Alan

Schoki,
Not sure if its calibrate as much as just measure - so if you should find by measuring that you are 10% off - how can you compensate for this on the speedo (except in your head...) is there a feasible way? Does the bosch hammer let you set a value related to 1 rotation length of tire in the digi dash?

Bernard - I dont see how you could get 19" wheels to have the same rolling diameter as 17" wheels - there isn't enough tire height to give away... right? 18" to 17" maybe I could see being feasible.

*** OK edited here - I played with the Miata calculator (Thanks Randy) and see that 19/30 can get you in the same diameter range as 17/40 even with somewhat wider tires - Bernard I get it - there is enough tire left - but I see why larger wheels are delicate - less strength of the rim due to the larger diameter AND also much less tire to take up the slack - at least in the sizes we need to run. ***

Alan

heinrich

You know, I used to have a good friend with bull terrier pets. He would swing a tyre sidewall, and both would grab onto it and never ever let go, no matter how long or how hard he swung it.

mark kibort

I use 305x 35 x 18 which are near 26.5" diameter and then i use a 335x30x18 which is more like 26" and then i also use the hoosier 305x30x18, which is about 25.2" in diameter. i vary my diameters around 1.2", with asolutely no difference that you can ever THINK of noticing. all you do is shift the speed and hp ranges in each gear (and torque to the wheels at any vehicle speed) up and down by about 5%.
(kind of like the difference of the gear of a 2.22:1 1984 vs a 2.2:1 S4 gear box diff)

I did a radar test vs speedo and found that with the 27" 305s, at 70mph, im radared at 70mph. looks like porsche did a little safety factor so we wouldnt get as many tickets. stock tires are around 24.9" (225x50x16)

305/30-18 3.6in 12.6in 25.2in 79.2in 800 0.0%
225/50-16 4.4in 12.4in 24.9in 78.1in 811 -1.4%

305/30-18 3.6in 12.6in 25.2in 79.2in 800 0.0%
305/35-18 4.2in 13.2in 26.4in 83.0in 764 4.8%



MK

mark kibort

He wasnt swinging hard enough!
at some speed, the g-forces could pull their teeth out.
Im sure my hammer throwing buddy from Track and field days, could throw a 20lb dog about 200feet at the end of a 3foot wire. (near 500lbs of force at the dog's teeth)

Mk

piccardo

N T B has a calculator. you just put in the stock set-up and the desired set-up and it tells you how it will affect your spedo

jakermc

Mark -

You stopped your proof a little early in the process. As you described, and I agree, acceleration = power/(mass x velocity).

Now breakdown the formula for power (hp) which is a function of rpms and torque: power = rpm x torque / 5252

Thus: acceleration = ((rpm x torque) / 5252) / (mass x velocity)

As you describe, and I agree, gearing 'changes the sweetspot' which is another way to say it changes the torque curve, making low rpm acceleration more difficult.

Perhaps you don't notice this in a high hp/weight ratio 928 track animal, you will notice in a lower hp vehicle. As I stated, how important a trade-off this is to you is one of personal opinion, but I believe my math is correct.

mark kibort

Not exactly. ( i dont quite understand the last paragraph.maybe you can re write it and make it a little more clear) I think you are talking about a lower rpm launch of a taller gear (because you would be operating at a lower hp range now at the start) which only effects launch in 1st and none of the other gears. Anyway, it doesnt matter what the torque curve, its all about putting hp to the wheels over any speed range. gear spacing dictates this, regardless of the final drive ratio, or engine torque for that matter. Its all about HP for which torque is a factor of.

So, with a taller ratio, all you end up doing is shifting the hp vs speed curves up a proportional amount. meaning , I can show at which speed range are optimal for which final drive ratios.

Since you agree that acceleration is proportional to power, you understand that accleration goes down inveresly proportional to velocity. (if hp is constant and at any particular velocity) constant would mean a infinitely variable gear box.

With gear boxes, max acceleration at any speed will occur at a point at which the engine is closest to the max HP. (not max acceleration in a particular gear which happens at max torque, which is not the max acceleration at that speed, as you could use a lower gear and get rpms at more near max hp for greater acceleration) not to digress, but gears sets and final drive ratios which shift the optimal range of these curves as they allow for the application of the engine's HP, do not make hp, they optimize the application of hp over any range of speeds.

the answer to the overall question, is how many Hp/seconds you produce over a speed range. the gears that optimize this, is the fastest way to get from one speed to another. a lower final drive doesnt always do this. and to your final point, doesnt do this in a lower hp car either, in fact, just the opposite. you need to better optimize the gears on a lower hp car as often those cars can also have a peakier hp curve, or you want to better optimize the hp you do have to compete with the higher hp cars. so, i disagree with your statement for those reasons. (see GT3 close ratio gear box, to better utilize the lower hp to weight vs the vipers in speed world challenge . viper 3200lbs/575hp vs porsche 2900lbs/415hp, but close gear ratios. rear end gears only optimize the HP utilization for a given set of speeds dictated by the car and the track.

an easy way to graphically see this, is go to the computer simulator. you can see how final drives effect any kind of car with any weight , hp and speed ranges.

Mk

acceleration = power/(mass x velocity)

Think of HP as comparitively equivilant to torque through all the gears (rear end and trans ratios) at ANY particilar speed.

Glen McCartney

Here is link to a little box (http://www.blackrobotics.com/yb_home.htm) that you can install to correct speedo when using non stock size tires. I have been thinking about it since I installed the 265/40/18's on the car. For now, just being careful as the speedo is low by about 6 MPH, when speedo shows 65, actually traveling at 71.77. Here is link to a calculator that shows the new speed with different size tires, http://www.csgnetwork.com/speedocalibcalc.html.

Alan

Glen,
Thanks - I'd have expected Porsche to have provided a means to do a correction factor too (possibly in firmware and not user accessible - except perhaps to the likes of John Speake etc...).

But I guess this wil likely work as an alternative.

Alan

mark kibort

with 305s vs stock 225s, there is less than 1.4%. thats less than 1mph at 60mph. dont think any of the tires you are looking at are much different. you show a 10% difference below. 265x40x18s are not 10% different than most anything we can use. i dont know where those numbers come from. it would actually be in the range of 5%

again, i was on 305x30x18s and they are only 1.4% difference than 225x50x16s. a 265x40x18 is a tall tire, almost too tall on an 18" rim. usually, we go to 35s when near 275 on a 18" rim. its 26.
5" diameter, almost the diameter of my 305x35s!! (and they are tall!)

anyway, its still 5% and that would be only around 3mph difference. if you used a 35 series, it would be near stock. the good news even for you, is that it seems like porsche put in some safety room and thats why i use 305x35x18s and my speedo is dead on accurate now.

dont understand the fuss.

MK

wds928

Randy, thanks for the link. That helps a lot. I'm looking at a tire only 1/2" to 6/10th's taller than my stock 255's

So with the taller tires, I may be a half-tick or less slower to 60, but I'll get better gas mileage at steady cruising.

When I got back home this evening, Tire Rack had called asking about my tire choice....seems their database thinks it bigger than stock and just wanted to confirm. Hope they arrive on Friday so I can get them mounted in time to leave for SITM.

Thanks everyone for the input.

mark kibort

since 0-60 requires two gears, there is some trade offs, but certanly 0-55 would be a tini bit faster (probaby still unmeasureable though)
55mph is redline in an S4 in 1st gear

MK