Lesson of Less weight in the rear of the car

[Mike Gaines]

On my recent diet program I took 183 lbs out of the car. About 120 lbs was taken out the car from the rear axle BACK. Replaced the 70lb steel deck lid with a 10 lb fiberglass deck lid (from Sled City) for a 60 lb reduction, Took out 1 of my 2 Optima Batteries for a 45 lb reduction and removed a steel "tub" that my old 10 gallon poly fuel cell was dropped into. That "tub weighed about 30 lbs. I replace the 10 gallon poly fuel cell with a 5 gallon aluminum fuel cell.
So...I have always ran 17lbs of air in my Hoosier Stiff Sidewall slicks (with tubs) and used a 3600rpm chip in my transbrake so I had left all that the same for my first pass.
When I launched the car it just blew the tires off....the tires broke traction violently, the tach went instantly tp 7000 and hit the rev limiter 7000rpm chip, I let off the gas and got back in it and it broker traction again...so I just drove in thru.
It had a 1.862 60 foot, rather than a 1.400 60' and went 10.927 rather than 10.15.

I called Hoosier and they said the solution was rather simple. They said with taking 120lbs out of the car BEHIND the rear axle was no different than adding about 5 lbs of air into the tires.
So...he said the car thought it had around 21lbs of air in the tires (relativitly thinking) so of course it would break traction.
He said to lower the air pressure in the tires to about 13lbs, or so, and everything would be ok.
Can I have racer's opinions of this please....seems logical to me.
Thanks

 

[kiwigtx]

I'm no expert, but I thought that you would have been running 12-13 pounds in the rear anyway. I ran 12 pound in my GTX - stock config, but all the weight behind the rear axle needs some pressure. The guys in hotter E-Body cars can get away with as low as 8 pounds.
Since your car is a decent sized B, I would be running 12-13 pound as a starter.

 

[hangn0ut]

Makes sense but I wonder about the math. How did he come up with 5lbs? Taking out to much air makes for some evil handling. The pattern on the tires will get you close. Did they show too much air? A car that's heavy up front percentage wise can be a bear to hook up.

 

[malex]

On my recent diet program I took 183 lbs out of the car. About 120 lbs was taken out the car from the rear axle BACK. Replaced the 70lb steel deck lid with a 10 lb fiberglass deck lid (from Sled City) for a 60 lb reduction, Took out 1 of my 2 Optima Batteries for a 45 lb reduction and removed a steel "tub" that my old 10 gallon poly fuel cell was dropped into. That "tub weighed about 30 lbs. I replace the 10 gallon poly fuel cell with a 5 gallon aluminum fuel cell.
So...I have always ran 17lbs of air in my Hoosier Stiff Sidewall slicks (with tubs) and used a 3600rpm chip in my transbrake so I had left all that the same for my first pass.
When I launched the car it just blew the tires off....the tires broke traction violently, the tach went instantly tp 7000 and hit the rev limiter 7000rpm chip, I let off the gas and got back in it and it broker traction again...so I just drove in thru.
It had a 1.862 60 foot, rather than a 1.400 60' and went 10.927 rather than 10.15.

I called Hoosier and they said the solution was rather simple. They said with taking 120lbs out of the car BEHIND the rear axle was no different than adding about 5 lbs of air into the tires.
So...he said the car thought it had around 21lbs of air in the tires (relativitly thinking) so of course it would break traction.
He said to lower the air pressure in the tires to about 13lbs, or so, and everything would be ok.
Can I have racer's opinions of this please....seems logical to me.
Thanks

---
Sorry to hear that the weekend didn't go so well Mike. Less air pressure is a start but it won't end there.. To be honest I would have been surprized if you didn't have these problems because you took out a load of weight and upset the apple cart. Do you have any videos of your car leaving the line? You've likely got a chit load of separation going on back there.

 

[Tex013]

Yes that wojld be the right direction to drop psi as you have taken rear ,good,weight out . But as Malex says you will probably need a better rear shock . Definately a double adjustable . You are past Ranchos , iir thats what you have .

Tex

 

[mopar 3 B]

Am surprised soft sidewalls were not memtioned. Would like to see how the tire reacts to 12 lbs..

 

[dvw]

I know people who runs less but mine are always 13.5 psi. And mine is a taller tire to boot, 31". Try it, should help.
Doug

 

[hangn0ut]

You know this calls for a little at home test and tune. Put some rubber down in front of your house or even in your driveway if you can. Get out and check the contact patch. Adjust accordingly. Much more convenient than making passes at the track. Just be sure the neighbors are cool with it. LOL

 

[hemiman426]

put the car on scales

 

[bearman]

you need a video of whats happening now and compare to what it was. everyone has a good ideas. my thought would be to change shocks and make them stiff lower tire pressure and put the battery back in. do a couple launches if it is better then pull battery back out repeat launches this will take some time to get it set up to work for you. diets are to only put the pds where needed. to much diet can take you the wrong way. can you adjust the suspension and lower the back of the car so on launch it pushes down harder and longer for the launch. of course this is just guess work from me as i am learning from all you guys on how to do it. good luck please keep us updated.

 

[kidmopar]

are the shocks on back adjustable as well as the front?if so since u took 120 lbs out back its probably too stiff as well as needing to lower tire pressure, make the rear softer and lower tires to 15 lbs first if it helps, loosen the front
shocks to help transfer the weight to the rear, whatever hurts go back the other way , one change at a time or u loose the rabbit

 

[Dodge 330]

When we do true street tire events I ADD 120 lbs. in the rearmost part of the trunk near the bumper. It is amazing what weight in the right place does for traction.

 

[dvw]

With the right suspension set up you can be pretty nose heavy. Mine has 320lbs more on the nose. Went 1.255 and 1.258 last night. Leaving at 2800 rpm on the foot brake.
Doug

 

[65 3 times]

With the right suspension set up you can be pretty nose heavy. Mine has 320lbs more on the nose. Went 1.255 and 1.258 last night. Leaving at 2800 rpm on the foot brake.
Doug

That's amazing Doug. Great Car & Driver. Dave

 

[oldbee]

You do have tubes in the slicks? Less weight needed in the driver's seat I think,lol.

 

[Mike Gaines]

You do have tubes in the slicks? Less weight needed in the driver's seat I think,lol.

Yup, I have tubes in my stiff sidewall Hoosiers.

 

[bearman]

mike, stiff or soft does it matter on launches i am guessing yes but why. i'm thinking soft to stick better and hard for tire spin control. what do tubes have to do with it.

 

[malex]

Yup, I have tubes in my stiff sidewall Hoosiers.

Mike.. how's it going sorting out your car. Getting anywhere?

 

[Mike Gaines]

My next race is Oct 19 20 21. The California NHRA Hot Rod Reunion at Famoso.
I had always run 17lbs of air in my Hoosiers and the car HOOKED like crazy, pulling the front wheels about 12'' for at least 20 feet with a 1.40 60 foot.
My first pass with the weight reduction and still 17lbs of AIR, it just did not hook at all....just tire spin.
After talking with the tire techs at Hoosier they said just to lower my rear tire pressure from 17lbs to 13 lbs after taking 120lbs of weight out of the car from the rear axle back.
They were not surprised that I couldn't get traction leaving 17lbs of air in the rear tires after taking 120lbs out of the car from the rear axle back.
They said put in 13lbs and it should hook.
We will see.....

 

[bearman]

good luck sir, we will be waiting for results. hit that number you are wanting