Spin Question

[jackson]

Am I the only one who has extreme issues at the 3 1/2 on the long course?  This happens at around 195 and I have short shifted or coasted for the shift in that part of the course to try and avoid a HARD snap to the left.  We added 500 pounds of lead in the back in attempt to avoid this, but I'm afraid this might be destroying the "lawn dart" effect that may have kept me from crashing before.  I guess I'll see in September!

[hotrod]

Where in the back did you put the weight, in front of the rear axle, over the rear axle or behind the rear axle?
Does that snap to the left occur when you get back on the power?

Larry

[bucketlist]

It does not take much to upset a vehicle that is on the ragged edge of traction and pushing a large aero load that is in front of the center of mass. As soon as that center of drag gets just slightly off to the side, and the side force exceeds your steering/lateral traction on both axles, it wants to push the car around and there goes your snap spin.

larry

I don't know roadsters at high speed, but I know canoes at low speed. One very effective way to steer a canoe is to add a little drag on the side you want to steer toward. Maybe the offset cage is a constant drag just waiting for the ragged edge condition hotrod described to make its contribution.

[rambler jack]

Interesting idea. Rudder effect as with a plane or boat. Even more-so with offset windshield.

[Tman]

Interesting idea. Rudder effect as with a plane or boat. Even more-so with offset windshield.

Interesting Ideas. I was looking at 911 last night pondering this same thing. Driver offset to the left, big ol mags hanging out the right. Might be helping balance the aero?

[jww36]

The Barbee Boys roadster (driver on left) was prone to want to pull to the left. Someone told them a year or two ago, it was an aero thing because of the driver wind screen, cage, etc. on left side. What I did to anticipate that was put the slightly taller tire on the left rear. During my runs, my car did not seem to want to "pull" one way or the other as far as drifting goes. But it did want to "dance" or "float" as previously mentioned.

As far as coil overs go, on Monday @ Speedweek, I took the coil overs off and put solid struts on. No rod ends on struts so they really made for a solid rear end as they eliminated any side to side movement as well. If I do go back to the coil overs, I will go to a Watts Link set up and sh.t can the panard rod.

[John Burk]

As John W mentioned the off center cage moves the center of drag to the left . Drive shaft torque tries to loads the left rear tire which puts the center of thrust to the left so one tends to cancels the other . When thrust changes that upsets the balance .

[Skip Pipes]

If you have low rate springs in the front(or coil-overs close together) with no sway bar in the front and low rate springs in the rear with a stiff sway bar in the rear,.... all the torque is in the rear sway bar, the body will roll but the weight on the rear tires will not shift side to side!!!

 ....in fact you can lift one of the front tires off the ground and see almost no shift in the rear side to side distribution!!!!!!!!!!!!!

That is the way the contrivance special is set up

Akk

Hi Akk,

Could you give me a ball park idea of an effective in/lb spring rate for a rear sway bar?

Thanks,
Skip Pipes

[johnneilson]

Skip,

design the sway bar with as large a dia as possible. Look at Sprint car setups, 1 1/4" dia.

Use a hollow bar and you will have the ability to stack up 2 bars.

Another issue in suspension would be the end stops of travel. Having the suspension move to the end and get hit with a infinite spring rate (solid) will introduce a wild ride.

FWIW, I like Burks idea of a IRS to isolate the driveline torque from the rear axle/wheel loading. Brings back memories of stuffing higher rate springs under the right side of the nova to launch it straight.

J

[akk]

Skip,
Working from memory the front coil overs are 275#/in each spaced about 16 in apart... the rear springs are not on coil overs (using jack screws for easy adjustment when adding weight) 500#/in each...the sway bar in the rear was the stiffest I could fit 1 1/8 hollow from speedway (I think?).

What is important is the lack of sway resistance in the front. The rear will absorb engine torque regardless of the stiffness of the sway bar and rear springs. The stiffness of the sway bar just determines how much the body will roll for a given engine torque.

If the front doesn't resist body roll, the weight on each of the rear tires is not affected by engine torque!!!!!!

[jl222]

  One thing not mentioned [I think] is how cars become unstable with lift and roadsters might be worse than others.

  I've posted before how Jim Hall [Chaparrall] and Chevrolet were one of the 1st to use spoilers to prevent

hi-speed instability not allowed on roadsters though BUT Shane Mcgann's 243mph D blown street roaster

 has the stock rear fuel tank

  A combination of lift --loose our wet salt on one side our both could cause loss of traction and tire spin which causes an oversteer condition.

  I noticed when we washed the car, a salt build up around front and back of the  rear valve stems [1/4-3/8''] must have been from wet course on friday never happened before

                    JL222



 

[Interested Observer]

In an effort to tighten up incorrect use of vehicle dynamics technical terms, note that “oversteer” and “loss of traction” or “tire spin” cannot coexist.  While loss of traction may well induce an uncontrolled yaw motion of the vehicle (spin), that motion is not oversteer.

Oversteer is a condition that arises from mismatched slip angles of front and rear tires before they break loose, and is a result of the elastic deformation of side-loaded tires.

Troy, see Jim Hall’s primer on vehicle handling (or any other vehicle dynamics reference).

[dw230]

Shaen Magan. Reply #40.

DW

[Stan Back]

And the answer is . . . ?

(Or the question.)

[Stan Back]

Oh, damm, I couldn't hold off any longer!


Here's some numbers I know:

San Berdoo Roadsters Street Roadster . . .
1 D/STR BV record, 4 C/STR BV records
111" wheelbase
2800 pounds
45/55 weight distribution
250 pounds more on the left rear than the right
Open rear end
210 MPH - never spun except one run with a posi.

Steve Batchelor's Monza
Owned the BCD/Classic Gas and Fuel Altered records  (go ahead and look)
Same engines, shorter wheelbase (100"?)
2800 pounds
40/60 weight distribution
Open rear end
245+ MPH - never spun

According to present knowledge, we were doing lots wrong(?).

Stan Back


(Something to be said for 1/2-length pitman arms.)