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      03-01-2013, 01:08 PM   #149
CSBM5
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Drives: 2011 M3, 2001 M5, etc
Join Date: Nov 2005
Location: Raleigh, NC

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Rich,

How is your front bar working out? I tried to run some numbers on the stock versus various aftermarket options, but the stock bar is hollow, and without cutting one in half, I don't know the inside diameter. So I ran across a FAQ on Turner's web site on the H&R front bar where someone supposedly did the calculation for a 27mm solid H&R versus stock, and they claim that one is 1.425 times the stiffness of the stock one. I assume the moment arm is equivalent to the stock bar.

I'm looking at all of this since I ended up buying a Dinan 28mm solid front bar on sale for 40% off, and I'm wondering if this will prove to be too much front bar on street tires. Interestingly, Dinan adds this bar to their spring kit (i.e. moving from their stage 1 to stage 2) without changing the rear bar, so clearly a car with just their springs versus springs + front bar is going to be a good bit different, but they must feel it isn't too much bar. All that is an aside...

So if I assume Turner's H&R vs stock calcs are correct, then the Dinan 28mm bar (same moment arm as stock) should be: 1.4252 x (28^4 / 27^4) = 1.65. Hence it is 65% stiffer than the stock bar.

So far just driving it a bit since putting it on last night, it feels fantastic, and slaloms are going to be great I think. I do note the aggressive outside front loading on sharp turn-ins, so I'm wondering how much steady state understeer is increased. I have the pins out and struts all the way in for max negative camber already btw. I will find out this weekend in any event, but I was just curious what you think of front bar stiffness increases vs stock so far.

Since my last post, I've been talking with numerous different sources about street tires versus wheel widths (a couple of multi-time national trophy winners, and a couple of tire engineers), and the net is that it seems to be a consensus opinion that a street tire will not respond anything like a Hoosier A6 to "fat-tire-narrow-wheel-stuffing". Hence I'm going to stick with the overall recommendation to try to match wheel width to tread width, so I'm going to try the new BFG Rival in 245/40 and 275/35 sizes. I did get the TRMotorsport wheels from Tirerack (8.5/9.5). The 275 Rival isn't available until June, so for now I bought the wheels with 245s all around. I know it's going to be a pain to put down power and probably make the car tail happy, but it's only for a few events until June, and then I will have an extra set of 245s for the front.

I think ideally I would like to have had a 255/35 up front, but from tests various folks have done, even a small bit of oversizing street tires seems to be slower -- just the opposite of R-comps. Perhaps it is related to the very stiff sidewall construction of R-comps versus the street tire -- I would very much like to get a fuller understanding of the whole subject (and I'm working on that -- background is in mechanical engineering, have been studying tires for 6-7 years now, daughter is a tire engineer now too).

Regards,
Chuck
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Current Stable:
2011.5 M3 sedan 6MT, Silverstone, no sunroof, no EDC, but most other options
2001 M5 LeMans Blue/Silverstone
2006 330i 6MT Red/black, ZSP, Nav, etc.
2001 Audi S4 6MT; 1996 Volvo 850R wagon