Dyno Results with Bigger Tires?
04-03-2006, 08:28 AM
#1
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Dyno Results with Bigger Tires?
Anyone?
Would like before & after HP/TQ numbers with changing to bigger tires as the ONLY change! Preferably in the 500-600 hp range... but any % difference will help.
I dynoed this weekend and picked up 21 hp with TST PM3 Comp... over my previous TST PM3 regular. Only other change (since last dyno) was tires... 285's to 315's.
Did tires hurt the numbers as many think/claim? And how much?
RJ
Would like before & after HP/TQ numbers with changing to bigger tires as the ONLY change! Preferably in the 500-600 hp range... but any % difference will help.
I dynoed this weekend and picked up 21 hp with TST PM3 Comp... over my previous TST PM3 regular. Only other change (since last dyno) was tires... 285's to 315's.
Did tires hurt the numbers as many think/claim? And how much?
RJ
04-03-2006, 11:02 AM
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Was it on the same dyno? At your power level 21hp is about a 3.5% increase. 3.5% could be a lot of things, better air, different correction factor, variance between two dynos, altitude change.
04-03-2006, 11:20 AM
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RJ...first off your dyno date is wrong bud 
i would like to know whats up too with this, i am putting 315's on this week (i hope) and im dynoing april 29, i want to know true HP...so if I have to take some stock size tires to the dyno day with me i will, that way i can get a better reading.
i would like to know whats up too with this, i am putting 315's on this week (i hope) and im dynoing april 29, i want to know true HP...so if I have to take some stock size tires to the dyno day with me i will, that way i can get a better reading.
04-03-2006, 11:21 AM
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You can calculate the effect of the larger tires if you want to.
It's all moment of intertia/
Assuming this is a Dynojet, then size alone has NO effect on dyno readings-- on paper. The change in gearing is linear-- what you lose in tq at the wheels is offset by the rate of tq application from the higher gear. The net result is sort of like testing in 6th instead of 5th gear on a 5600-- but less than that.
If the moment of intertia stays the same (even if size does NOT), then the DynoJet will give you the same numbers.
But larger tires weigh more. In order to have the same moment of intertia, larger tires would have to weigh LESS. This compounds the problem.
Using the formula of I=mr^2, you have:
I= inertia
m=Mass
r= radius
Mass is mass of wheel (which hasn't changed any) and mass of tire (which HAS changed). So let's assume that the OEM wheels weigh ~40#.
Using Toyo's numbers, their 315s weigh 67lb each. The Michelin 285s you had on before (LTX m/s, right?) aren't listed for weight, so we'll approximate that using a Toyo D-rated 285, which is 53.1#.
So, in our example, the 285 wheel/tire weights 93.1#
The 315 combo weighs 107#
The radius in the formula is the radius of the center of gravity. Obviously, this is not the radius of the tire.
What is the radius of the center of gravity?? We'll have to guess using logic. For example, we can assume that the center of gravity of the WHEEL is *not* the halfway point of the wheel because the RIM has so much of the wheel's mass. So, with 16" wheels (diameter), we know that that the CG lies OUTSIDE where it would like of your wheels where just a flat disc. On a flat disc, the CG is the point where half the mass is inside that measurement, and half is outside.
The "logical" CG of a 16" disc (8" radius) is the halfway point, or a 4" radius. in reality, it the halfway measurement times the square root of two (1.414214). That means the "real" CG of a 8" radius disc is 5.66"
Add to that the fact that the wheel isn't a disc and has more mass at the rim, and it's easy to see how the actual CG of the wheel could be at a radius of 6.5" or so, even on an 8" radius wheel. Let's call it 6.5" for a guesstimate.
Now we have to turn to the tires. We know the CG of the wheel is constant, so all we need to find is the change in CG for the 315 compared to a 285, add that to the CG for the wheel, and compute.
The CG for the 285 tire can be approximated also. Since the sidewall is 75% of the 285 section width, we know the section height. We also know that the CG will be pretty much the middle radius times sqrt of 2. Then we have to convert to inches. That gives us 285*0.75/2*1.414214= 151.14mm or 5.95"
This is 5.95" relative to the 16" rim (8" radius), so the actual CG of the tire is 13.95"
How about the 315 tire? Repeating the calculations for the 315 tire gives us a CG of 167.05mm or ~6.58". Adding in the 8" radius of the wheel gives us 14.58"
Since the same wheel was used with both the 315 and 285 tires, we can ignore the CG of the wheel. We aren't trying to find the CG necessarily-- we want the CHANGE in CG.
But the change in CG is not equal to the change in just the CG of the tires. The CG of the wheel/tire ASSEMBLY will be the average of the CG radii of the wheel and of the tire.
So, for the 285, we have (13.95+6.5)/2= 10.225"
For the 315, we have (14.58+6.5)/2= 10.54"
These are the radii we have to use for our I=mr^2 calculation.
Now we bring back the weights of the tires/wheels.
Remember that earlier we found the the 285 wheel/tire weighs 93.1#
and the 315 combo weighs 107#.
So:
285: 93.1#*(10.225)^2= 9733.66
315: 107# *(10.54)^2 = 11886.8
All that to say that the 315 tire represents about a 22.12% greater rotational inertia than the 285 tire does!!
This, of course, ignores the effects of tire pressure and treadpattern. This is NOT rolling resistance-- it's rotational inertia, or resistance to change in wheelspeed.
Also you CANNOT correct your dyno numbers by this amount, because the rotational inertia of the tires is only one of MANY factors. If you knew the exact contribution of the tire/wheel rotational inertia to the whole, then you could correct for it using this number.
The only way you could find this out is if you had a dyno that would bolt right to the wheel hubs and allow you to take off the wheels to get a baseline
Justin
It's all moment of intertia/
Assuming this is a Dynojet, then size alone has NO effect on dyno readings-- on paper. The change in gearing is linear-- what you lose in tq at the wheels is offset by the rate of tq application from the higher gear. The net result is sort of like testing in 6th instead of 5th gear on a 5600-- but less than that.
If the moment of intertia stays the same (even if size does NOT), then the DynoJet will give you the same numbers.
But larger tires weigh more. In order to have the same moment of intertia, larger tires would have to weigh LESS. This compounds the problem.
Using the formula of I=mr^2, you have:
I= inertia
m=Mass
r= radius
Mass is mass of wheel (which hasn't changed any) and mass of tire (which HAS changed). So let's assume that the OEM wheels weigh ~40#.
Using Toyo's numbers, their 315s weigh 67lb each. The Michelin 285s you had on before (LTX m/s, right?) aren't listed for weight, so we'll approximate that using a Toyo D-rated 285, which is 53.1#.
So, in our example, the 285 wheel/tire weights 93.1#
The 315 combo weighs 107#
The radius in the formula is the radius of the center of gravity. Obviously, this is not the radius of the tire.
What is the radius of the center of gravity?? We'll have to guess using logic. For example, we can assume that the center of gravity of the WHEEL is *not* the halfway point of the wheel because the RIM has so much of the wheel's mass. So, with 16" wheels (diameter), we know that that the CG lies OUTSIDE where it would like of your wheels where just a flat disc. On a flat disc, the CG is the point where half the mass is inside that measurement, and half is outside.
The "logical" CG of a 16" disc (8" radius) is the halfway point, or a 4" radius. in reality, it the halfway measurement times the square root of two (1.414214). That means the "real" CG of a 8" radius disc is 5.66"
Add to that the fact that the wheel isn't a disc and has more mass at the rim, and it's easy to see how the actual CG of the wheel could be at a radius of 6.5" or so, even on an 8" radius wheel. Let's call it 6.5" for a guesstimate.
Now we have to turn to the tires. We know the CG of the wheel is constant, so all we need to find is the change in CG for the 315 compared to a 285, add that to the CG for the wheel, and compute.
The CG for the 285 tire can be approximated also. Since the sidewall is 75% of the 285 section width, we know the section height. We also know that the CG will be pretty much the middle radius times sqrt of 2. Then we have to convert to inches. That gives us 285*0.75/2*1.414214= 151.14mm or 5.95"
This is 5.95" relative to the 16" rim (8" radius), so the actual CG of the tire is 13.95"
How about the 315 tire? Repeating the calculations for the 315 tire gives us a CG of 167.05mm or ~6.58". Adding in the 8" radius of the wheel gives us 14.58"
Since the same wheel was used with both the 315 and 285 tires, we can ignore the CG of the wheel. We aren't trying to find the CG necessarily-- we want the CHANGE in CG.
But the change in CG is not equal to the change in just the CG of the tires. The CG of the wheel/tire ASSEMBLY will be the average of the CG radii of the wheel and of the tire.
So, for the 285, we have (13.95+6.5)/2= 10.225"
For the 315, we have (14.58+6.5)/2= 10.54"
These are the radii we have to use for our I=mr^2 calculation.
Now we bring back the weights of the tires/wheels.
Remember that earlier we found the the 285 wheel/tire weighs 93.1#
and the 315 combo weighs 107#.
So:
285: 93.1#*(10.225)^2= 9733.66
315: 107# *(10.54)^2 = 11886.8
All that to say that the 315 tire represents about a 22.12% greater rotational inertia than the 285 tire does!!
This, of course, ignores the effects of tire pressure and treadpattern. This is NOT rolling resistance-- it's rotational inertia, or resistance to change in wheelspeed.
Also you CANNOT correct your dyno numbers by this amount, because the rotational inertia of the tires is only one of MANY factors. If you knew the exact contribution of the tire/wheel rotational inertia to the whole, then you could correct for it using this number.
The only way you could find this out is if you had a dyno that would bolt right to the wheel hubs and allow you to take off the wheels to get a baseline
Justin
04-03-2006, 11:45 AM
#5
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Thread Starter
Originally Posted by fiveology
Was it on the same dyno? At your power level 21hp is about a 3.5% increase. 3.5% could be a lot of things, better air, different correction factor, variance between two dynos, altitude change.
Thanks GB. Fixed it.
RJ
04-03-2006, 11:54 AM
#6
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Thread Starter
Originally Posted by HOHN
You can calculate the effect of the larger tires if you want to.......
It's all moment of intertia/
Using the formula of I=mr^2, you have:
I= inertia
m=Mass
r= radius..........
..... it the halfway measurement times the square root of two (1.414214). .... Then we have to convert to inches. That gives us 285*0.75/2*1.414214= 151.14mm or 5.95"........
It's all moment of intertia/
Using the formula of I=mr^2, you have:
I= inertia
m=Mass
r= radius..........
..... it the halfway measurement times the square root of two (1.414214). .... Then we have to convert to inches. That gives us 285*0.75/2*1.414214= 151.14mm or 5.95"........
Your explanation is why I was hoping for the simple answer... "I dynoed before & after going to 315's and lost X HP"!
Actually, with decent HP (400+)....My mind "thinks" larger tires MAY not cost max HP (once you get them spinning).
I feel sure the 315's account for a more gradual climb (slope) to the initial curve on the dyno chart... taking longer to get to max numbers.... which is why bigger is not better at the drag strip. I'm not convinced the line on the dyno graph won't go as high?
What I'm really trying to calculate is "how much extra HP/TQ is in the TST Comp vs the TST..... as well as understanding the effects of larger tires.?
RJ
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04-03-2006, 11:59 AM
#9
Registered User
Originally Posted by HOHN
You can calculate the effect of the larger tires if you want to.
It's all moment of intertia/
Assuming this is a Dynojet, then size alone has NO effect on dyno readings-- on paper. The change in gearing is linear-- what you lose in tq at the wheels is offset by the rate of tq application from the higher gear. The net result is sort of like testing in 6th instead of 5th gear on a 5600-- but less than that.
If the moment of intertia stays the same (even if size does NOT), then the DynoJet will give you the same numbers.
But larger tires weigh more. In order to have the same moment of intertia, larger tires would have to weigh LESS. This compounds the problem.
Using the formula of I=mr^2, you have:
I= inertia
m=Mass
r= radius
Mass is mass of wheel (which hasn't changed any) and mass of tire (which HAS changed). So let's assume that the OEM wheels weigh ~40#.
Using Toyo's numbers, their 315s weigh 67lb each. The Michelin 285s you had on before (LTX m/s, right?) aren't listed for weight, so we'll approximate that using a Toyo D-rated 285, which is 53.1#.
So, in our example, the 285 wheel/tire weights 93.1#
The 315 combo weighs 107#
The radius in the formula is the radius of the center of gravity. Obviously, this is not the radius of the tire.
What is the radius of the center of gravity?? We'll have to guess using logic. For example, we can assume that the center of gravity of the WHEEL is *not* the halfway point of the wheel because the RIM has so much of the wheel's mass. So, with 16" wheels (diameter), we know that that the CG lies OUTSIDE where it would like of your wheels where just a flat disc. On a flat disc, the CG is the point where half the mass is inside that measurement, and half is outside.
The "logical" CG of a 16" disc (8" radius) is the halfway point, or a 4" radius. in reality, it the halfway measurement times the square root of two (1.414214). That means the "real" CG of a 8" radius disc is 5.66"
Add to that the fact that the wheel isn't a disc and has more mass at the rim, and it's easy to see how the actual CG of the wheel could be at a radius of 6.5" or so, even on an 8" radius wheel. Let's call it 6.5" for a guesstimate.
Now we have to turn to the tires. We know the CG of the wheel is constant, so all we need to find is the change in CG for the 315 compared to a 285, add that to the CG for the wheel, and compute.
The CG for the 285 tire can be approximated also. Since the sidewall is 75% of the 285 section width, we know the section height. We also know that the CG will be pretty much the middle radius times sqrt of 2. Then we have to convert to inches. That gives us 285*0.75/2*1.414214= 151.14mm or 5.95"
This is 5.95" relative to the 16" rim (8" radius), so the actual CG of the tire is 13.95"
How about the 315 tire? Repeating the calculations for the 315 tire gives us a CG of 167.05mm or ~6.58". Adding in the 8" radius of the wheel gives us 14.58"
Since the same wheel was used with both the 315 and 285 tires, we can ignore the CG of the wheel. We aren't trying to find the CG necessarily-- we want the CHANGE in CG.
But the change in CG is not equal to the change in just the CG of the tires. The CG of the wheel/tire ASSEMBLY will be the average of the CG radii of the wheel and of the tire.
So, for the 285, we have (13.95+6.5)/2= 10.225"
For the 315, we have (14.58+6.5)/2= 10.54"
These are the radii we have to use for our I=mr^2 calculation.
Now we bring back the weights of the tires/wheels.
Remember that earlier we found the the 285 wheel/tire weighs 93.1#
and the 315 combo weighs 107#.
So:
285: 93.1#*(10.225)^2= 9733.66
315: 107# *(10.54)^2 = 11886.8
All that to say that the 315 tire represents about a 22.12% greater rotational inertia than the 285 tire does!!
This, of course, ignores the effects of tire pressure and treadpattern. This is NOT rolling resistance-- it's rotational inertia, or resistance to change in wheelspeed.
Also you CANNOT correct your dyno numbers by this amount, because the rotational inertia of the tires is only one of MANY factors. If you knew the exact contribution of the tire/wheel rotational inertia to the whole, then you could correct for it using this number.
The only way you could find this out is if you had a dyno that would bolt right to the wheel hubs and allow you to take off the wheels to get a baseline
Justin
It's all moment of intertia/
Assuming this is a Dynojet, then size alone has NO effect on dyno readings-- on paper. The change in gearing is linear-- what you lose in tq at the wheels is offset by the rate of tq application from the higher gear. The net result is sort of like testing in 6th instead of 5th gear on a 5600-- but less than that.
If the moment of intertia stays the same (even if size does NOT), then the DynoJet will give you the same numbers.
But larger tires weigh more. In order to have the same moment of intertia, larger tires would have to weigh LESS. This compounds the problem.
Using the formula of I=mr^2, you have:
I= inertia
m=Mass
r= radius
Mass is mass of wheel (which hasn't changed any) and mass of tire (which HAS changed). So let's assume that the OEM wheels weigh ~40#.
Using Toyo's numbers, their 315s weigh 67lb each. The Michelin 285s you had on before (LTX m/s, right?) aren't listed for weight, so we'll approximate that using a Toyo D-rated 285, which is 53.1#.
So, in our example, the 285 wheel/tire weights 93.1#
The 315 combo weighs 107#
The radius in the formula is the radius of the center of gravity. Obviously, this is not the radius of the tire.
What is the radius of the center of gravity?? We'll have to guess using logic. For example, we can assume that the center of gravity of the WHEEL is *not* the halfway point of the wheel because the RIM has so much of the wheel's mass. So, with 16" wheels (diameter), we know that that the CG lies OUTSIDE where it would like of your wheels where just a flat disc. On a flat disc, the CG is the point where half the mass is inside that measurement, and half is outside.
The "logical" CG of a 16" disc (8" radius) is the halfway point, or a 4" radius. in reality, it the halfway measurement times the square root of two (1.414214). That means the "real" CG of a 8" radius disc is 5.66"
Add to that the fact that the wheel isn't a disc and has more mass at the rim, and it's easy to see how the actual CG of the wheel could be at a radius of 6.5" or so, even on an 8" radius wheel. Let's call it 6.5" for a guesstimate.
Now we have to turn to the tires. We know the CG of the wheel is constant, so all we need to find is the change in CG for the 315 compared to a 285, add that to the CG for the wheel, and compute.
The CG for the 285 tire can be approximated also. Since the sidewall is 75% of the 285 section width, we know the section height. We also know that the CG will be pretty much the middle radius times sqrt of 2. Then we have to convert to inches. That gives us 285*0.75/2*1.414214= 151.14mm or 5.95"
This is 5.95" relative to the 16" rim (8" radius), so the actual CG of the tire is 13.95"
How about the 315 tire? Repeating the calculations for the 315 tire gives us a CG of 167.05mm or ~6.58". Adding in the 8" radius of the wheel gives us 14.58"
Since the same wheel was used with both the 315 and 285 tires, we can ignore the CG of the wheel. We aren't trying to find the CG necessarily-- we want the CHANGE in CG.
But the change in CG is not equal to the change in just the CG of the tires. The CG of the wheel/tire ASSEMBLY will be the average of the CG radii of the wheel and of the tire.
So, for the 285, we have (13.95+6.5)/2= 10.225"
For the 315, we have (14.58+6.5)/2= 10.54"
These are the radii we have to use for our I=mr^2 calculation.
Now we bring back the weights of the tires/wheels.
Remember that earlier we found the the 285 wheel/tire weighs 93.1#
and the 315 combo weighs 107#.
So:
285: 93.1#*(10.225)^2= 9733.66
315: 107# *(10.54)^2 = 11886.8
All that to say that the 315 tire represents about a 22.12% greater rotational inertia than the 285 tire does!!
This, of course, ignores the effects of tire pressure and treadpattern. This is NOT rolling resistance-- it's rotational inertia, or resistance to change in wheelspeed.
Also you CANNOT correct your dyno numbers by this amount, because the rotational inertia of the tires is only one of MANY factors. If you knew the exact contribution of the tire/wheel rotational inertia to the whole, then you could correct for it using this number.
The only way you could find this out is if you had a dyno that would bolt right to the wheel hubs and allow you to take off the wheels to get a baseline
Justin
WOW--- I have a headache now!!
04-03-2006, 02:21 PM
#11
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Thread Starter
Yup! Your right...as you usually are.
I assumed others had gone before and would get many answers. Thought I would save the $40.00.
Now kind of wish I had taken you up on your offer!
RJ
I assumed others had gone before and would get many answers. Thought I would save the $40.00.
Now kind of wish I had taken you up on your offer!
RJ
04-03-2006, 02:33 PM
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I gained 9hp on a dyno jet going down from 305/70E rated Nittos to 285/75 D rated Nittos. For what its worth a couple years ago when I was running a tst I went from 559 to 578hp upgrading to the competition version. Same dyno,same day about 10 mins apart. Tim
04-03-2006, 03:01 PM
#13
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Thread Starter
Originally Posted by Timinva
I gained 9hp on a dyno jet going down from 305/70E rated Nittos to 285/75 D rated Nittos.
The 9 hp is very interesting. Both 305/70's and 285/75's are 33" tires, per BFG Site. Don't understand any difference??
Will E tires spin the drum better than D's?
RJ