27" to 28" front tire swap

40Coupe · 10-08-2007, 02:14 PM

Does anyone have any first hand experience with front tire swaps? I'm curious what if any measurable effect going from a 27" to a 28" front tire has on ET?

G Schenck · 10-08-2007, 04:07 PM

It's not as much as you think. We have a car with 17 in. and went to 21 in. with no noticable effect on ET or reaction time. Everything we know tells you we should but we didn't see it. Best 60 w/ 17 is .987 , best w/21 is 1.000 should be the opposite, but wasn't. In all fairness times were not the same day or the same track, but was at Nat event tracks.

I guess there is a math expert out there that could formalize an answer for the additional rollout. I would spectulate that the slower the car, the more if would lower the ET.

Greg

Chris1529 · 10-08-2007, 04:18 PM

You really need to know what the tire actually measures before you can make an apples to apples comparison. The advertised sidewall diameter isn't what the tire measures.

So, you may go from a 27" tire from one manufacturer to a 28" tire from another manufacturer, but see no noticeable change of ET because the tires may be very close in diameter.

If you are wanting a tire to improve ET, make sure you buy the tire with the largest diameter. You will also have to run more air pressure that the manufacturer suggests to get the most effect from this change unless you have a very light car. And remember to stage as "shallow" as possible and be consistent with your staging.

40Coupe · 10-08-2007, 05:02 PM

Already have. 27" tire goes 84.5" cir. and new 28" tire goes 87". That's .8 difference in dia. Does that help?

Myron Piatek · 10-08-2007, 06:22 PM

Quote:

Originally Posted by Chris1529

You really need to know what the tire actually measures before you can make an apples to apples comparison. The advertised sidewall diameter isn't what the tire measures.

So, you may go from a 27" tire from one manufacturer to a 28" tire from another manufacturer, but see no noticeable change of ET because the tires may be very close in diameter.

If you are wanting a tire to improve ET, make sure you buy the tire with the largest diameter. You will also have to run more air pressure that the manufacturer suggests to get the most effect from this change unless you have a very light car. And remember to stage as "shallow" as possible and be consistent with your staging.

Chris,

I agree with it all, but while taller tires do add rollout, less air pressure also adds to the rollout which should help ET and 60'. On the other hand, a taller tire, particularly firmer with more air pressure, will have to rotate fewer times within a 1/4 mile and have less flex going down the track. But that's getting into tiny variables.

G Schenck · 10-08-2007, 06:33 PM

Starting line beams are suppose to be 5/8 " off the ground. So you need to see how much roll out you have at 5/8 " off the ground for each tire and the difference would be that much more of a rolling start.More rolling start equals lower ET. Then you have front end rising and or wheelees that might make a difference but most cars are moving foward before moving up.

Greg

Chris1529 · 10-08-2007, 07:20 PM

ok

Joe Sprint Bros · 10-09-2007, 04:25 PM

I agree with Myron, using blocks and starting at low press, increase and measure blocks. My experience showed the 28's reached a point of diminishing returns, as the sidewalls began to expand instead of circumference. You should also notice a "sweet spot" where pressure doesn't change the blocks much, important for consistency. Running 20 psi instead of 35 reduced vehicle reaction time .02-.03 and didn't seem to have effect on mph. Other cars may react differently.. Joe.

Michael Beard · 10-09-2007, 05:27 PM

>> Starting line beams are suppose to be 5/8 " off the ground.

The spec is 1-5/8", not 5/8". Although I swear that Mason-Dixon's were that low at one time!

I went the other way, a number of years ago when bracket racing at Quaker City Raceway, which had a crazy loose rollout. Went from 26" to 23" fronts, and only picked up about .015 in reaction time, much to my surprise.

Myron doesn't believe in the law of dimishing returns.

This is a guy that changes from 48psi to 50psi and tells me it changed his light.

That being said, you have to watch out for not only the tire diameter/circumference, but also the construction of the tire. Street type radials, street bias-plys, and drag frontrunners may all react differently to tire pressure changes. It may be worth some time to do some static measuring, comparing pressure changes.

10-08-2007, 02:14 PM	#1
40Coupe Junior Member Join Date: Mar 2007 Posts: 91 Likes: 0 Liked 0 Times in 0 Posts	27" to 28" front tire swap Does anyone have any first hand experience with front tire swaps? I'm curious what if any measurable effect going from a 27" to a 28" front tire has on ET?

10-08-2007, 04:07 PM	#2
G Schenck Senior Member Join Date: Mar 2007 Location: Harahan, La. Posts: 521 Likes: 41 Liked 81 Times in 35 Posts	Re: 27" to 28" front tire swap It's not as much as you think. We have a car with 17 in. and went to 21 in. with no noticable effect on ET or reaction time. Everything we know tells you we should but we didn't see it. Best 60 w/ 17 is .987 , best w/21 is 1.000 should be the opposite, but wasn't. In all fairness times were not the same day or the same track, but was at Nat event tracks. I guess there is a math expert out there that could formalize an answer for the additional rollout. I would spectulate that the slower the car, the more if would lower the ET. Greg

10-08-2007, 04:18 PM	#3
Chris1529 VIP Member Join Date: Dec 2006 Location: West Virginia Posts: 1,366 Likes: 6 Liked 70 Times in 29 Posts	Re: 27" to 28" front tire swap You really need to know what the tire actually measures before you can make an apples to apples comparison. The advertised sidewall diameter isn't what the tire measures. So, you may go from a 27" tire from one manufacturer to a 28" tire from another manufacturer, but see no noticeable change of ET because the tires may be very close in diameter. If you are wanting a tire to improve ET, make sure you buy the tire with the largest diameter. You will also have to run more air pressure that the manufacturer suggests to get the most effect from this change unless you have a very light car. And remember to stage as "shallow" as possible and be consistent with your staging. __________________ Chris Bowman The Mountain State Mustang 1984 Mustang GT350

10-08-2007, 05:02 PM	#4
40Coupe Junior Member Join Date: Mar 2007 Posts: 91 Likes: 0 Liked 0 Times in 0 Posts	Re: 27" to 28" front tire swap Already have. 27" tire goes 84.5" cir. and new 28" tire goes 87". That's .8 difference in dia. Does that help?

10-08-2007, 06:33 PM	#6
G Schenck Senior Member Join Date: Mar 2007 Location: Harahan, La. Posts: 521 Likes: 41 Liked 81 Times in 35 Posts	Re: 27" to 28" front tire swap Starting line beams are suppose to be 5/8 " off the ground. So you need to see how much roll out you have at 5/8 " off the ground for each tire and the difference would be that much more of a rolling start.More rolling start equals lower ET. Then you have front end rising and or wheelees that might make a difference but most cars are moving foward before moving up. Greg

10-08-2007, 07:20 PM	#7
Chris1529 VIP Member Join Date: Dec 2006 Location: West Virginia Posts: 1,366 Likes: 6 Liked 70 Times in 29 Posts	Re: 27" to 28" front tire swap ok __________________ Chris Bowman The Mountain State Mustang 1984 Mustang GT350 Last edited by Chris1529; 10-08-2007 at 09:14 PM.

10-09-2007, 04:25 PM	#8
Joe Sprint Bros Junior Member Join Date: Nov 2004 Location: Virginia, USA Posts: 46 Likes: 0 Liked 0 Times in 0 Posts	Re: 27" to 28" front tire swap I agree with Myron, using blocks and starting at low press, increase and measure blocks. My experience showed the 28's reached a point of diminishing returns, as the sidewalls began to expand instead of circumference. You should also notice a "sweet spot" where pressure doesn't change the blocks much, important for consistency. Running 20 psi instead of 35 reduced vehicle reaction time .02-.03 and didn't seem to have effect on mph. Other cars may react differently.. Joe.

10-09-2007, 05:27 PM	#9
Michael Beard VIP Member Join Date: Sep 2002 Location: Columbus, OH Posts: 4,060 Likes: 0 Liked 13 Times in 9 Posts	Re: 27" to 28" front tire swap >> Starting line beams are suppose to be 5/8 " off the ground. The spec is 1-5/8", not 5/8". Although I swear that Mason-Dixon's were that low at one time! I went the other way, a number of years ago when bracket racing at Quaker City Raceway, which had a crazy loose rollout. Went from 26" to 23" fronts, and only picked up about .015 in reaction time, much to my surprise. Myron doesn't believe in the law of dimishing returns. This is a guy that changes from 48psi to 50psi and tells me it changed his light. That being said, you have to watch out for not only the tire diameter/circumference, but also the construction of the tire. Street type radials, street bias-plys, and drag frontrunners may all react differently to tire pressure changes. It may be worth some time to do some static measuring, comparing pressure changes. __________________ Michael Beard - NHRA/IHRA 3216 S/SS Last edited by Michael Beard; 10-09-2007 at 05:30 PM.