I don’t feel flamed or offended. I enjoy a good debate, and hopefully, I am up to the challenge!
SportsCarDesigner wrote:
You seem to be trying hard to fit theory to an anecdotal experience. Fitting 35mm offset wheels to your Miata should give you the same general situation as the Elise, F1 and C5 shown. If you have problems with it, look elsewhere for answers; it's not likely the +10mm scrub radius at fault. Gordon Murray ain't stupid.
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MV8
I am suprised you would compare a F1 to a miata. Gordo did not design my car. The fellow that was part of the team that did has provided an explanation. Do you have a better one? Many may say something is bad, but few provide a solution.
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SportsCarDesigner wrote:
A former Lotus chassis engineer explained that keeping the (static design) scrub radius a little bit positive is better than zero because it keeps the state the same as the tire squirms from normal road irregularities and minor steering corrections... you don't want the scrub radius constantly flopping from pos to neg as you drive, alternately loading the joints in different directions when you just want to go straight. Makes sense to me.
King pin inclination causes the contact patch to move outward when you steer off-center; to a positive scrub radius situation. So it's not zero scrub radius vs. non-zero... it's zero migrating to positive, positive migrating to more positive, or negative migrating to less negative and possibly crossing over to positive. Which of these three situations is constant in sign?
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MV8
Thank you for making my point. Less kpi equals less migration of the scrub radius, positive, negative, or any combination. This means less static scrub radius is required to prevent “oscillation” between positive and negative, if any static scrub radius is required at all.
The kpi is a compromise. Adding kpi for parallel parking a vehicle with 7 inch wide rims that weighs less than 1,500 lbs should be very low on the list of objectives.
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SportsCarDesigner wrote:
I'd point out that none of the engineers involved with the four examples I gave were hindered by using donor car parts, nor were they limited to existing wheels available from Tire Rack; they could manufacture any geometry, KPI, scrub radius, caster they want... they don't lack performance knowledge; they are closely linked to the (factory effort) experience from multiple F1 WCs, multiple LeMans overall and class victories, etc. They have test tracks, instrumentation, contracted test drivers, and the means to test anything they think up. When they build their highly visible "halo" high-performance road cars they all settled on KPI around 10°, caster around 5°, and three out of four have significant positive scrub radius. I hate to discourage thinking for oneself, but I can't ignore the conclusions of the aforementioned
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MV8
Those unhindered engineers are still compromising in many ways. A certain size car needs a specific amount of tire for the intended use. That tire needs a specific rim width. There are advantages to maximizing the distance between the upper and lower balljoints, so now we have a spindle buried in a high offset wheel with the joints close to the rim’s inside diameter. The wheel must turn so many degrees. Since we don’t want the control arm links to contact the rim, the distance between the control arm to chassis mounting points is limited, which affects the loads transferred into the frame at those points, affecting how strong the frame must be. Increasing kpi out of necessity due to the wheels and tires makes the job easier. If they have a lot of positive scrub, it is probably because they didn’t want to increase the kpi any more than they already had. The F1 is a perfect example, with its 9 inch wide, high offset front wheels.
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SportsCarDesigner wrote:
Don't assume everything you read is right... I've seen chassis engineers and tire engineers ridicule Herb Adams' book (quote:
"I view it as the absolute worst book on suspension design ever written. I figure the fewer people read it, the fewer will get hurt.).
My copy is hidden away somewhere in storage, so I haven't looked at it for years (not sure what it is that upsets them), but I'd be a little more kind... he was racing at a time of rapid change 40 years ago when no one really quite understood what they were doing (compared to now, when the best halfway understand almost half of what they're doing)... and trying to boil down all the complex interactions to simple explanations does tend to produce rubbish.
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MV8
Regarding Herb Adams, I am curious as to who you are quoting? Herb does an excellent job of explaining what is going on in layman terms. While his conclusions may not be correct for every possible scenario that a professional engineer could dream up, they don’t need to be to fit the scope of the book.
Our conversation seems to come down to this. You feel there should lots of kpi and scrub radius for their own sakes and not as a result of physical limitations, while I think kpi and scrub should be minimized. I don’t think this is something we can agree on.
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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS
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