Forum: HoseHeads Sprint Car General Forum (go)
Moderators: dirtonly  /  dmantx  /  hosehead

I'm going to apologize up front because I know I'm not going to explain it well.  Watching the Knoxville documentary, there was a lot of slow motion video and one thing I've always wondered stood out.  The amount the rear tires grow at full speed.  Basically what I'm talking about is if the average person looked at the profile of the sidewall of the rear tire of a sprint car while it is stopped, it looks like it is very low on air (which compared to passenger car tires they are).  But at full speed, the sidewall is flat from the edge of the wheel to the tread.  Again, I know I'm not explaining it well, so if you're not following what I'm saying watch this video https://www.foxsports.com/motor/video/1255639107510 from the 40 second mark forward.  I know they are top fuel dragsters but I would think the tires are doing the same thing and growing in circumferance.

Obviously when teams are checking stagger the tires are still.  So my question is, does anybody at all care how much the tires grow when they are at full speed?  Or is it one of those things where teams roll into a track and say "15 inches of stagger with this gear always works"?  Some things I wonder about are, if the LR and RR would expand at the same rate then the stagger argument would be null.  But my thought would be because of the stagger the RR is traveling faster than the LR which would make me think the RR would grow more than the LR.  Also if the tires are growing, I would think this would change the gear ratios.

So lets say a track is really fast one night and the next night it is rubbered down and everyone is on the bottom one night, for arguments sake, lets say they are going 3 seconds slower on the second night.  Obviously due to the difference in track surface they would be choosing different compounds.  But would they also be choosing different tire sizes (changing amount of stagger) due to tires growing at different rates.  Am I overthinking this or is this something teams actually think about?

Being a mechanical engineering graduate who chose that path because of racing, I love this question!

I can't speak to what the teams think about in terms of the growth of the tires because I'm not on a team.  I'm sure there are several crew chiefs who have thought about this though.  I think your observations make sense.  The LR is definitely going to grow less than the RR because the sidewalls are different heights (they have to be to get the RR to be so much bigger than the LR in a static state and still use the same diameter wheel).  The difference in width between LR and RR might also play into that, as well as air pressure, tire compound, tire temperature, and wheel speed.  The trouble is that you can't measure the stagger at speed, and no sprint car team is going to be able to figure out a reliable way to do that which doesn't cost too much money to even think about (some kind of digital technology).  So you have to stick with what you can measure in the pits.  What can be done, however, is for Hoosier to use a tire dyno at the factory and measure the tire growth with a laser setup in a controlled environment.  I have no idea if they do that, but as an engineering exercise I think it would be very beneficial to them.  The data they gather could then be distributed to the teams to help them make decisions on tire choices.

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bascially what we do with our car, is that we start with what we think will work. Then the driver tells us what the cars feeling like and we go from there. We're not a big team my and means and only have 3 different RR's and 3 different LR's , so we dont have a lot to choose from lol 

When guys say they struggle figuring tires out this is a significant part of the equasion! The sidewalls vary A LOT on the tires.the RR and LR dont grow at the same rate either. The different LR compounds are HUGE changes to the car set up also. A 360 LR sidewall is a floppy donkey dick compared to the outlaw brickwall sidewall. It's not that the guys dont understand whats happening, it is that they arent able to set the car up to get the same feel and drive on for the different tire. 

You would shit your pants if you saw the difference between a RD12 and an H12 at the end of the straightaway on a big half mile at 150MPH!

When you say "rubbered down" you probably mean dryslick. When rubber goes down the track will be faster and have more grip than than a wet tacky track. If the track is black and shiny it will be dryslick with limited traction. If it is black but dulling up that means rubber is probably going down and speeds will go up. I see and hear people referring to a dryslick track as rubbered up or rubbered down all the time i figured I'd clear that up if they weren't sure

That's interesting, I've never heard that.  I'm assuming the RD12 and H12 are both Hoosier tires?  What's the reason for the different sidewalls?

I'm sorry, I'm well aware of the difference between a rubbered down track and a dry slick track.  It was way past my bedtime when I was typing this lol.

Interesting question but from my experience... checking the memory tachometer for "max RPM" and analyzing where the "track" is going to go regarding conditions... is what most teams use to decide rear end gear. 

The main thing to say about Hoosier tires is they suck.  Rollouts are all over the place.  That number on the side is just an approximate.  RRs can vary as much as 4 inches.  LRs as much as 2.  They sometime shrink, sometimes grow during a race.  They sometimes seal over on yellows and sometimes don't.  The sidewalls can be different stiffness which affects how much they roll over in the corners and how much they grow down the straightaways.  Basically the quality control at Hoosier is non-existent anymore.  

I often wonder just how much tire treating goes on in sprint cars.  It's a huge deal in late models.  Why wouldn't sprint cars do it too?  All series and tracks have rules against it.  But how many times have we seen a sprint car DQ'd due to tire treatment?  I can't remember it.  It happens to the best LM drivers at the biggest races.  There's no way I'm going to believe that sprint car guys don't do it too.  Kind of like illegal motors in ASCS.    

This has been about 3 years ago, went to a go-cart shop to buy some tire detergent

soap.  Guy said do you also want some  tire softener.  I said no, that you can really get busted at Knoxville if caught useing it,  Guy said he sells some to some Knoxville teams.  Knoxville did bust one team for tire softner.  It does cost track alot to get a tire tested.  I have heard about $300.00.  You do not just test tire, you have to test dirt,  Dirt has oil, tire,brake fluid, anti  freeze and every thing else thaty gets on tires.