier frames are slightly more narrow but that they`re otherwise identical. This might explain some of the differences in #s too.

On the caster issue... I`ve done a great deal of on road testing with our different suspension packages,much of it determining the best alignment specs for each. The adj. upper arms made it easy to try a really wide range of specs. I tried as much as +12* caster with my Stg.2 equipped Monte. There was very little difference in "centering" vs. say +6* and the car became much more vague and sluggish in transitions (S turns and such). It also picked up an odd twitchyness at times on rougher surfaces. For that car and combination +4.5* ended up giving the best overall results (only 1* more than stock specs). The car`s handling is nice and crisp,turn in is good and it`s still stable and well centered. Initially I would have thought +6* or so would have worked better but it wasn`t the case. Our G-5 package test car feels a little bit better at +5.5*,which may have to do with it`s much faster steering ratio,spring rates etc.
One note about track testing. It`s great fun and I`m going to get a lot more time in doing it..but..we should keep in mind that the track is not the street and we`re dealing with street cars here. Some combinations we`ve tried that worked very well on nice smooth track like roads have been lousy (even scary) on rough roads with potholes,ripples and off camber turns and such. Of couse you lucky stiffs that live in the southwest or California etc. can pretty much disregard this. ;) Marcus

[Mean 69]

we should keep in mind that the track is not the street and we`re dealing with street cars here.

Err, who is "we?" Not me!? Just kidding. 'course, I never thought I'd need rain tires either!

M

[dennis68]

Who has a street car here??? My wife drives a street car (or SUV). I would be happy with Said's car from Sunday and some headlights and license plates.

[Salt Racer]

...It also picked up an odd twitchyness at times on rougher surfaces...Our G-5 package test car feels a little bit better at +5.5*....

I'm almost certain that difference in scrub radius has a lot to do with it as well. GM did a fantastic job designing C5 knuckles and hubs.

I'm currently running +6.0* on my Riviera. Feels better on street compared to stock +4.0* Never ran it at track with stock alignment so I have no comparable result. Scrub radius is currently 2.38", which isn't terrible for a '60s car. I might try +7.0* at Buttonwillow just to aid camber angle a little.


Dennis,
The spindles you currently have must be a lot taller than what we thought. But I'm pretty sure you'll be getting the new ones made anyway, so just be sure to take accurate measurements on chassis pickup points. Those are what determines new UBJ/LBJ locations. You can still re-measure your existing spindles for academic purpose.

If you still have access to Perf Trend, try backing out a little on KPI. 14* is very steep. Its drawbacks may not be worth the small gain in scrub radius.

I don't know what your long term plan is with your car, but scrub radius can be reduced later when you can afford new rims and longer A-arms. If you have the slightest intention of doing this somewhere in your head, plan ahead and redesign spindles accordingly so you wouldn't have to have two sets of custom spindles made.

[kmracer]

wow. you guys rock.

[David Pozzi]

I think I'll move this thread to the Tech section. Thanks for finding this one!

[kmracer]

yea, there was actually a link to it on lat-g. great stuff. this is the reason i love this site.