[flash]

yeah - lowered the way i am, it smacked the bodywork - i seriously looked at those, even though i thought they were a lot less attractive than i wanted (looked at the RC5 too, and actually like that one) - i really wanted a billet, and not cast rim though, for reasons of strength



i was searching for a while for rims - might have just cleared with the smaller front tires though, but i was really looking for a 9 and a 255 - the initial thought was to match front and rear



i will be paying more attention to the tire weight next time around though - lol - at the rate i'm going though, it's going to be 2 years before i have to think about tires - i really need to drive the car

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[badberry68]

[quote name='thingo' date='Jul 21 2006, 07:39 PM']

Offsets might be wrong for your car but don't think bob would have a problem with 8.5 et50 10et 50 or 60, guess the tyre sizes are wrong for you, no problem with a 968

[right][post="24465"]<{POST_SNAPBACK}>[/post][/right]

[/quote



Unsprung weight has more of an effect than 4X the actual weight difference. Isn't it about the largest contact patch with the least amount of sidewall ( to minimize deflection).

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[bibbetson]

One piece of information that hasn't been stated is that bigd_SD is driving San Diego autocrosses. Although these are open lot timed events through a coned course (for anyone not knowing what an AX is), they are run at the Qualcomm stadium which is huge with rather large elevation changes. Events at this venue hit third gear often at 70+ MPH in a 968, with first gear only used at the start. They are nearly equivalent to a low speed track event such as Street of Willow at Willow Springs.



Many have stated that venue will change what is optimal, so I thought I would throw it into the mix that the San Diego AX is not a typical autocross venue.

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[flash]

hmmm - right now, i'm good for over 70 in 2nd - not sure i'd go to third, but the point is well taken - i might actually do better with the wheels left alone, because then i could stay in 2nd, and not have to shift at all - that would probably make for faster times on a course like that

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[bigd_SD]

Flash - sounds like you've really got your driving dialed in. You should come down for a San Diego event and test your theory about staying in 2nd gear. The next event is Aug 19 and is open to all PCA members. There's another on Sep 16 and both are in the big lot at "The Q," so they'll be plenty fast.



You can register on-line at San Diego PCA.



This would also be a good chance to check out some very fast (top 10) water pumping drivers and check out how they have their cars set-up. <img src="/forum/images/smilies/968/smile.gif" class="smilie" alt="" />



David



[quote name='flash' date='Jul 24 2006, 01:34 PM']hmmm - right now, i'm good for over 70 in 2nd - not sure i'd go to third, but the point is well taken - i might actually do better with the wheels left alone, because then i could stay in 2nd, and not have to shift at all - that would probably make for faster times on a course like that

[right][post="24535"]<{POST_SNAPBACK}>[/post][/right][/quote]

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[flash]

sounds like fun at any rate - i'll see how that looks on the schedule - the other half is getting a bit annoyed at the amount of time i'm spending on the car, so..............



but i'll try

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[Straight Four]

guys, sometime when i get home ill get you my rotational mass calculator. if you go from 17 to 18 you can have no effect if the weight of teh rim doesnt change. or goes smaller. (depending on offset and weight etc...)



when i was goign to get rims for my m3zda i found out that if i had a 18 inch enkei rim that weighed 19 lbs there was no gain or loss in teh rotational mass on the car (what would affect yoru acceleration). but if i dropped down to the 16" enkei that weighed 14lbs the gain was the same as the car gaining 30 ftlbs of torque. the stock rims on my car (hatch of course) are 17's that weigh an almightly 22lbs each. As well, your tire weight affects acceleartion even more. in sizing tires for teh 17 i found that i could go from 22lbs per tire to almost 28 lbs per tire. race tires are heavier than stock tires (thicker sidewalls etc...). There is a reason that most of these race cars have 18's etc... they have 14 " brakes. as cool as an 18 looks i have to say rice unless its a really nice rim like kodiak or some such where they really work at lowering the weight of the rim. (friends 15x8 kodiaks weigh 10lbs. and are 3 peice billet Al). So sometime tonigth ill post a link to the excel weight calculator and you guys can go nuts figuring out which rims with which weights and which tires are best. (i spent two weeks playing with the thing every night before i decided that my budget didnt include buying 18inch 5zigen wheels that weighed 18lbs and was 1.5 inches wider than stock...)



flash the reason smaller cars feel this more is that when you have a smaller car that 10 ft lb gain from having a lighter rotational mass is more easily felt than a 3000 pound car. hp to lb etc.. and all that jazz..

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[flash]

that was a bit tough to follow, but i think i get what you're saying, and i think it agrees with what i've been saying



does that calculator take into account the location of the mass? there is a large difference in effect of hub mass versus outer rotational mass - you can add weight at the hub with a lesser effect than at the outside (like the tire) - it would be a very cool device if it did, and i would love to have it



and yes, i am painfully aware of the effect on lighter cars - that was a huge consideration on the mgs - it still has a significant effect on this car though, and i am still looking to lose weight there, primarily in the tires, where it would affect things most

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[Straight Four]

yeah my hankook ventus are 21 lbs versus the falkens that i was looking at were 26lbs.

it is only an excel file so it takes its best shot at approximating the effects. I am a ME student and i went through the functions after finding it and they are as close as i think excel can get without spending a year developing it. the basic inputs are car weight, rim size, rim weight, tire weight, car hp, and thats it. You compare two types and see the difference, if it shows -tq then the right rim is goign to have a negative affect on your performance while the left one is better. The tq is the approximation of how much torque it would take to rotate the wheel faster. (not sure how much faster think it is 10mph).



it checks three ways, just mass in a disc, mass located a certain percentage out to the edge and the final one uses the best type of approximation it can. Im sorry its hard to understand, im busy working as a COOP at Robert Bosch, Anderson, SC and i only take a few seconds to rush through these posts. More detail and better english to follow tonight.

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[flash]

lol - cool - no worries



it's been way too long for me to remember how to create such a tool - too many brain cells gone now - gives me a headache doing that much math anymore



love to get my hands on it though

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[Jason Judd]

Just came across this thread, so everything may have been discussed and solved by you all, but here's my $.02.



I've always been under the impression that a bigger tire patch was a good thing...and that one of the best ways to accomplish that was to get a wider and larger diameter wheel under the fender; of course with the correct aspect ratios.



So, I've been able to put 9" X 18" up front (with 265/35/18 MPSC tires) and 11" X 18" in the rear (with 295/30/18 MPSC tires).



My rims are BBS monoblock magnesium, so they are super light. They made a huge difference and make the car feel downright sprightly.



I do have a fair amount of neg. camber up front...I'm pretty sure it's set at -3 degrees, we have the Racer's Edge camber plates and a strut brace, too.



Just another data point...



Jason

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[Cloud9...68]

Or, one could forget about all the nonsense of tradeoffs between contact patch size vs weight, and just go with These!



I haven't been able to find a price on these anywhere, but I suspect that when you're ready to place your order for these babies, you simply check the box that says "empty your kids' college fund now"



Seriously, 13.23 lbs for 18-inchers. Pretty amazing. And while they're not exactly the prettiest things, they're not really hideous either. They'd be great track wheels, at any rate. Hopefully, as other manufacturers start producing carbon fiber wheels, the price will come down. Of course, I'm dreaming the same thing about ceramic disks, too...

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[Straight Four]

download this to caculate the effect of wheel weight. http://www.mazda6tech.com/files/rotational.xls



that should be the excel file i was talking about.

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[Jason Judd]

Pretty cool spreadsheet!



Jason

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