Slipping the tire to avoid tire shake

I have a question concerning slipping the tire during launch. I currently run a 125-inch FC chassis currently using an altered body. It is a converter car and my ET is in the 7.15 range.

My problem is consistency during launch. I have my wheelie bar set at 5.5-inches and added 50-lbs to the front of the car. I adjusted my rear tire pressure to 5.0-PSI and the car will carry the front wheels, hit the bar and go strait. However, some runs the tire will shake when the car plants the wheelie bar, sometimes sending the car in one direction or the other.

I was told to slip the tire on launch by lowering the pressure to 4.5-PSI. My understanding is that the center of the tire would pull away from the track allowing the tire to slip. I tried this method and it worked for 3-runs then the problem came back.

I tried to lower the wheelie bar to 3-inches (4.5-PSI tire pressure) to slip the tire more but this resulted in tremendous tire shake.

With the bar raised back up to 5.5-inches I made a drastic change (tend to do these things when running out of track time) and raised the air pressure to 7.0-PSI. I thought that this would make the tire grip harder in the center but less on the sides allowing the tire to slip. I felt the increase in air pressure would prevent the tire from shaking. The result was the car smoked the tire on launch.

Here are my questions-

- Using 5.0-PSI as a base line should I increase or decrease tire pressure? How much?
- Track temp is 125F on the runs I recorded successfully. However, I have not done a good job at recording this variable because I did not understand how it affected the tire pressure. Do you change air pressure for different track temps and why?
- How does wheelie bar height come into the equation?
- I run a nostalgia class and the use of electronics or data recorders is prohibited. What variables can I track that will help in tuning decisions on tire pressure.
- I currently use the 33X17X15 Goodyear tires. A 34.5 diameter tire using a 16-inch wheel will fit but with body modifications. Is it possible to make this combination work without going to the 34.5-16 inch tires?

Thanks in advance to anyone who can help.

 

Not sure if I can help, but I think you were going in the right direction on tire pressure, maybe just went too far. Would go up 1/4 pound at a time from 5. Less pressure would increase footprint so is doesn't seem like slip would increase until pressure was low enough for the tire to wad up on launch. Not sure how much power you're running, know from the other thread you're running an Indy wedge setup, but I would think you've got enough tire for a light altered. Any chance you're running with the NPCA group?

 

We have found that for tracks with good traction we increase the tire pressure to try and get it to slip a little. Have found that the car will run a quicker 60 foot with a little slip. As stated 5 is a good starting point and increase with small amounts. Wheelie bar will come into the equation too. If it is too low it will unload the tires. But I guess it will also depend on engine position and power you are making. I would remove the weight from the front of the car, lower the wheelie bar a fraction and than up the tire pressure a little each run. These are my thoughts but I am still learning. If anyone has a differing opinion I would love to hear it as I am always keen to learn new things. Thanks.

 

We added .25 psi to solve our problem. Going down would make it worse I would think. Adding air will help the tire slip a little more thus you should be able to get the tire speed up faster.

 

slipping tire to avoid shake

Thanks for the information in all replies. Yes, the car is an Indy wedge set up. I run with the ProNostalgia association. The class is for front engine dragsters, altereds, and 1970-earlier door cars and funnycars. The format is similar to top comp; however, there are two 8-car fields. To get in the fastest "A" field I need to run my car all out or I will not qualify. All replies listed increasing tire pressure (I will do this). One reply had mentioned lowering the wheelie bar. Could you tell me what is the height (to all who offered information) any of you run your bar at? What is the bar distance (from rear end)? Do you run twin or single wheel wheelie bars?

Thanks again for your help.

 

Have noticed a lot of funny cars with the wheelie bar set real low, about 1'' of the deck, these are single bar set ups, have a fair bit of flex and would be 7 to 8 feet in length.

 

Tire Shake

Well here I go...I'm sure this will create a few responses....We had a 3.89 rear gear in the Nitro Madness car last year with a 1.81 low first gear = 7.04 overall first gear ratio. The car was fairly consistent, but would get loose occasionally...would spin or shake sometimes. Then we switched cars. The new chassis came with a 4.29 Strange top loader, so I decided to leave the rear end ratio the same. Well, if you do the math, it now was 7.76 overall first gear. The new car either shook violently or spun the tires no matter what we did. If the track was good, it shook...if the track was bad, it spun. Tire pressure, wheelie bar, weight bar, etc. did little to correct the problem. Since the Lencodrive had a 1.38 first section and a 1.31 second section, Frank Parks (Neil & Parks) suggested we leave in 2nd gear. I re-plumbed the air lines to engage the 1.31 unit when the air was turned on. We now had a 1.38 first gear and 4.29 rear for an overall first gear ratio of 5.92 and only shift once. Well - it worked great! The call leaves consistently with a .996-1.000 - 60' time...4 teens in the 8th and 6.40's in the 1/4....every run is very predictable. I know that you did not ask a gear ratio question, but consider taking a little ratio away at the hit and the tire problem should correct itself..I cant describe how well the launch is now - smooth and straight! Hits the wheelie bar nice a soft - picks the front end up and sets it down smoothly....just a blast to run this new car! I do not have a perfect ratio for first gear figured out...maybe 6.00 to 6.50 somewhere might be good...but 8.00 is definately too low...this will be another Q & A session on this board over the off season...

 

Do not want to steal this thread, but I wonder how this would apply to my hooptie? bruno/ lenco, 2 speed, 1.68 first gear, 4.10 rear gear, 33.5 tire, 3200# door car. Leave at idle approx 1900 and usually like you say very soft and may slightly shake but drive thru it. A couple weeks ago on same set up, car shook so violently nearly stuffed it in the wall and broke 3 main caps on motor and a flex plate bolt. I had raised tire pressure to 7 psi when normally 6.75. Do you think to much tire speed from raising air pressure 1/4 psi? Could my first gear be to tall? thanks for any input.
PS cars 60 ft always been poor in the 1.10 to 1.12 range with a best of 2.97 330 ft and 4.48 660 at 3200#.

 

As far as ratio goes...your overall first gear is a 6.89. I don't have an answer to the original question - tire pressure, wheelie bar, etc. I don't think that 1/4 pound more in the tires should shake the car to pieces...if you had a 1.40-1.50 lenco ratio just laying around, with the 4.10 rear - then the first gear would be 5.75 - 6.15 range. I think this would calm it down a little...but let's get some other ideas from the experts before changing ratios....

 

I won't comment on all the ratio stuff. The wheelie bar can help get wheel speed when it touches the track. For every action there is an opposite and equal reaction. Lowering the tire pressure within a range lessens the traction. Lowering the tire pressure allows the tire to grow larger decreasing the footprint. So it really depends on how far out it shakes. If you are trying to unhook tire early the bar can finely tune if the tire pressure is right. Now depending on tire brand, rollout, bar up, bar down, more clutch, less clutch........careful records and someone with a video camera can help you out. Usually if the tire sticks, it will manhandle the car. If it gets better when everyone is complaining about less traction that should point you in the right direction. If there is too much tire you maybe out of tuning range.

 

Mr Gaulter,

I have avoided putting on the web how we are running our car this year. NOW the cat is out of the bag!!!!

With the World Fuel Altered Nationals coming up in Aug at Eddyville, please refrain from helping the competition beat us!

Rapid {Disclaimer: This post is for harrassment purposes only,,in other words it was a joke from the driver to the car owner}

 

Meow.......

 

i had the same problem
with my blown fiat i didnt know witch dirrection it was going to go i keep messing with the tire pressure and then went t a clutch got rid of the converter good luck

 

Insanity,
We had the same shake issues as you. Our car is a bit different as it's a 3,000 pound doorslammer going a just a little bit faster. We run a convertor too. We lowered our 1st gear ratio as not to shock the tires as hard. We raised tire pressure from 5.5 to 7.0 - 7.5 and switched to Hoosires. Since making these changes the car has yet to shake so violently right at the hit runs have had to been aborted. It still shakes (quivers) a bit as all fast cars do but now it's out about 100 feet or so and able to be driven through. I should also mention we ended up lowering the wheelie bar because after we got rid of the shake right at the hit, it would plant so hard it would hit the bar really hard and unload the tires. If the problem continues to persist I would give Niel Chance a call in reagrds to your convertor.

 

Slipping tire to avoid tire shake

Thanks again to all who have posted information on this thread.

 

Why has no one mentioned real wheel speed numbers (ds rpm)?
I run a clutch so i am not sure if those numbers correlate to a convertor or not.
1600 @ .250 in
2300-2600 @.500 in
is my sweet spot

 

Underboost, I don't have a computer on the car, or a driveshaft sensor to acquire that info.

Will,

I think your numbers are pretty close. I have started creeping up in pressure this year. Last year nothing mattered, this year it has worked fine from 4.75 all the way up to 6-lbs {highest so far] on the good tracks.

Late in the year last year, 7.5 lbs made our car drive like it was on an ice skating rink, when we were on a real good track. Which seems to support your ideas on pressure.

Rapid

 

How high of pressure can you go before the traction starts to decrease, because only the center of the tire is making contact with the ground?

 

Joe,

I was waiting for Will to post back in here. Our 23T car last year seemed like 7.5 was too much. I haven't gotten there yet with our new Coupe, so I can't say for sure. It probably depends on how hard you hit the tires, etc!

Rapid