Gearing Ratio? Aero Limited?
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Gearing Ratio? Aero Limited?
I'm a bit miffed. Looking at the gear ratio for the Turbo, based on my usual tire heights (25.6 inches) and a 6900 rev limiter I get the following speeds on stock gear ratios (except 3rd is slightly off because it's straight cut).
1st - 40 mph
2nd - 73 mph
3rd - 108 mph
4th - 136 mph
5th - 166 mph
6th - 201 mph
These are all tested on track (mostly Road Atlanta) I don't use 1st or 2nd gear on track, ever. Useless.
3rd and 4th are exactly right. Car hits rev limiter if I'm not shifting at that point, so I usually shift at 106-107 and 134-135 mph if I'm on point. It gets weird at 5th gear though because I have data verified 170-171 mph on multiple occasions and can do 167-168-169 regularly pending my aero setup.
Speaking of aero setup, it gets even weirder though because depending on how much front DF I run, the rev limit changes. I have several set of big/small Joe T canards and another smaller set of canards to go with my 3 inch splitter. If I run with no canards or the very small ones and I can reach 171 and not feel the rev limiter but run out of track before I have to brake.
However if I put lets say two sets of Joe T canards on, a big set and a small set, I can hit the rev limiter at 164 mph.
Then again if I run just one set of small canards and max rear wing angle I can reach 164 but won't hit the rev limiter.
I've heard of Aero limited cars before, and rpm limited cars, but does an aero limited car hit the rev limiter or just won't go any faster, and why? I can understand 1mph high, but 5-6 mph high?
All speeds are GPS verified. Could my tune have had a higher rev limiter in 5th gear only? It would take an extra 200 rpm to reach the speeds I've reached.
1st - 40 mph
2nd - 73 mph
3rd - 108 mph
4th - 136 mph
5th - 166 mph
6th - 201 mph
These are all tested on track (mostly Road Atlanta) I don't use 1st or 2nd gear on track, ever. Useless.
3rd and 4th are exactly right. Car hits rev limiter if I'm not shifting at that point, so I usually shift at 106-107 and 134-135 mph if I'm on point. It gets weird at 5th gear though because I have data verified 170-171 mph on multiple occasions and can do 167-168-169 regularly pending my aero setup.
Speaking of aero setup, it gets even weirder though because depending on how much front DF I run, the rev limit changes. I have several set of big/small Joe T canards and another smaller set of canards to go with my 3 inch splitter. If I run with no canards or the very small ones and I can reach 171 and not feel the rev limiter but run out of track before I have to brake.
However if I put lets say two sets of Joe T canards on, a big set and a small set, I can hit the rev limiter at 164 mph.
Then again if I run just one set of small canards and max rear wing angle I can reach 164 but won't hit the rev limiter.
I've heard of Aero limited cars before, and rpm limited cars, but does an aero limited car hit the rev limiter or just won't go any faster, and why? I can understand 1mph high, but 5-6 mph high?
All speeds are GPS verified. Could my tune have had a higher rev limiter in 5th gear only? It would take an extra 200 rpm to reach the speeds I've reached.
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Interesting observations. While I can't vet for the explanation below, it's an interesting problem that I can't help as an engineer, take a stab at.
I don't believe the rev limiter is what is fluctuating as it's a true electro-mechanical parameter driven by the ECU. I can't see the driveline changing/slipping either so the wheel revolution at your engine rev limit is likely a constant here as well.
So it comes down to some variable in the tire vs ground speed that's causing the gps verified differences.
My guess would be that it comes down to effective tire diameter at speed. Centrifugal force would cause an expansion of the tire at high speeds (like top fuel drag cars). At a fixed wheel rpm (that's correlated to the engine rev limiter), the ground speed could be higher given more centrifugal expansion.
Where downforce come into play is that it's adding more load onto the tires. This effectively compresses, or limits the centrifugal expansion, of the tire at speed. Which would possibly account for the ground speed difference between the two configurations.
Those aero mods must be quite effective!
I don't believe the rev limiter is what is fluctuating as it's a true electro-mechanical parameter driven by the ECU. I can't see the driveline changing/slipping either so the wheel revolution at your engine rev limit is likely a constant here as well.
So it comes down to some variable in the tire vs ground speed that's causing the gps verified differences.
My guess would be that it comes down to effective tire diameter at speed. Centrifugal force would cause an expansion of the tire at high speeds (like top fuel drag cars). At a fixed wheel rpm (that's correlated to the engine rev limiter), the ground speed could be higher given more centrifugal expansion.
Where downforce come into play is that it's adding more load onto the tires. This effectively compresses, or limits the centrifugal expansion, of the tire at speed. Which would possibly account for the ground speed difference between the two configurations.
Those aero mods must be quite effective!
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Interesting observations. While I can't vet for the explanation below, it's an interesting problem that I can't help as an engineer, take a stab at.
I don't believe the rev limiter is what is fluctuating as it's a true electro-mechanical parameter driven by the ECU. I can't see the driveline changing/slipping either so the wheel revolution at your engine rev limit is likely a constant here as well.
So it comes down to some variable in the tire vs ground speed that's causing the gps verified differences.
My guess would be that it comes down to effective tire diameter at speed. Centrifugal force would cause an expansion of the tire at high speeds (like top fuel drag cars). At a fixed wheel rpm (that's correlated to the engine rev limiter), the ground speed could be higher given more centrifugal expansion.
Where downforce come into play is that it's adding more load onto the tires. This effectively compresses, or limits the centrifugal expansion, of the tire at speed. Which would possibly account for the ground speed difference between the two configurations.
Those aero mods must be quite effective!
I don't believe the rev limiter is what is fluctuating as it's a true electro-mechanical parameter driven by the ECU. I can't see the driveline changing/slipping either so the wheel revolution at your engine rev limit is likely a constant here as well.
So it comes down to some variable in the tire vs ground speed that's causing the gps verified differences.
My guess would be that it comes down to effective tire diameter at speed. Centrifugal force would cause an expansion of the tire at high speeds (like top fuel drag cars). At a fixed wheel rpm (that's correlated to the engine rev limiter), the ground speed could be higher given more centrifugal expansion.
Where downforce come into play is that it's adding more load onto the tires. This effectively compresses, or limits the centrifugal expansion, of the tire at speed. Which would possibly account for the ground speed difference between the two configurations.
Those aero mods must be quite effective!
[massive edit]
interesting observation on the OP's part.
sounds plausible tho my guess is centrifugal expansion effects are really complicated with transients. ignoring transients, i'd think that around 180° away from the contact patch, the expanded radius would be similar regardless of weight. but then again at that speed, transients are probably a very big deal. edit edit edit edit: semi serious question. wonder what is the resonance frequency of the tire vs. the rotation frequency. i.e. how fast would one need to get going to produce a standing wave in tire expansion?
a high speed camera could probably answer it. i'm guessing waaay faster than we're driving tho, else it'd be a well known effect.
to the puzzle at hand. probably going to restate the second post a lot, but i think there are some subtlteties. also, this is a thought changing in time, not intended to be a definitive thy.
probably two dominant effects (for a constant tire pressure, etc.):
1) decrease in weight due to centrifugal force
2) change of radius at contact patch due to weight. weight is going to be the sum of gravity * mass + downforce + centrifugal force.
regarding (1). i haven't put much thought into this before, but my initial thought is that it's a little different than just the tire 'expanding' due to centrifugal forces. my thought is there is a bit of a non-obvious subtlety (at least for me), even when excluding transients: it's an unbalanced force thing. for a tire to remain round while rotating, a force toward the center must be applied. for everything except approximately the contact patch, this is carried from the tread area through the sidewall, wheel, and ultimately to the car, i.e. the car is holding it in a round trajectory. many of these forces cancel, and all of them would cancel to a zero net force on the car if the car weren't on the ground. however, because it is on the ground, for approximately the contact patch, this force is applied by the ground, not the car. so there is an unbalanced force on the car such that approximately whatever force is required to keep a contact patch sized (mass) piece of rubber going in a circle will produce an upward force on the car, reducing its weight which goes into the sum for (2). the second post may be addressing this, but it's the first time i've put some thought into it. kinda works backwards to me like bicycle spokes (tension not compression...)
(2) i suspect that to a pretty good approximation, the effective radius seen when the tire rotates is a function of the car's weight. and the radius the car sees is the distance from the hub to the ground, doesn't matter what the tire expands to away from the ground. one can make it very complicated, but more weight -> smaller raidus. so (1) has the effect of reducing weight as some function of speed, downforce has the effect of increasing weight as some function of speed.
there is probably also a # (3): the slippage and bending of rubber, but i suspect it's largely explained by 2. i.e. equations for radius change due to rubber bending and slippage will largely close to the effective radius being the distance from the center of the hub to the ground.
i suspect the OP is changing things such that the downforce is growing faster than (1) as a function of the car's speed.
interesting observation on the OP's part.
sounds plausible tho my guess is centrifugal expansion effects are really complicated with transients. ignoring transients, i'd think that around 180° away from the contact patch, the expanded radius would be similar regardless of weight. but then again at that speed, transients are probably a very big deal. edit edit edit edit: semi serious question. wonder what is the resonance frequency of the tire vs. the rotation frequency. i.e. how fast would one need to get going to produce a standing wave in tire expansion?
a high speed camera could probably answer it. i'm guessing waaay faster than we're driving tho, else it'd be a well known effect.to the puzzle at hand. probably going to restate the second post a lot, but i think there are some subtlteties. also, this is a thought changing in time, not intended to be a definitive thy.
probably two dominant effects (for a constant tire pressure, etc.):
1) decrease in weight due to centrifugal force
2) change of radius at contact patch due to weight. weight is going to be the sum of gravity * mass + downforce + centrifugal force.
regarding (1). i haven't put much thought into this before, but my initial thought is that it's a little different than just the tire 'expanding' due to centrifugal forces. my thought is there is a bit of a non-obvious subtlety (at least for me), even when excluding transients: it's an unbalanced force thing. for a tire to remain round while rotating, a force toward the center must be applied. for everything except approximately the contact patch, this is carried from the tread area through the sidewall, wheel, and ultimately to the car, i.e. the car is holding it in a round trajectory. many of these forces cancel, and all of them would cancel to a zero net force on the car if the car weren't on the ground. however, because it is on the ground, for approximately the contact patch, this force is applied by the ground, not the car. so there is an unbalanced force on the car such that approximately whatever force is required to keep a contact patch sized (mass) piece of rubber going in a circle will produce an upward force on the car, reducing its weight which goes into the sum for (2). the second post may be addressing this, but it's the first time i've put some thought into it. kinda works backwards to me like bicycle spokes (tension not compression...)
(2) i suspect that to a pretty good approximation, the effective radius seen when the tire rotates is a function of the car's weight. and the radius the car sees is the distance from the hub to the ground, doesn't matter what the tire expands to away from the ground. one can make it very complicated, but more weight -> smaller raidus. so (1) has the effect of reducing weight as some function of speed, downforce has the effect of increasing weight as some function of speed.
there is probably also a # (3): the slippage and bending of rubber, but i suspect it's largely explained by 2. i.e. equations for radius change due to rubber bending and slippage will largely close to the effective radius being the distance from the center of the hub to the ground.
i suspect the OP is changing things such that the downforce is growing faster than (1) as a function of the car's speed.
Last edited by nitrojunky; Dec 11, 2014 at 09:34 AM.
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Interesting observations, I had no idea of that effect. I would think a racing tire would be more resistant to said forces though....
How much expansion effect are we talking about at speed?
How much expansion effect are we talking about at speed?
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Has to be, that's 10 mph off. But that's a long way off for 1 gear. How tall are your tires?
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[massive edit]
...snip...
(2) i suspect that to a pretty good approximation, the effective radius seen when the tire rotates is a function of the car's weight. and the radius the car sees is the distance from the hub to the ground, doesn't matter what the tire expands to away from the ground. one can make it very complicated, but more weight -> smaller raidus. so (1) has the effect of reducing weight as some function of speed, downforce has the effect of increasing weight as some function of speed.
...snip...
(2) i suspect that to a pretty good approximation, the effective radius seen when the tire rotates is a function of the car's weight. and the radius the car sees is the distance from the hub to the ground, doesn't matter what the tire expands to away from the ground. one can make it very complicated, but more weight -> smaller raidus. so (1) has the effect of reducing weight as some function of speed, downforce has the effect of increasing weight as some function of speed.
Last edited by Highlander; Dec 11, 2014 at 11:57 AM.
996TT should have a rear tire contact patch of about 35.5 in^2 with 30psig in its tires. (weight on each should be around 1064.5)
i expect the weight of the contact patch portion of the tire to be around 1.79 lbs for this size of contact patch. (contact patch proportion of an assumed 80 pct of MPSS 295/35 18 28 lb weight in tread area)
at 170 mph, it would require 3180 lbs of weight to offset the centrifugal force and maintain the same contact patch size. assuming no downforce, i expect the contact patch will go from 35 in^2 to around 11.9 in^2. so it'd go from something like a 3" long contact patch to maybe a 1" long contact patch. from this one can approximate the change in radius, but i'm curious how much the shape of the patch also changes.
anyway, what i get for a the resting condition is a length of contact patch which implies that the effective radius goes from 13.5" to 13.400". for the 170mph scenario, if the contact patch reduces to about 1/3, i get a radius of about 13.49". so i expect the effective radius grows a little more than 1/2 percent. due to stretching, the tire itself may balloon more in the non-contact patch area, but i don't think it'll lift the car more than maybe 1/10 inch.
however, the area is a function of the weight, and therefore my (1) and (2) items are not independent; including the downforce is going to definitely be an ugly nonlinear function even for a back of the envelope calc and even ignoring probably a lot of effects... that said, how much downforce do you think you produce?
Last edited by nitrojunky; Dec 11, 2014 at 12:11 PM.
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Wow, cool, thanks for the education gents. I even asked my shop mechanics, who are multiple championship winning car supporters in professional racing and they all looked at me like Scooby Doo. Of course, they are used to tuning on super hard sidewall slicks, which makes sense why they are so hard. To prevent this issue. Hoosier DOT's are much much softer.
I did have success running a taller slick that was pretty much the same width as a 315 hoosier I normally ran. But now I'm running a 335 hoosier, which is the same height as the 315 hoosier, but it's still the same height and there is zero room to run a taller tire like a 245.
Crap, I wish I could run a taller tire. I'm literally holding speed for a second or two to avoid shifting up in 2 turns. I know it's costing me time.
Will more tire pressure help with this.
I did have success running a taller slick that was pretty much the same width as a 315 hoosier I normally ran. But now I'm running a 335 hoosier, which is the same height as the 315 hoosier, but it's still the same height and there is zero room to run a taller tire like a 245.
Crap, I wish I could run a taller tire. I'm literally holding speed for a second or two to avoid shifting up in 2 turns. I know it's costing me time.
Will more tire pressure help with this.
well crap. just did a super simple calculation.
164/171 ~= 0.92. this means your new high aero radius needs to be 0.92 * your original radius. but that means squashing the tire ~1" vertically for it to account for this big change. i just don't see that happening.
maybe there's some very slight clutch slip because things are hotter and harder everywhere with the increased aero??
edit: scratch that, misentered some numbers in the calculator. 164/171 =~0.96 -> squishing the tire maybe 1/2 inch. downforce could still be a real effect on this. i'll run a couple more numbers.
edit edit: i need to do some actual work; will see if i can put together a plot of estimated radius at 165 mph given different downforce scenarios this evening. keep in mind this is super estimated; to do this right probably requires much better data than i'm grabbing from the internets + some computer modeling.
164/171 ~= 0.92. this means your new high aero radius needs to be 0.92 * your original radius. but that means squashing the tire ~1" vertically for it to account for this big change. i just don't see that happening.
maybe there's some very slight clutch slip because things are hotter and harder everywhere with the increased aero??
edit: scratch that, misentered some numbers in the calculator. 164/171 =~0.96 -> squishing the tire maybe 1/2 inch. downforce could still be a real effect on this. i'll run a couple more numbers.
edit edit: i need to do some actual work; will see if i can put together a plot of estimated radius at 165 mph given different downforce scenarios this evening. keep in mind this is super estimated; to do this right probably requires much better data than i'm grabbing from the internets + some computer modeling.
Last edited by nitrojunky; Dec 11, 2014 at 12:24 PM.
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assuming my theory is correct, i did a quick back of the envelope calculation for part (1), at least regarding what order of magnitude of forces are involved and what it does to the contact patch size.
996TT should have a rear tire contact patch of about 35.5 in^2 with 30psig in its tires. (weight on each should be around 1064.5)
i expect the weight of the contact patch portion of the tire to be around 1.79 lbs for this size of contact patch. (contact patch proportion of an assumed 80 pct of MPSS 295/35 18 28 lb weight in tread area)
at 170 mph, it would require 3180 lbs of weight to offset the centrifugal force and maintain the same contact patch size. assuming no downforce, i expect the contact patch will go from 35 in^2 to around 11.9 in^2. so it'd go from something like a 3" long contact patch to maybe a 1" long contact patch. from this one can approximate the change in radius, but i'm curious how much the shape of the patch also changes.
anyway, what i get for a the resting condition is a length of contact patch which implies that the effective radius goes from 13.5" to 13.400". for the 170mph scenario, if the contact patch reduces to about 1/3, i get a radius of about 13.49". so i expect the effective radius grows a little more than 1/2 percent. due to stretching, the tire itself may balloon more in the non-contact patch area, but i don't think it'll lift the car more than maybe 1/10 inch.
however, the area is a function of the weight, and therefore my (1) and (2) items are not independent; including the downforce is going to definitely be an ugly nonlinear function even for a back of the envelope calc and even ignoring probably a lot of effects... that said, how much downforce do you think you produce?
996TT should have a rear tire contact patch of about 35.5 in^2 with 30psig in its tires. (weight on each should be around 1064.5)
i expect the weight of the contact patch portion of the tire to be around 1.79 lbs for this size of contact patch. (contact patch proportion of an assumed 80 pct of MPSS 295/35 18 28 lb weight in tread area)
at 170 mph, it would require 3180 lbs of weight to offset the centrifugal force and maintain the same contact patch size. assuming no downforce, i expect the contact patch will go from 35 in^2 to around 11.9 in^2. so it'd go from something like a 3" long contact patch to maybe a 1" long contact patch. from this one can approximate the change in radius, but i'm curious how much the shape of the patch also changes.
anyway, what i get for a the resting condition is a length of contact patch which implies that the effective radius goes from 13.5" to 13.400". for the 170mph scenario, if the contact patch reduces to about 1/3, i get a radius of about 13.49". so i expect the effective radius grows a little more than 1/2 percent. due to stretching, the tire itself may balloon more in the non-contact patch area, but i don't think it'll lift the car more than maybe 1/10 inch.
however, the area is a function of the weight, and therefore my (1) and (2) items are not independent; including the downforce is going to definitely be an ugly nonlinear function even for a back of the envelope calc and even ignoring probably a lot of effects... that said, how much downforce do you think you produce?
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I'll take a stab at this. 
My guess is sometimes rev limiters want to see greater than a certain rpm for a given amount of time. As an example the ignition won't cut out for 0.5 seconds after you hit the specified maximum rpm.
In lower gears and at lower speeds, your rate of acceleration doesn't very that much at different aero setting, therefore you hit a similar speed when in the same amount of time after the specified maximum rpm is achieved.
By the time you're in 5th gear, different aero setting have a significant effect on the rate of acceleration due to the increase drag. With a lower drag setup, you can accelerate to a higher speed in the amount of time between reaching the maximum specified rpm and the hitting the ignition cut out. In a higher drag setup, your rate of acceleration is significantly less, therefore you gain less speed between hitting the maximum rpm and the ignition cut out.
It could happen.
Later, Steve
My guess is sometimes rev limiters want to see greater than a certain rpm for a given amount of time. As an example the ignition won't cut out for 0.5 seconds after you hit the specified maximum rpm.
In lower gears and at lower speeds, your rate of acceleration doesn't very that much at different aero setting, therefore you hit a similar speed when in the same amount of time after the specified maximum rpm is achieved.
By the time you're in 5th gear, different aero setting have a significant effect on the rate of acceleration due to the increase drag. With a lower drag setup, you can accelerate to a higher speed in the amount of time between reaching the maximum specified rpm and the hitting the ignition cut out. In a higher drag setup, your rate of acceleration is significantly less, therefore you gain less speed between hitting the maximum rpm and the ignition cut out.
It could happen.
Later, Steve
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Heavychevy, I have run hoosiers and to 250kmh and not experienced this but I will say I don't have the same aero. I would look to increase rev limit regardless to cover the extra legs you need. I have mine at 7400 but shift at 7200. The extra rpm is for the main straight where I need the legs or I end up in that no mans land of do I shift up or do I bounce off rev limiter. So I increased it.
I have just purchased Joe T canards and splitter and thinking of big wing options so I find this post interesting with regard to effect of aero.
I think to seriously answer it I would want load sensors on each corner to see exactly the effect of what the aero is doing at what speed otherwise we are just guessing values. My first thought was tyre distortion like others...but it must be a lot.
I have just purchased Joe T canards and splitter and thinking of big wing options so I find this post interesting with regard to effect of aero.
I think to seriously answer it I would want load sensors on each corner to see exactly the effect of what the aero is doing at what speed otherwise we are just guessing values. My first thought was tyre distortion like others...but it must be a lot.
