FrontEngineDragsters.Org Forum
Drag Racing Discussions => Front Engine Dragsters => Topic started by: jspell on July 21, 2016, 04:36:24 AM
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When discussing air pressures for slicks half the time people tell me more air equals more traction and the other half says less air equals more traction?
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Too many variables to make a hard and fast rule----tire construction, rim width, car weight, and the big one--what pressure you are starting out with as a base to determine more or less.
Roo
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Thanks Roo...generally speaking wouldn't increased pressure equal more wheel speed and possibly more slippage?
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But too little and the tire spends the time at the hit wrapping around the rim rather than moving the car forward.
There is no clear answer. you must find what works for you given the track prep. Tires are so different from the days of low pressure tires and little track prep that "wheel speed" gets the car moving quicker thru the 60' because there is more traction available. In some respects, look at NHRA SS racing, the cars move off the line quicker doing a wheelstand then not doing one.
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Thanks Bruce...makes sense!
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I have been running my car for about a year--I have tried everything with tire pressure--I have MT 10.5wx33 on a 10 inch wheel. I have found out that to get my car settled down and working well most of the time that it is a balancing act
Track Prep is major factor--on a well prepped track I promise you I can make great runs back to back--a test night prep or slick track and all bets are off
Leave RPM--it is a big deal--my car would 60 ft 1.19 leaving hard--but...it was too violent--now I leave almost off idle and it likes it though I am settling for a 1.22 60 ft now--big deal
Wheelie bar setting--this is so important--figuring how to mix this with the other factors is what it is all about
Tire pressure--well..my car does not care much where it is set after I got all the other things working together--I started out 11 lbs and now am down to 6.5 honestly it is fine up to about 8.5 Yes there is some difference but I have found that the other items seem more important
The combo of leave from low RPM, wheelie bar approx. 2 3/4 to 3 3/4 range, and about 6.5-7.0 tire pressure and if that track is good I can run it like a bracket car Bad tracks threw me off for the longest time and it kept me from getting zeroed in--we avoid all but the best prepped tracks if at all possible--I simply got tired of Army runs--Left, Right, Left
Hope this helps
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Thanks Dave...all good stuff
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We run a 103" altered small block altered on MT 10.5wx33 on a 10" wheel as well. A lot of people consider the W tires to be door car tires but MT lists them as a general use tire. We generally run them at 6 pounds but have gotten away with 4 in a pinch. Been as high as 7 with no problems. Leaves off the transbrake at 4200. No problems unless the wheelie bar is to high then it's a little violent.
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If the wrinkle is under the tire at launch and not in front (5 O'clock Position), does that mean underinflated?
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Jspell, your buddies that say more pressure will get you more traction are right depending on rim width, and what pressure you are starting out with. And your other buddies that say less pressure are right also depending on what pressure you are starting with, like going from 8-9 lbs to 7lbs may get you more traction because the high pressure makes the tire too rounded. Also street tires get you more traction when lowered. But you have a FED with slicks.
I have been racing a JrFueler for 12-13 years, running 31x12x15 Goodyears. We started out using 10" rims, which is good for tire growth, then I built another set on 11" rims to run when the sun went down at Bakersfield [ Finals], when the track gets loose. Then as I found more power we needed more bite, so then I built a set with 12" rims for night time Bakersfield, all with D2585's. Yes the 12" rims ened up with less tire growth, but with more power we were going 186 to 189 mph with 4.71 or 4.88 gears. We run 5 3/4 to 6 1/4 lbs of pressure. I have a RacePak computer so I can see our driveshaft speed and curve. But one good indication of what is happening with my tires is using my infra-red temp gun, the one I use to check head temps before staging the car, and read the temps across the tires right after the run. When running lower pressures to get/ keep the tires spining abit to keep from pulling the motor down out of the RPM range it wants to run with-in, it will be hotter toward the edges, because the tires will concave abit with lower pressure. Like 2400 drive shaft speed about .400 into the run. And 6 lbs will lower the driveshaft/ tire speed.
This last week-end we raced a Div 7 race in Sonoma, with the goal of just experimenting, 1'st run was a 7.10 @186 on the D2585's, but the next 4 runs were with MT 33x10.5wx15, 1'st run at 7 1/4 lbs then kept lowering the pressure, to 7#'s then 6 3/4 on the last run, after going to a tighter converter, and lowering the launch from our normal 6500 to 5400, and added another 15 lbs on the nose. We ran a 1.01 sixty foot, and a 7.17. So I will try more weight on the nose, raise the the wheelie bar even higher like 5" and a liitle tighter converter, like 8500 rpm stall. I have been trying to not hit the wheelie bar and unload the slicks, and get a much better driveshaft curve, but we need to get the balance better [ ie enough weight on the nose].
I know some JrFuelers running less power than us running 7-7 1/2 lbs with 2585's, so they are getting more tire speed with the higher pressures, and benefit is your reaction times will go down because the tires will react quicker, but if you are having a red-lighting problem go with the lower pressures, it takes a little more time for the tire to windup.
I hope this helps.
Jon
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What Jon says is true, however I always go back to this point, What Class are you running?????? With Jrfuel as Jon runs little things make a big difference because there is no ET restriction and you have to run quicker than the other guy. Index is all about being consistent at the leave and tune for the ET.
Ive run 5 different tracks in a season and have not changed anything with reguards how the car leaves. This is what works for for me and just tune it to hit the index
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Thanks Bruce, you are right about the difference in index racing, what we do is changing all the time because of the constant searce for a better combination. Like we were surprised how much different the car worked when we removed only 7.5 pounds off the nose.
Jon
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John & Bruce thanks for the valued information. I'm alos experimenting with nose weight and was amazed at how much 12lbs in front of the spindle C/L affected the attitude of the launch.
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Because I do not run any overhang all my weight is behind the front axle, all 135# of it and I still carry the front wheels 4-6inches for 20 feet at the hit. In doing that it plants the tire harder because the car is trying to drive underneath the weight.. But again I say---its what works for me