Million Dollar Skunksworks Kirkham

[David Kirkham]

Time for the moment of truth...we weigh the car.

[David Kirkham]

Here you can see we are aligning the car. The red plates are the alignment plates. (That is why they have the degree'd angle gauge on them to set your caster with). The red alignment plates are sitting on top of some dark grey plates...our scales. You want the weight of each of your front corners to be the same (and, of course, you want the weights in the rear to be the same as well). If one corner is heavier than its corresponding side, it will have different traction than the opposite corner. So, for really accurate and cool alignments, you finely tune the springs by tightening or loosening them to get the weights equal.

[David Kirkham]

And now the moment we have all been waiting for...well, at least me.

This is a picture of the scales. The car (in its current configuration) weighs 1771 pounds! Granted, it still needs a body and some other parts, but we put a windshield in and another seat and it weighed in at 1800 pounds. The body weighs about 100 pounds or so and hopefully there is not 100 more pounds of lights and such to go on the car. Our original goal was to have the car weigh below 2000 pounds and I think we have reached that goal. Our current cars weigh 2150 pounds, so we have lost 150 pounds somewhere...now, if I could just do the same!

Also, notice the weight distribution side to side. In the front it is perfect 433 pounds each and the rear is within 1% of each other. The 5 pounds heavier on the driver's side is because we had the wires crossed. It is actually 5 pounds heavier on the passenger side because the battery is on that side. The front to rear weight distribution is a nearly perfect 51% rear and 49% front.

[David Kirkham]

Master Jeremy doing the 4 wheel alignment fine tuning.

[David Kirkham]

[David Kirkham]

Here you can see how we ran the exhaust out the back and placed the mufflers by the gas tank. There is NO other place to put the mufflers!

[David Kirkham]

Will post more pics tomorrow! Thanks for checking in!

David

[David Kirkham]

I just found another pic.

This is a shot of the underside of the front suspension. You can see the milled in brake line supports on the front control arms. You can also see how long the front lower control arms are.

Also, we placed the fuel pump right up next to the motor so it was easy to access and not hidden by belly pans and the like.

Finally, you can see how we boxed in the front suspension box to make it really rigid.

[xlr8or]

David,
With the lower arms being so much longer than the uppers, won't that cause a lot of camber gain in bump and droop?

Also, is the oil filter as close to the chassis as it appears in the photo?

[David Kirkham]

Originally Posted by xlr8or

David,
With the lower arms being so much longer than the uppers, won't that cause a lot of camber gain in bump and droop?

Also, is the oil filter as close to the chassis as it appears in the photo?

Yes, the oil filter is very close to the chassis, but you are looking at the bottom of an "I" beam so it looks closer than it actually is.

Actually, suspension geometry is an extremely complex problem. I will attempt to explain it without getting too technical. I am pretty good, but frankly, if you really want to get extremely technical, I will have to get Thomas to chime in--he is an extraordinarily gifted mechanical engineer...I just know how to make things.

You want your control arms to be as long as possible (think F1 or Indy car). The reason for that is--for a given wheel displacement (movement) the camber change is minimized. Camber change is when the top of the wheel moves in or out relative to the bottom of the wheel when looking at the wheel from the side of the car. In most cars, you want negative camber--ie, the top of the wheel moves IN as the suspension compresses UP. If you think about it, as the car leans into a turn and loads the outside tire, you want the top of the wheel to camber IN so the tire contact patch stays FLAT on the road. If there were no camber gain, then the wheel would tip up on its edge as the car leans and you would loose contact patch area and hence grip.

This is a gross simplification of the kinematics of suspension. There is a LOT more that goes into this. The angles of the control arms have a great effect on camber gain as well--especially the upper control arm as it is shorter. (This is what is known as the classic SLA or short arm long arm suspension that is always talked about in the car mags.) If the upper control arm is angled steeply up, then its relative length is "shorter" as it has already started to swing through the steep part of the circle.

As such, our upper control arm is only slightly angled UP (wheel end if HIGHER) than the frame end).

Now, caster, bump steer, Ackerman angle, static camber, roll centers, jacking, squat and dive, wheel rates, and a million other things come into play here. We literally spent hundreds an hundreds of hours messing with the suspension points to get all of this optimized. Of course, everything has its own set of trade offs as well, so what may work well on a smooth track (lots of static negative camber) may stink on the street--where we have to avoid pot holes, speed bumps, road gators, and boogey men who have wandered into the road.

I hope I haven't confused you and that I answered your question.

David

[1ntCobra]

Originally Posted by David Kirkham

...and drive we did!

I think you need to post a video of the car being driven in this go-cart state.

[RedBarchetta]

Did you corner weight/balance the car with a full tank of gas?

[1ntCobra]

Originally Posted by RedBarchetta

Did you corner weight/balance the car with a full tank of gas?

That got me thinking, the 427 S/C had a 42 gallon gas tank probably meant for long endurance races. And that is not probably the type of race that the 427 raced in much. So would you actually want to fill the tank all the way for a shorter race? And if you balanced the car with a full tank of gas, would it still handle correctly on a nearly empty tank of gas? I kind of remember reading something about such a problem with a Cobra, but don't remember where I read it.

[David Kirkham]

All those measurements were taken with the gas tank empty. Certainly gas will change the equation. But gas is (by its nature) a variable. On serious cars, we set them up with 1/2 tank of gas as a happy medium.

We don't do base weights of the car with gas as tank size and the amount of gas can change your baseline significantly. So, like many people, we "weigh" the car with all fluids and the gas tank empty.

David

[David Kirkham]

Originally Posted by 1ntCobra

That got me thinking, the 427 S/C had a 42 gallon gas tank probably meant for long endurance races. And that is not probably the type of race that the 427 raced in much. So would you actually want to fill the tank all the way for a shorter race? And if you balanced the car with a full tank of gas, would it still handle correctly on a nearly empty tank of gas? I kind of remember reading something about such a problem with a Cobra, but don't remember where I read it.

The gas tank is always a pretty nasty problem in a race car. You want the tank to be as close to the center of gravity as possible. That way, as the fuel load changes, the CG doesn't change.

Putting the gas tank behind the rear axle is a really bad place to put it. If the gas is behind the rear axle, it actually "levers" over the rear axle and takes weight off of the front wheels as it tries to lift the front wheels up. This, of course, transfers "extra" weight to the rear wheels in the process. You end up with a car that has the CG constantly changing. Not to mention, having the gas tank in the back is more dangerous in a rear end collision. At least it makes for a fun ride!

You can put the gas tank above the rear differential. (A pretty good place to put it). The gas tank is centered over the pivot point of the car (the differential is the actual pivot point the car steers around) and so this lowers the north/south polar moment of inertia. In other words, the car "points" quicker into a turn. Of course, this also moves the gas tank closer to the CG of the car...which is always good. The Shelby Series One put the gas tank there.

There ain't no free lunch, however. By putting the gas tank over the differential, you automatically have raised the gas tank high into the air. This, of course, causes problems in roll. As your car goes into a corner, it rolls. The higher the weight is up in the air, the "easier" it is for it to make your car roll too far--and the harder it is to control the roll as you loose fuel.

That is why they put the "ship in a bottle" gas tank in the Ford GT down the center of the cockpit tunnel--very low in the car. I think that is a really good place for it. It is well protected; placed very close to the CG so changing gas level doesn't upset the car too much; it is low so the roll centers don't change.

No free lunch again. It is extremely hard to package, and about impossible to service without lots of swear words and taking the entire car apart. Also, a center tunnel gas tank is impossible in a front engined car--the transmission has to go there.

David

[David Kirkham]

[David Kirkham]

[David Kirkham]

[David Kirkham]

[David Kirkham]