Make a Aspire rear axle lighter?

[Doug Brown]

In my quest to get my BP drag racer lighter, I've been looking over the Aspire rear beam that I have (for the big rear brakes - I am not using any front brakes) to see what I could lighten up.

So here are my basic thoughts - any huge safety objections to my ideas? Remember, RED is ONLY used for 1/8 & 1/4-mile drag racing and is NOT a street or road race car.

1. Remove the "torsion bar" from inside the rear beam

2. Drill a series of lighting holes in the rear beam

I'm nearly done making the "solid" rear shocks which will be much lighter since no springs are used with them.

What are the lightest rims that will fit onto the Aspire rear brakes?

Thanks for comments and suggestions!

[FestYboy]

There shouldn't be any issue with your proposed mods, given the circumstances.

The lightest wheels I can think of would be either 2 donuts, or a pair from a first gen Prius (the one that looks like an Echo) or Insight (again, 1st gen)

[Aaronbrook37]

Civic VX Rims are a solid choice as well if you're open to 13x5. They're aluminum and under 10lbs each.

[bhazard]

Wouldn't it be better to have brakes only in front rather than only rear? Not only are they a lot more effective, but then if you happen to lock up and you only have rears you'll prolly end up facing the other direction.

I was also think the whole keep as much weight on the front axle as possible but that's unsprung.

[blake4591]

In my quest to get my BP drag racer lighter, I've been looking over the Aspire rear beam that I have (for the big rear brakes - I am not using any front brakes) to see what I could lighten up.

1. Remove the "torsion bar" from inside the rear beam

Easiest way to take out the bar is to drill in from the outside with a 3/4" drill bit. Below is a picture of how I did this with my beam.




You might have to weigh parts to confirm this but festiva front brakes might be lighter than the rear aspire drums. A quick check says the aspire drum is 11.5lbs (probably only 3lbs or 4lbs once turned down to just the hub part) and a festiva rotor is 7.5 lbs . The festiva caliper is pretty light and the aspire backing plate is heavier than you would think. Either way if it is within a couple pounds I think the weigh right on the drive axle would work better.

[mikemounlio]

My enkei rpf1 rims are 8.4 pounds. Super light weight rims. Then get some 165 50 14 econo tires.

[wwwdotgov]

The Import fwd racers usually strap skinny 3.5" wheels on the back like these for less rolling resistance.

http://www.ipgparts.com/store/Lenso-XPD-15x3.5-Drag-Racing-Wheels-Honda-Acura-Civic-Integra-Mini-Black-Gold-Silver-Gun-Metal.html

Most donuts are limited to 50mph. You need real skinny tires to pass tech and be safe.

[Doug Brown]

Thanks for all the replies.

My thinking on the brakes is:

If I run the front disc brakes, I have the mass of the rotor to overcome to get the engine to rev quickly. So if I remove the rotor, the engine can rev much more quickly. Theoretical more functional horsepower.

The same principle applies for an aluminum flywheel vs. a steel flywheel. Engine revs more quickly and produces more HP.

As for rear brakes only, you just have to be careful with them. In the mid 60's to the early 70's, all of the Plymouth & Dodge factory A/FX cars plus others ran without front brakes to lighten the cars. All of the rail type dragsters & funny cars have always only run rear brakes (and parachutes), so it is a proven concept.

As for using the rotor and caliper weight as "ballast" for extra traction, if I follow Advancedynamix correctly, if all this weight was put IN FRONT of the axle - like with the reinforced radiator frame or a battery in the bumper - you would gain more traction, and horsepower because you are not turning all of this mass.

Yes? No? I'm a loony (heard that before...)

Everybody who has answered my various threads have really been helping me. THANKS!!!

[Team Lightning]

Will there be enough weight on the rear to allow enough grip to keep from sliding the tires under decel?

[bhazard]

That's what I'm saying. When you hit the brakes the weight is transferring forward, get too light in the back and it's gonna come around.

[Doug Brown]

OK, since people have expressed concerns about me destroying RED because of no front brakes, I do have additional plans.

1. The rear of a Festiva is pretty heavy, weight transfer & stripping the car or not. I think I'll be OK weight wise to give the rear wheels traction for braking. Especially if I use a "normal" size tire width instead of bicycle tires. Keep it light, but give it some tread width to spread out the braking/grip forces.

2. At the end of the 1/4-mile, foot comes off accelerator and there is "engine & aerodynamic braking". I can also use the gearbox and downshift. I plan on lightly applying the brakes, not putting the pedal to the floor.

bhazard - since you are really fast, how much does your car slow down by aerodynamics, engine compression and downshifting at the end of the 1/4-mile without using your brakes? Ever tried it? Did it get you slowed down enough to be able to safely turn off the track?

3. If all the above fails, since I am not modifying the front uprights or hubs, I can - if need be - just bolt the rotors and calipers back on (and the brake lines, etc).

4. I could (with time and expen$e) put Wilwood snowmobile or similar lightweight rotors and calipers up front. Then I could get rid of all the brakes in the rear. This is sort of my most aggressive plan B.

So I am listening to everyone's concerns...just have to get RED finished and take him to some long straight road where there is nothing to hit and give it a try!

Ideas and suggestions always welcome !!!

[bravekozak]

Buy a manual propotioning valve to separate the front and rear brake lines and adjust it so that the front wheels lock a split second before the rear wheels. Do it with actual wheels and tires under panic stop. Have someone watching.

Magnesium wheels are the lightest.

[Grey Vw]

I respect your effort and dedication, but this is so backwards in so many ways.

Since you insist so much go see what the Honda guys do in the rear. It's practically bicycle wheels in the back held by a couple tubes.

Also pull your e-brake at 100mph and see how well that slows you down. I can lock the rear tires rolling in the driveway and still slide a bit.

[bhazard]

Putting fatter tires in the rear in order to get more braking traction, at least to me, would probably have the opposite effect. As Charlie has found out through actual testing and experience, its not the size of the contact patch, but the contact pressure. A fatter tire will just spread the pressure out more.

Also my car would usually blow a bunch of oil out the exhaust if I engine braked hard after a pass, so I usually just went to neutral and let the brakes do the job.

[Advancedynamix]

Another problem with this plan is 80 percent of a FWD cars effective braking is done by the front brakes. That's because that's where the weight is transferred while braking. If your rear wheels lock up (which they will), the car will be extremely unstable at speed.