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Footbrake and front tire rollout
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DRR Sportsman
Picture of botmbulb
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quote:
Originally posted by JPosey:
I wish there was a rule of thumb on R/T vs rollout.


Typically, 1" of tire diameter is worth .007 in reaction time.
 
Posts: 490 | Location: Hammonton, N.J. | Registered: March 06, 2001Reply With QuoteReport This Post
DRR Trophy
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Thanks for the info botmbulb! With this knowledge, tire swapping is going to be a wasted endeavor in my application. I did a Google search this morning found on Class Racer one racer said 2 inch shorter was worth .015 in reaction time. I guess in need to practice a couple extra bumps from shallow and try that.
 
Posts: 102 | Location: Work | Registered: April 12, 2010Reply With QuoteReport This Post
DRR Trophy
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So when the car is actually leaving the line would'nt the rollout be different then when its just being pushed out by hand? Because of the effect the weight transfer has on it ?
 
Posts: 65 | Location: Larwill,In | Registered: September 18, 2011Reply With QuoteReport This Post
DRR S/Pro
Picture of Lenny5160
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quote:
Originally posted by Mark Cmar:
So when the car is actually leaving the line would'nt the rollout be different then when its just being pushed out by hand? Because of the effect the weight transfer has on it ?


Surely, but lower tire pressure will give you a more shallow stage position to begin with. A flatter tire will be wider 2" off the ground than a fully inflated tire.


Tony Leonard
 
Posts: 3249 | Location: Inver Grove Heights, MN | Registered: March 18, 2004Reply With QuoteReport This Post
DRR Trophy
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i get that but wouldn't as the tire comes up under launch actually be smaller diameter than if it were if it had more air in it ?
 
Posts: 65 | Location: Larwill,In | Registered: September 18, 2011Reply With QuoteReport This Post
DRR S/Pro
Picture of Lenny5160
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Probably, but the difference is tiny compared to the difference at 2" with the weight on the tire.

Inflate one front tire on your car to 30 psi and one to 20. I'm guessing you can visually see a difference in the tires, specifically the shape at the bottom.

Now take them off the car and lean them up against the shop wall. Can you visually see the difference now? Guessing not.


Tony Leonard
 
Posts: 3249 | Location: Inver Grove Heights, MN | Registered: March 18, 2004Reply With QuoteReport This Post
DRR Trophy
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not seeing a difference against the shop wall is my point
 
Posts: 65 | Location: Larwill,In | Registered: September 18, 2011Reply With QuoteReport This Post



DRR Trophy
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meaning more than half of the 2 inch difference is gone
 
Posts: 65 | Location: Larwill,In | Registered: September 18, 2011Reply With QuoteReport This Post
DRR S/Pro
Picture of Lenny5160
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quote:
Originally posted by Mark Cmar:
meaning more than half of the 2 inch difference is gone


The difference was there when you staged the car, is my point.


Tony Leonard
 
Posts: 3249 | Location: Inver Grove Heights, MN | Registered: March 18, 2004Reply With QuoteReport This Post
DRR Trophy
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im missing something here
 
Posts: 65 | Location: Larwill,In | Registered: September 18, 2011Reply With QuoteReport This Post
DRR Trophy
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what im saying is , if you get 2 inches more static rollout while staged and 1 inch of it is gone when the back of the tire exists the stage beam . the amount of slowed reaction , assuming around 12 inches of total rollout that you'd get from the first inch is pretty small . im guessing maybe 7 thousands
 
Posts: 65 | Location: Larwill,In | Registered: September 18, 2011Reply With QuoteReport This Post
DRR S/Pro
Picture of Lenny5160
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quote:
Originally posted by Mark Cmar:
what im saying is , if you get 2 inches more static rollout while staged and 1 inch of it is gone when the back of the tire exists the stage beam . the amount of slowed reaction , assuming around 12 inches of total rollout that you'd get from the first inch is pretty small . im guessing maybe 7 thousands


Sounds like you get it just fine. When I say 2" I am referring to the starting line beam height; I don't think you're typically getting an extra 2" in tire width from pressure adjustments.

I prefer to adjust R/T with RPM rather than pressure because there isn't a lot of room to work there, IMO.


Tony Leonard
 
Posts: 3249 | Location: Inver Grove Heights, MN | Registered: March 18, 2004Reply With QuoteReport This Post
DRR Trophy
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Ike and i went through all this stuff in the 70s and 80s Hence the first accurate and reliable Delay Box was born
 
Posts: 65 | Location: Larwill,In | Registered: September 18, 2011Reply With QuoteReport This Post
DRR Trophy
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It was always a private joke between us the He needed more rollout and i needed less. And the top bulb with his quartz timed Box took care of us both LOL
 
Posts: 65 | Location: Larwill,In | Registered: September 18, 2011Reply With QuoteReport This Post



DRR Trophy
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I miss my friend
 
Posts: 65 | Location: Larwill,In | Registered: September 18, 2011Reply With QuoteReport This Post
DRR Sportsman
Picture of FootbrakeJim
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quote:
Originally posted by Mark Cmar:
i get that but wouldn't as the tire comes up under launch actually be smaller diameter than if it were if it had more air in it ?

Maybe because at a lower PSI, the centerline of the spindle is lower (closer to the ground), so to lift high enough to clear the beams requires more vertical travel of the spindle, (which would also slow the R/T at lower PSI). Confused


Dan "Jim" Moore
Much too young to feel this damn old!!
 
Posts: 1102 | Location: Farmersville, TX  | Registered: December 05, 2002Reply With QuoteReport This Post
DRR S/Pro
Picture of Eman
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This is a front heavy 8 sec car with street radial tires. The frt. radial tire will squat so much if you lower air pressure it will widen and lengthen the footprint and increase the roll out. Inflating the tire to max pressure will help with RT but hard to say how much with a 16" radial.
I would try and borrow a set of wheels and tires from someone and see what difference it makes. Surely someone has a set of mounted front runners you could make a couple of passes with
 
Posts: 1572 | Location: E TN | Registered: February 13, 2009Reply With QuoteReport This Post
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