Roll centers, weight distribution, caster...

Re: Roll centers, weight distribution, caster...

Postby Mopar 151w2 » Thu Jul 19, 2012 2:02 pm

Bouncy is not from too stiff a spring - more likely is a shock, either dead or way too stiff - way too much pressure with slicks will dribble like a basketball (no shock to damp out oscillations - think tire shake./wheelhop)
Yes, I understand the weight bias - My old car was 57% rear, the current one is 55%, Kevin's is 54%, their old car was similar. Spring rates were comprable, in proportion. The old mod was a handfull because it was initally missing the front swaybar, and the rear springs were way too stiff - stock car guys would describe it as "neutral in, loose in the center, wreckin' loose off". 8-)

As far as your swaybar(s) go, they are both likely to be waay too big - and the rate goes up at the 4th power of the diameter, so if you can see a little difference, it makes a BIG difference in the stiffness. It may be possible to change the leverage/motion ratio to change the bar rate, and even to adjust /fine tune the rate. Also, if you are scrounging up a new swaybar, remember that the arm lengths make a difference too.
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Re: Roll centers, weight distribution, caster...

Postby DennisRacing3 » Sat Jul 21, 2012 12:56 pm

Well my goal now is to make it to philo. We will see what happens. I think that may be the best for tuning since you get a gazillion runs comparatively. So I can make small changes every run and learn from it. Im hopefully going to have lots of extra parts to see what fits best. Thanks guys for the info.
Ill keep everyone posted. Maybe I will get around to making a project log.
Racing on the edge isnt dangerous its a game
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Re: Roll centers, weight distribution, caster...

Postby sdwarf36 » Sat Jul 21, 2012 3:42 pm

I still say bring it over + lets measure everything up and see where things are now. There very well could be an "ah-ha!" that we can find now.
The problem with testing at Philo: Odds are if there is plenty of runs its because its raining. :(
And if you get your set up right---great. If you get it wrong, you're in the woods. Not much in between.
Heres a thought-isn't CART doing autocrosses at Thompson?
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Re: Roll centers, weight distribution, caster...

Postby Chief Geek » Wed Jul 25, 2012 9:06 pm

Josh

Your point about the rear weight bias having an impact on the relative front/rear spring stiffness is totally right. Your light front end probably won't like having springs that are 4 times as stiff as the rears, but I'll bet, you eventually wind up with a 2:1 ratio or more.

Kevin's point about spring rate reflected at the wheel being important, not the rate of the spring is crucial. Geometry matters. For your MacPherson strut rear end, the spring stiffness at the wheel is almost exactly the same as the spring alone. The only modifier is the little bit of incline on the strut (it doesn't stand perfectly vertical).

Your front spring is at a mechanical disadvantage to the wheel so the rate at the wheel is lower than the spring's own rate. Also, my memory says that your front coil-overs are inclined inward at the top. This means the spring and damping rates are variable and stiffen as the suspension compresses (progressive). Having the spring rate and damping rate change may not be a good thing, particularly as our hillclimb roads were last paved before you were born. This also means that when you change the ride height, you're changing the level of "progressive-ness". If there is an easy way to bring the front coil-overs closer to vertical, I'd do it if for no other reason than to reduce the number of variables you have to deal with.

We all know formula cars have complex bell-crank/pushrod or pullrod connections between the hub and spring/shock. The thing that controls where the pivot points are and where the components attach to the crank is the geometry to minimize this "variable-ness". Simon, my old co-worker/handling genius, was always looking at XY charts with wheel movement as X and spring/shock length as Y. The ones he liked for non-ground-effects cars were always pretty linear. Does anyone in NEHA have any graphs like this that they'd be willing to create/share. John and/or Kevin?

Paul

PS I hope someone finds my ramblings helpful/interesting. I've done a bit of this engineering as an assistant, but I've never adjusted a car that I drove, so I have no idea what any of this feels like. Also, I'm not sure how changing the settings on a F3 style car on a runway relates to a Dennis chassis on a road that is more... um... 3D. Who knows, maybe a Dennis will be the next McLaren.
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Re: Roll centers, weight distribution, caster...

Postby Mopar 151w2 » Thu Jul 26, 2012 6:55 pm

I would beg to seriously differ with the first paragraph. Neither Kevin or I made these numbers up out of thin air - and the calculations we use factor in spring/shock angle, motion ratio of the control arms, and, where appropriate, even wheel offset and diameter. Also, the roll couple figure Kevin used is just about optimum - and his car is very close to neutral. Swaybar rate is as critical as spring rate to roll couple. "Soft spring/big bar" setups are particularly problematic on bumpy courses like ours, and they are known for lacking "feel", and being difficult to balance.

When it comes to the angle of shock mounting - what you need to avoid is the shock going "over center" (past perpindicular to the arm it is attached to),at the bottom of travel, which will make the rate decrease as the suspension compresses.

As far as the relative ages of pavement on the hills, you might be surprised.....
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Re: Roll centers, weight distribution, caster...

Postby Chief Geek » Fri Jul 27, 2012 6:58 pm

Mopar151w2

I totally agree and I'm sorry if I made anything sound like the work you two have done on #25 was in any way arbitrary. The proof is in the pudding. No one could guess their way into making #25 work as well as it does.

I'm sure we'd all love to see the chart or table you guys have for reflected spring and roll bar stiffness at the center of the contact patch. I.ve never seen a position/spring rate graph that was perfectly linear, there were always curves in them and they all tended towards progressive-ness (rising spring rate as the suspension compresses). You're right, if the spring goes over-center (making a regressive rate), I don't know what would happen, but I'd bet it's bad.

I trust I'll see all of you at Okemo 2 next weekend. There's probably something I'm not seeing when thinking through #3. With luck, someone will shine a light on it for me.

Paul
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Re: Roll centers, weight distribution, caster...

Postby honda#72 » Sat Jul 28, 2012 9:29 am

Mopar 151w2 wrote: "Soft spring/big bar" setups are particularly problematic on bumpy courses like ours, and they are known for lacking "feel", and being difficult to balance


John, so I don't forget to ask you when I see you, what ratio of spring to bar would you consider to be a Soft spring/ big bar setup ?

Chris
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Re: Roll centers, weight distribution, caster...

Postby Mopar 151w2 » Sat Jul 28, 2012 10:51 pm

I'd have to look at numbers for a while - but I'd guess that if 60% - 70% of the wheel rate is spring - that is a fairly conventional setup. An interesting factoid - when comparing live axle rear spring rates, you have to account for a 20-30% difference in roll rates from coil springs to leaf springs - because leaf springs twist as the car rolls, the (torsional) rate adds to the spring rate.
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Re: Roll centers, weight distribution, caster...

Postby Shenglehoffer » Mon Jul 30, 2012 11:54 pm

That's because leaf springs are superior technology!
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Re: Roll centers, weight distribution, caster...

Postby Mopar 151w2 » Tue Jul 31, 2012 5:40 pm

As springs, perhaps. As locating members, not so much..... ;)
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