How do wheel/tire size affect speed
#61
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We dont want to turn this into a gearing discussion, but lower gears, especially 1st , is sometimes more effective out of the start, BUT, gearing doesnt make hp , only more effective use of the HP you have available. It all depends on your operational speed range. This is easly seen with the drag simulators by doing a gear change and seeing the effects over varied speeds.
Lots of trade offs through the gears. 80 to 120 might be a sweet spot for a certain gear ratio, you only can get with a taller rear end. gear spacing doesnt change, but when it does, like in the Cup car GT3, you get better use of the available hp through closer gears, not lower gearing. I dont think this is an issue here as its only a 1.5% max difference in diameter we think. (although he has stated that there is no difference in diameters)
anyway, you dont nessarily get better acceleration overall with lower gearing. It depends on the speed range. generally, we want to just hit the rev limiter at the end of all main straights. I think with autocrossers, they can pretty much use only 1st gear and maybe 2nd, as their top speeds never are faster than 60-70mph I would imagine.
Lots of trade offs through the gears. 80 to 120 might be a sweet spot for a certain gear ratio, you only can get with a taller rear end. gear spacing doesnt change, but when it does, like in the Cup car GT3, you get better use of the available hp through closer gears, not lower gearing. I dont think this is an issue here as its only a 1.5% max difference in diameter we think. (although he has stated that there is no difference in diameters)
anyway, you dont nessarily get better acceleration overall with lower gearing. It depends on the speed range. generally, we want to just hit the rev limiter at the end of all main straights. I think with autocrossers, they can pretty much use only 1st gear and maybe 2nd, as their top speeds never are faster than 60-70mph I would imagine.
For what it's worth, when I switch between 17's and 18's, I have to slightly adjust my suspension, as the geometry is slightly different.
Also, a smaller tire diameter will produce an effective gearing change, so will improve acceleration.
There's a reason why a lot of good autocrossers (in stock classes) look for the smallest diameter wheel/tire combo they can legally run.
Also, a smaller tire diameter will produce an effective gearing change, so will improve acceleration.
There's a reason why a lot of good autocrossers (in stock classes) look for the smallest diameter wheel/tire combo they can legally run.
#63
Rennlist Member
You didnt tell me if you are having to input the new tire diameter for speed sensing or is it all GPS. Any accelerometer readings? If the GPS and ground speed match up, how are they correlated with tire size. (I understand that the GPS part will be based on how you are tracked via satalite) I guess it really doesnt matter if the time to a certain postion on the track is now different and we want to know why. I cant think of anything else right now.
Im really leaning toward some type of engine power issue or difference.
mk
Im really leaning toward some type of engine power issue or difference.
mk
#65
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#66
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Easiest way for me is to hit the "print screen" button.
Go to MS Paint.
Edit--Paste
Save the pic and then post on here. If you have trouble posting you can send it to my email address and I'll get it up for you.
#67
Rennlist Member
Like speedo input?? If so, you will be off there by the amount you might be concerned about. as I said, I just happen to have the exact same tires on the same sized rims as you do! 26" exactly is the diameter of the tire with a bubble level. If the speed sensor is for the stock tire on the car, certainly that would be an overall issue. I forgot the hight of my 275s, but they were well under 26".
Now, hard to argue with the time clock and GPS . Stilll puzzled.
mk
Now, hard to argue with the time clock and GPS . Stilll puzzled.
mk
#69
Three Wheelin'
What is considered matching up well? Almost dead nuts or off by a bit. Any error will easily show up as 2 mph. They should be nearly identical.
The other thing I've noticed with the Motec, if calculating track map from wheel speed and lateral accel, is that there will definitely be an error associated with sizes. Therefore if your ground speed isn't correct then you're track maps won't match up either. Are your distances for the straight identical, they should be?
#70
Three Wheelin'
Its measured by measuring the number of rotations of the wheel. Therefore its back calculated using the rolling radius to give you wheel speed. If the radius you've entered isn't correct then the calculated wheel speed won't match the actual ground speed.
#71
I mainly use gps speed and don't worry about changing wheel size imputs to maintain correct ground speed.?.
Joe, while in i2 I tried hitting the print scrn button and nothing happens. It does not give me a window to save that screen or anything.
I don't have MS Paint.
Joe, while in i2 I tried hitting the print scrn button and nothing happens. It does not give me a window to save that screen or anything.
I don't have MS Paint.
#72
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Then you just need to open up a graphics program and hit Paste and it should pop up.
MS Paint comes batched in all the Windows machines I've ever used.
Hit the Start Menu
All Programs
Accessories
It should be there. The name on my computer is just 'Paint' If not you could try another graphics program but I haven't ever checked to see if it worked on anything other than MS Paint