The STX thread!
05-09-2012, 09:50 PM
#2501
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rick jung built a wing for my xp fc. i put the thing into a tree before i ever put the wing on it. putting the wing on the 8 would be pretty funny but it'd push me out of stx if people are being hardasses about it. although given that it's a 9.5" chord and 67" wide, I don't think it meets the old rule about providing no aerodynamic effect at solo ii speeds lol.
05-10-2012, 08:52 AM
#2505
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I'm pretty sure that the dash doesn't know, was just saying as a comparison is all. I always forget the math behind it.
And forget tire size, I'm pretty sure Team is running NOS and is sponsored by Vin Diesel. :X
And forget tire size, I'm pretty sure Team is running NOS and is sponsored by Vin Diesel. :X
05-11-2012, 07:13 AM
#2509
Changing the tire diameter is the equivalent of changing the rear gear ratio. The OE speedo works off an assumption of the OE tire size diameter so anything different creates an error between indicated vs. actual vehicle speed
(actual tire diameter or radius/OE tire diameter or radius) x indicated speed = actual speed
(OE tire diameter or radius/actual tire diameter or radius) x gear ratio = effective gear ratio
The OE tire diameter is 25.9". The tire diameter I'm using is 23.8". My memory was bad. That size indicates 72 mph @ redline, but the actual speed is 66 mph because despite having an actual 4.44:1 gear ratio it creates an effective gear ratio of 4.83:1
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(actual tire diameter or radius/OE tire diameter or radius) x indicated speed = actual speed
(OE tire diameter or radius/actual tire diameter or radius) x gear ratio = effective gear ratio
The OE tire diameter is 25.9". The tire diameter I'm using is 23.8". My memory was bad. That size indicates 72 mph @ redline, but the actual speed is 66 mph because despite having an actual 4.44:1 gear ratio it creates an effective gear ratio of 4.83:1
.
05-11-2012, 07:58 AM
#2510
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^this assumes the speedometer is accurate with OEM tire size, which is true for some cars but definitely not a given. Not sure about RX-8's as I've never checked mine against GPS or radar display, but each of my past BMW's were consistently off - the indicated speed was always 2-5% higher than the actual speed (with OEM tire sizes installed).
05-11-2012, 10:22 AM
#2511
Most speedometers read within 1 - 2 mph at interstate speeds. How worn were the tires?
tire diameter changes with wear and most street tires have 9/32" of tread thickness down to the wear bars from new x 2 = 9/16" total diameter change from new to replacement time. That is 3% of change on a 25.9" OD tire. The actual accuracy is actually the loaded radius though, which there are also variables such as air pressure, temperature, weight, bumps in the road, actual road surface distance vs true straight line distance, etc. Civilian GLS is not as entirely accurate as some people make it out to be either. We are talking competition so it is just a rough approximation of not overly important value. In the end your actual speed at any given point is not entirely relevant to reaching the finish line first.
The calculations above simply get you back to an approximate reference against the factory setup.
tire diameter changes with wear and most street tires have 9/32" of tread thickness down to the wear bars from new x 2 = 9/16" total diameter change from new to replacement time. That is 3% of change on a 25.9" OD tire. The actual accuracy is actually the loaded radius though, which there are also variables such as air pressure, temperature, weight, bumps in the road, actual road surface distance vs true straight line distance, etc. Civilian GLS is not as entirely accurate as some people make it out to be either. We are talking competition so it is just a rough approximation of not overly important value. In the end your actual speed at any given point is not entirely relevant to reaching the finish line first.
The calculations above simply get you back to an approximate reference against the factory setup.
Last edited by TeamRX8; 05-11-2012 at 10:24 AM.
05-11-2012, 10:29 AM
#2512
rick jung built a wing for my xp fc. i put the thing into a tree before i ever put the wing on it. putting the wing on the 8 would be pretty funny but it'd push me out of stx if people are being hardasses about it. although given that it's a 9.5" chord and 67" wide, I don't think it meets the old rule about providing no aerodynamic effect at solo ii speeds lol.
They'd no more be hardasses than someone else would be an idiot thinking the rules don't matter and they can do as they please and negatively denounce anyone who opposes their self-serving viewpoint ...
05-11-2012, 10:44 AM
#2513
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Tread depth was anywhere from new (10/32") to near wear bars (~3/32"), which accounts for ~1.5% change in indicated speed during the life of the tire. Regardless of tire wear (even on brand new OEM size tires - in other words, maximum OD), indicated speed on most cars I've owned is consistently higher than actual speed. The exception is a Honda I currently own, which indicates within +/-1% of actual speed. Again, I haven't tested my RX-8 on OEM tire size.
Last edited by quisk; 05-11-2012 at 10:47 AM.
05-11-2012, 02:02 PM
#2514
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The rules state that regional events can make exceptions for aero in stx. Since I haven't been to a regional or national SCCA event, I'm not sure how much people are going to care. Feel free to call me an idiot for trying to find an excuse to gain an unfair advantage. Didn't mean anything negative by calling SCCA guys hardasses btw - if I was really into autocrossing, and especially if I had money on the line in terms of sponsorships/contingencies, I'd be a hardass too. No reason not to be.
Last edited by Litreocola; 05-11-2012 at 02:30 PM.
05-11-2012, 03:32 PM
#2515
I'm not calling anyone an idiot, and certainly not you. I was just making a point.
If your region allows it then fine, but that's likely not the case and if it were allowed you can bet everybody in STX at the top would have one on their car.
Otherwise the rules are the rules. the people who insist on upholding them are not jerks/wienies/etc.. The people who break them are not jerks/cheaters/etc.. The car is either found legal or not legal. There doesn't have to be any emotional drama associated with the situation, but everyone always tries to induce it anyway.
If your region allows it then fine, but that's likely not the case and if it were allowed you can bet everybody in STX at the top would have one on their car.
Otherwise the rules are the rules. the people who insist on upholding them are not jerks/wienies/etc.. The people who break them are not jerks/cheaters/etc.. The car is either found legal or not legal. There doesn't have to be any emotional drama associated with the situation, but everyone always tries to induce it anyway.
05-11-2012, 03:43 PM
#2516
Tread depth was anywhere from new (10/32") to near wear bars (~3/32"), which accounts for ~1.5% change in indicated speed during the life of the tire. Regardless of tire wear (even on brand new OEM size tires - in other words, maximum OD), indicated speed on most cars I've owned is consistently higher than actual speed. The exception is a Honda I currently own, which indicates within +/-1% of actual speed. Again, I haven't tested my RX-8 on OEM tire size.
05-11-2012, 04:24 PM
#2518
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I'm not calling anyone an idiot, and certainly not you. I was just making a point.
If your region allows it then fine, but that's likely not the case and if it were allowed you can bet everybody in STX at the top would have one on their car.
Otherwise the rules are the rules. the people who insist on upholding them are not jerks/wienies/etc.. The people who break them are not jerks/cheaters/etc.. The car is either found legal or not legal. There doesn't have to be any emotional drama associated with the situation, but everyone always tries to induce it anyway.
If your region allows it then fine, but that's likely not the case and if it were allowed you can bet everybody in STX at the top would have one on their car.
Otherwise the rules are the rules. the people who insist on upholding them are not jerks/wienies/etc.. The people who break them are not jerks/cheaters/etc.. The car is either found legal or not legal. There doesn't have to be any emotional drama associated with the situation, but everyone always tries to induce it anyway.
05-11-2012, 05:20 PM
#2519
^this assumes the speedometer is accurate with OEM tire size, which is true for some cars but definitely not a given. Not sure about RX-8's as I've never checked mine against GPS or radar display, but each of my past BMW's were consistently off - the indicated speed was always 2-5% higher than the actual speed (with OEM tire sizes installed).
05-14-2012, 09:50 AM
#2520
Accleration run
Did a 2nd gear acceleration run to see how things look.. the road was pretty smooth but picked up lots of peaks and drops. One is around 5500 rpm where I have a noticeable flat spot part throttle. Sampling rate was 10hz. Horizontal axis (X) is in mph.
Anyone else try this?
Anyone else try this?
Last edited by JeffH; 05-14-2012 at 09:54 AM.
05-15-2012, 07:38 AM
#2524
Glad to see you're back on pace. My vote is that it was the tires and not the wheel diameter. Hopefully you can get more than 50 runs out of the Toyos. For me they go against one of the best things about ST.
05-15-2012, 10:43 AM
#2525
That is a tough crowd up there. I used to autocross in those parts. I usually wound up in top 10 for pax, but never topped the board. That's what you get for using street tires in stock class I guess. Good job.