125" hard tail altered questions

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Matt@RFR
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125" hard tail altered questions

#1 Post by Matt@RFR » Sun Sep 26, 2010 5:51 pm

This is a new car to us, and neither my dad (driver) or I (crew chief) have any experience with drag race cars. I don't know if an 8 second altered was the best choice for a first car. :D

The car:
125" hard tail 23T altered
1550 pounds WITH driver
Hoosier 14x32 D5
4.11 gear
1.82 powerglide
385" SBC making approximately 550 HP.
5500 RPM stall
Best run so faris 8.83 @ 150

When we bought the car, it was run at 1/8th mile tracks only, so we put a 4.11 gear in it, but left the launch rev limiter where the previous owner had it set, which was 3000 RPM. Tire pressures were also copied from the old owner at 4.75 psi.

What we found is that the car porpoised pretty bad, pulling the front tires about 2" 3-5 times before it settled down, and it had the appearance of slapping the front tires on the track. General 60' times with those settings were about 1.28 to 1.25.

We have since raised the tire pressure to 5.5 psi with no other changes, and the car immediately went .02 - .03 faster in 60'. It also has much, much less tendancy to hop under braking.

Now we are starting to raise the launch RPM in 200 RPM increments. Each time we see a reduction in 60' times, and each time it pulls the front tires a tiny bit more, and skips them a little less. We are at 4400 RPM now and the front tires don't seem to slap the track so much as gently skip across it now. Best 60' time so far is a 1.203.

Are we on the right track here? Does our 5.5psi tire pressure seem inline? I still want to keep raising the launch RPM right up until we launch it on the converter, but I thought I would bounce this off you guys and see what you have to say. Again, we are very new to all of this, and there are very few hard tail cars in our area, and even fewer that have such low horsepower... most are fuel altereds and I can't see any of their parameters being usefull for our car.

Thanks guys,
Matt

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BillyShope
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Re: 125" hard tail altered questions

#2 Post by BillyShope » Sun Oct 03, 2010 6:37 am

Wonderful that you and your dad can work together on something like this.

Does the car have a front suspension? If it's a beam front axle, does it use a leaf or coils?
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Matt@RFR
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Re: 125" hard tail altered questions

#3 Post by Matt@RFR » Sun Oct 03, 2010 9:37 am

Hi Billy. Yeah, Dad and I are having a LOT of fun with this.

I knew I would forget something usefull about the car; It has no front suspension, using fabricated spindle mounts (3-point contact to the chassis), and no front brakes. We have the wheelie bars, which are the bottom-flat-bar style, set only 2" off the ground because I'm really afraid a big wheelie will hurt the front end as we tune the launch. So far it has done no such thing though.

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BillyShope
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Re: 125" hard tail altered questions

#4 Post by BillyShope » Sun Oct 03, 2010 10:06 am

Matt@RFR wrote:Hi Billy. Yeah, Dad and I are having a LOT of fun with this.

I knew I would forget something usefull about the car; It has no front suspension, using fabricated spindle mounts (3-point contact to the chassis), and no front brakes. We have the wheelie bars, which are the bottom-flat-bar style, set only 2" off the ground because I'm really afraid a big wheelie will hurt the front end as we tune the launch. So far it has done no such thing though.
For safety reasons, I'd suggest adding a front suspension. This would be the equivalent of a large increase in wheelbase. In other words, that long dragster wheelbase, with its low torsional stiffness, decreases the front roll stiffness, which, in turn, helps to equalize the rear tire loading. The addition of a front suspension to your short wheelbase altered will help in keeping it going straight...and keeping it safe.
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bracketracer
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Re: 125" hard tail altered questions

#5 Post by bracketracer » Sun Oct 03, 2010 7:34 pm

Don't bother with the front as it would make it a lot better to add a 4 link to the rear with a nice set of double adjustable shocks. Plus it will make the ride way more comfortable :smt003 :smt003

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Re: 125" hard tail altered questions

#6 Post by BillyShope » Mon Oct 04, 2010 7:00 am

As you know, the driveshaft torque tends to unload the right rear and load the left rear. The reaction to this driveshaft torque is absorbed by the transmission/engine mounts. The reaction torque is equal in magnitude, but opposite in sense, to the driveshaft torque. So, if that reaction torque finds its way back to the rear, it will completely cancel the driveshaft torque and the rear tires will remain equally loaded.

Unfortunately, the reaction torque has 2 paths. It can go to the front or it can go to the rear. What happens is that some goes each way. It's split in proportion to the relative roll stiffness of the front and rear suspensions. You presently have no front and rear suspensions, so it's split in proportion to the relative front and rear torsional stiffness. This is where that 300 inch dragster wheelbase comes in. It essentially decouples the front end, allowing most of the reaction torque to go to the rear, keeping the rear tire loadings essentially equal.

So, with your limited wheelbase length, you need to "soften" that front end with the addition of a front suspension.

With fully suspended cars, the ARB has become very popular. This greatly increases the rear roll stiffness, thus improving rear tire loading. If your dragster was fully suspended, you could consider tricks like that. But, to keep the front end solid and add a rear suspension would obviously be going in the wrong direction.
http://www.racetec.cc/shope

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Re: 125" hard tail altered questions

#7 Post by John_Heard » Mon Oct 04, 2010 11:50 am

BillyShope wrote: So, with your limited wheelbase length, you need to "soften" that front end with the addition of a front suspension.
Billy - might want to advise him that you've never actually driven, built or worked on a car like his prior to making recommendations like that.

Matt@RFR
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Re: 125" hard tail altered questions

#8 Post by Matt@RFR » Mon Oct 04, 2010 7:07 pm

With a 1.203 60' (and dropping), I don't think there is any reason to be changing the chassis and making the car heavier. The wheelie bars were set equal side to side on a very flat garage floor, and when they hit at the track, the marks they leave start at the same spot and end at the same spot. The car is not rolling over on the right rear at ALL. The thing just squats and goes. It even picks up the front wheels pretty evenly.

My original question was if we are on the right track for tuning THIS chassis. If we wanted a plush ride, we woulda bought a roadster. No matter, we'll keep messing with it and get it perfect soon.

bracketracer
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Re: 125" hard tail altered questions

#9 Post by bracketracer » Thu Oct 07, 2010 5:53 am

But, to keep the front end solid and add a rear suspension would obviously be going in the wrong direction.
http://www.racetec.cc/shope[/quote]


Wrong!!!There are more Altered's out there than you can count with a 4 link or ladder bar with no front suspension!!

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BillyShope
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Re: 125" hard tail altered questions

#10 Post by BillyShope » Thu Oct 07, 2010 7:21 am

bracketracer wrote:There are more Altered's out there than you can count with a 4 link or ladder bar with no front suspension!!
And the majority is always correct. Did you vote for Obama?
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Re: 125" hard tail altered questions

#11 Post by bracketracer » Thu Oct 07, 2010 1:05 pm

Billy.........No commit for John?????

This is like beating a dead horse.....

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Re: 125" hard tail altered questions

#12 Post by BillyShope » Thu Oct 07, 2010 5:38 pm

bracketracer wrote: This is like beating a dead horse.....
I agree.

I didn't feel John's comment merited a reply. If engineers had to first ride a rocket, we'd have never made it to the moon.
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Re: 125" hard tail altered questions

#13 Post by John_Heard » Thu Oct 07, 2010 8:00 pm

BillyShope wrote: I didn't feel John's comment merited a reply. If engineers had to first ride a rocket, we'd have never made it to the moon.
Didn't need a reply from you Billy, everyone else knew exactly what I was talking about.

If they hadn't built and tested prototypes to prove their theory's, nobody in their right mind would have got on that rocket. If memory serves me correctly, we built a LOT of rockets before strapping one to somebody's butt and pointing it at the moon.

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