Tire growth

[masracingtd1167]

Any way to determine how much a tire grows at speed ? Thinking about the 2585 Good Year in particular . When I went from a 4.56 to a 4.86 my car picked up . It crosses at around 9400 right now so I am thinking what's a couple of hundred more rpm gonna hurt ! So I am thinking of putting a 5.0 gear in it . My car is around 100 more lbs than a B/ND would be .

[digster]

 Bill I don't think the percentage of tire growth number is  linear. Rim size and tire pressure will be big factors. Perhaps one of the experts will chime in with a formula. I'm interested too. I just switched to the 2585 and 12 in rims.

[JrFuel Hayden]

Yes, slicks grow at speed, and yes it depends on wheel width, tire pressure, tire style, size, and tire brand. Looking at finish line photos it looks like around 2" with the Goodyear 31x12x15 on 12" rims.
One of our JrFuel racers, didn't like running the GY D2585 on 12" rims, because they didn't grow enough, so he ran 11" rims unless he needed more bite and then ran 12". He was using a shorter stroke 387 ci SBC, and his motor was out 50", different combo than ours.
My experience has been mostly with 31x12x15 GY on 12" rims because we seem to need the 12" traction. When I'm deciding on gears and tires, I use one of the online formulas. Because I use a RacePak I can see what RPM we get thru the lights, I have found the computer and the online formula is pretty correct, because of converter slipage it makes up for tire growth. I found I just need to add 1/2" to the 31" tire to get the numbers to add-up to the RacePak.
Just figure the tire size with no or very little growth to get your gear change RPM. If you don't have a data computer, try using a GoPro aimed at your Tach, or get a play-back tach. 

Jon, 800-624-3803

[coupemerc]

Bill, A couple of years ago I got this from Todd Swaim...

You can expect the following increase in diameter in inches for the 31.0x12.0-15 tire.
100 MPH = 2”
150 MPH = 3.3”
200 MPH = 4.6”

Those numbers seem high when compared to my Racepak data. My data agrees with what Jon posted. The tire could very well grow like Todd says but because of converter slip, add about 1/2-3/4" to the 31" diameter and the speed/rpm equations work well.

[masracingtd1167]

Thanks for the information Jon and Chuck . If I go by a half to three quater it comes out really close to where I am right now . 

[wideopen231]

I checked the wallace deal and found that rducing rpm by amount its looing thru converter slippage made close other wise its way off. Just info for anyone messing with it.

[LZ]

Hey what an opportune thread.

Cant thank you all enough. I was just taking a WAG that tire growth cancelled convertor slip.
I am just a "we are going to have fun car" nothing like what you guys are doing but hope that when we can get driver and car sorted. We can someday go NE this thing.

In my particular situation car has a 12 bolt in it with 4:33's on a 31" goodyear. I really really want to keep same tire. Changing to the 4:11 it still kind of puts my RPM a little higher then with talking over with my friend Matt (Frontenginedragsters - Pro formance racing engines ) what we are aiming for.  There in lies my issue. They do not make a pro gear any higher then 4:11 for a 12 bolt.  I would need to put a Nine inch in it to get the little higher gear, thus lowering my engine CL to adjust for different hypoid angle. More money out of short supply .  Have the aluminum blank sitting here for engine plate and obviously can't start cutting it till this is determined, amongst related issues. And busy at work.
 Have the cam and heads to support the RPM. But am pond scum so need to stay on the easy side of set up. 4.15 arm on a 542 - 460 ford.

Oh well guess these are good problems
Thanks everyone again for sharing.
Luke

[wideopen231]

Hey I hope everyone in for the fun.T his stuff cost way too much not to be having fun along with the late nights,working all weekend and struggling to find another thousand bucks for new part. Wait I did say fun right?

[Paul New]

Hey what an opportune thread.

Cant thank you all enough. I was just taking a WAG that tire growth cancelled convertor slip.
I am just a "we are going to have fun car" nothing like what you guys are doing but hope that when we can get driver and car sorted. We can someday go NE this thing.

In my particular situation car has a 12 bolt in it with 4:33's on a 31" goodyear. I really really want to keep same tire. Changing to the 4:11 it still kind of puts my RPM a little higher then with talking over with my friend Matt (Frontenginedragsters - Pro formance racing engines ) what we are aiming for.  There in lies my issue. They do not make a pro gear any higher then 4:11 for a 12 bolt.  I would need to put a Nine inch in it to get the little higher gear, thus lowering my engine CL to adjust for different hypoid angle. More money out of short supply .  Have the aluminum blank sitting here for engine plate and obviously can't start cutting it till this is determined, amongst related issues. And busy at work.
 Have the cam and heads to support the RPM. But am pond scum so need to stay on the easy side of set up. 4.15 arm on a 542 - 460 ford.

Oh well guess these are good problems
Thanks everyone again for sharing.
Luke

If I remember correct Spud Miller ran into this quite a few years ago with his 8 3/4 Mopar he could not get a Pro Gear that would work and he sent a set of street gears some where to get them Annealed or Softened up a bit. Worth a shot I wouldn't want to change the way the car was built.

[dreracecar]

every gear set ratio has about a 200rpm change , so going from 4:33 to a 4:11 would be about a 400 rpm drop for the same ET given having the power to pull higher gear.

  Toms Differential in Idaho  is the guy to go to having the gears annealed to correct number for the application , Hes a real prick some time(most of the time) but has been doing it for 35+ years, knows his stuff and will not put up with others telling him how it needs to be done because your friend told you

[LZ]

Hey thanks Paul and Bruce:
Seems like I remember Bruce or someone talking about this some time ago about heat treating a set. Anyone have personal experience with this?

Bruce is this the place?
http://www.tomsdifferentials.com/

Thanks much
Luke

[LZ]

Bill sorry to be hijacking your thread.

The trap RPM is been high on my hit list at this stage of things.

thank you
Luke

[dreracecar]

Yep thats the "DUDE" (its how he refers to people)
Been running a HT gear since '95 and havent pulled the center out of the housing