Front tire size vs. ET in a Stocker....

[Wade_Owens]

Does anyone have any evidence of et difference after changing to a larger or smaller diameter front tire.

I have a couple different sized sets and access to 2 other sizes. Just curious if anyone knows of any difference....

Wade O

[Alan Roehrich]

In theory, a taller front tire gives you longer rollout, and less rolling resistance. Once we got the stocker to react, it did seem like we picked up just a little from the taller front tire. Nothing earth shattering or record setting mind you, but there's probably a little there.

[tj310]

On the flip side the taller tires are heavier and lifting the nose an inch may hurt aerodynamics. ---Trevor

[The Hawk]

With my V6 Stocker,I`ve never noticed much of a change in ET but the reaction time difference can be a struggle. If you go shorter,lower your starting line rpm.

[Rollins_2241]

Talking about the first 60' only, not downtrack aerodynamics or rolling resistance, 2" from say, 24" to 26", is worth about .012. This is not exact due to tire "squat" and depends on the air pressure you run in both sets (hopefully apples to apples of course).

But the math is pretty simple and has been done here before. Draw a circle with radius R, and a "beam" with height H. Then draw two right triangles using R and the chord made by a horizonal line through H, and use the Pythagorean Theorem. H is specified by NHRA but actually varies from track to track and depends also on where you stage (where the groove is) relative to the crown of the racetrack. The differential in rollout can be calculated when R changes. Then more difference equations can be used to turn it into ET. I spared you all that stuff.

[Bob Don]

A taller tire will allow more roll out before clearing the beam, resulting in a slightly lower ET(I went from a 28" to a 26" and lost about .05 but my reaction time was better because I still hit the tree at the same spot). A shorter tire will allow you to fine tune your reaction time. If you can cut a good light with a tall tire than that is the best of both worlds.

[tgriffith]

wade,,Ive done alot of testing on tire sizes.....what I have found through the years is it just really depends on your car.

Automaticly, the tire size of a larger rollout tire should result in in quicker ETs BUT and I say "BUT" how it actually works on the track is a different story.

the first thing to consider is how much rolling weight affects your car.....Big Hp cars dont see as much of a difference as a slower car.....in theory, Ive been told that for every 1 lb of rotating weight = 5 lbs of weight on the car and since its directly on the front,,,that makes a bigger difference on a lower HP car.

another big factor is transfer of weight......if you have a 25 tire vs a 28 tire on a big hp car, your taking nose weight off the car and in turn helping it hook better but how it works out on the track again is different because even though you have killed rotating weight,,,you have dropped the front end a couple of inches so what does that do???

here is an example on a car that I had,,,,I was having issues going red, so I stuck on 28`s in place of the 25s I had on the car,,,,,it got worse??? and also was slower??? didnt make much sense.....I also picked up .04 in 60ft but by 330ft, only .02 and by the 660 mark was even with a reduction of 1mph,,,,in the 1/4 was down .04 and almost 2 mph...

after testing numerous runs and switching back and forth,,,was getting consistent results and here is what I believe was happening,,,,,with the bigger tire,,,more rollout AND car was raised was planting the tires better....thats why the 60 was better than what it should have been....BUT the extra weight of the tires and the loss of aerodynamics cause an incremental et loss down track. ANd because I was getting the nose out off the ground and planting the tires better, the car reacted quicker and thats why the red light got worse.....this car was a 85 camaro with a 305...at the time it was E/F fuel injected at I think, about 3500lbs.

On my vega,,,,(bracket car) she will run about 5.90`s and the difference between 24`s and 26 is almost non existing,,,may .01 quicker in 60 and carries that down the line..

the best thing I can tell you is if you dont have to spend a ton and have access to different sets,,,the only way to know for sure is just test them. BUT if your looking for ET gain,,dont automaticly see a difference and settle with it. With the camaro, I did finally settle on 26`s and with some twiking with front end height, tire pressures got the car quicker than it was and tuned in the RT`s......also different manfactors have different weights and tire sizes.....for example,,,I have a set of new goodyears 27x5 that when on a rim, look like mudgripps......the same set of hoosiers are much smaller as width and weight

hope this helps

[Wade_Owens]

Some very good answers and just what I was looking for...

Thanks guys..

Wade

[Stewart Way]

Rollins_2241
Would like to see the calculations for future reference. Being a numbers type of guy I'm always curious. I don't see a way to calculate this since you have at least 3 unknown variables in each instance. The velocity when the 60' timer starts and the velocity when the 60' timer is tripped are 2 of them, and difference in rate of accel due to the changes is a 3rd. True, the geometry can give difference in the 60' distance which is not really exactly 60' and varies with the tire height but that alone does not translate to a time difference.
Also, no one has mentioned the possibility of the car lifting out of the beam and not rolling out. If that happens then the delta of the 60' distance is not able to be calculated and the other variables still exist.

With so many variables I think the only way to know is to test it as tgriffith stated.

Normal disclaimer: I could be wrong and my opinion can be swayed.