Spin Question

[johnneilson]

Thanks for the input. I just took the car off the scales.
Left front        639 lbs.
Right front       630 lbs.
Left rear          875 lbs.
Right rear         909 lbs.
Total weight of car with driver  3053 lbs. TOO FRICK'IN LIGHT??

Just an observation, but your wedge/diagonal should have made the car turn left.
LF 639 + rr 909 = 1548
rf 630 + lr 875 = 1505

this shows 1.4% wedge turning the car left.

It is not much, but still there.

Just another opinion, J

[jww36]

If any of you followed the building of this roadster in "Build Diaries", you'll remember I threatened to write a book, "How a Newbie Gets Started in LSR and Wants to Build His Own Car"! I thought after the car was built, the book would be done. NOT! The next chapter, and it's going to be a long one, is "What in the Hell is Wrong With My Car, and Why Does it Want to Spin".

Seriously, I appreciate all of the input, and I'm sure I'm not the only LSR following and learning from this topic. I'll try to answer some of the questions, and then I want to follow up on a few comments.

When I spun, I was in fifth gear, on it hard as I have mentioned, at the 2 mile. If I had to guess, I'd say I was around 200 MPH. As for my experience, I've got less then a dozen runs in total at El Mirage and Bonneville. But in those dozen runs, I have never once spun the tires or lost traction in acceleration. I have always driven this car and the Barbee Boys roadster (227.8 MPH in 2007) as if I had an egg between my foot and the throttle pedal.

Kiwi Paul mentioned preloading a lower rear four bar which is something I've never heard of, but then I'll admit I know as much about race car suspension and setup as I do about brain surgery.
The other interesting comment was about diagonal weights. The thing that will make both adding weight to this car and being able to adjust that weight is I have access to setup scales, and more importantly, I plan on making a mold for the ID of my 2" x 6"  lower frame rails and sliding the weight inside the lower frame rail. In this way the weight is contained and adjustable. Once I get it where it needs to be, I can put a bolt thru the frame rail to keep it from moving forward or back. All it takes is time.

[Glen]

Have you checked the tire roll out??

[jww36]

Glen;
If you're refering to the run out, I spent alot of time truing the wheels and then truing the tires after mounting, and then balancing the wheel/tire after tire truing. As far as the circumfrence of the rear tires, I used a tree tape to measure them and adjusted the air pressure (8 PSI) to make them equal size.
John

[Rex Schimmer]

It is very interesting how many different things can and do affect the possible reasons for a roadster to spin on the salt. John, I know on your last run that your replaced the rear shocks and springs with struts so the spring rate of your suspension, not counting the tires, was effectively infinite. Now only your tires were acting as suspension, and you have set one of them 8 psi higher than the other to balance the diameters but now you have increased the spring rate on that tire possibly by as much as 10% (this is based upon some tire data that I have for some road racing tires and I am not sure what the inflation pressure vs. the spring rate is for land speed tires.) This means that this tire will be carrying a larger percentage of the rear weight. Did it happen to be the right rear??

I have also done some "pie in the sky" calculations regarding rear weight/salt coefficient of friction/available hp and with your present rear weight about the maximum hp that you can apply to the salt without spinning the tires is around 520 hp (this is at 220 mph) if you increased the rear weight from the present 1800 (approx) lbs to 2500 lbs you could apply about 590 hp which if you take into consideration the altitude and air density, drive line loses, rolling resistance etc is pretty close to what you have for available horse power. These calculations have nothing to do with anything aero which means that if you bump against the "aero wall" where forward thrust equals aero resistance the addition of extra rear weight will not make you go faster than that limit. At that point you need to A. Make more hps or B. Reduce aero drag.

Rex

[akk]

You could use the search engine in this site and find a lot of information on roadster set up and spins.....rather than go through it all again I will summarise....run a spool...put enough weight on the rear end so that you don't have to peddle in second gear.....

if the rear tires are rolling (not spinning or slipping) you will not spin out!

[Dynoroom]

Rex, why would hp be a load indicator? Shouldn't it be X torque at the tire contact patch? Just wondering, it's like a guy saying his trans is good to 1000 hp. Not so as 350 lb/ft @ 15,000 rpm = 1000 hp and trans would probally live but 850 lb/ft might kill the trans (or break the tires loose) but still make 1000 hp as you well know.

[hotrod]

Roll out = tire circumfrence ie how far they travel in one complete revolution. If you have slightly different tire circumference, due to the physical tire size, its loading and tire pressure, with a solid spool the taller wheel will be constantly trying to turn the car toward the shorter drive wheel, as it will travel slightly farther with each axle revolution.

Lots of issues that interact here.

For example the Ack Attack motor cycle had problems at last years shoot out, it kept pulling to one side, and they eventually figured out it was due to a tire that had a slightly off center crown on its tread, so it wanted to lean the bike to the side slightly at speed. They replaced that tire with another that was known to be good and the bike ran true and went out the back door at about 394 mph.

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

[John Burk]

Front wheel drive cars rarely if ever spin .

[Rex Schimmer]

Mike,
What I look at is the rear tire weight of the car, lets say in John's case, 1800 lbs., then look at the maximum coefficient of friction between the tire and the salt, let's assume .4  for grins,  .4 x 1800= 720 lbs of thrust. Now we can take the "James Watt" definition of horse power, i.e. 1 hp= 33,000 lbs of force over a distance of 1 foot in one minute. Divide by 60 seconds/min and you get 550 lbs force/second. John's car is making 720 lbs of force so I divide 720 by 550=1.3091 hp/ft/sec. John is going 220 mph so now I multiply the 1.3091 x 220 x 5280(this gets it to feet/ hour) and then divide by 60 gets it to feet/hour and divide by 60 again gets it to feet/sec. So we have a force in pounds going in feet per second which equal horse power. In Johns case the number is 422 hp, this is the maximum horse power that can be applied to the salt with this rear wheel weight. I happen to know that John's motor is good for around 850 hp at STP so at the salt it is probably good for around 720 flywheel horse power (I used a .85 multiplier to adjust for the altitude, a WAG on my part) Now if you look at the 422 hp that John is limited to because of the lack of rear weight, and use a drive line efficiency of .9 and a rolling resistance of .9 you get a flywheel horse power of 520 hp. So John has (using my numbers an additional 200 hp available at 220 mph. So torque in this case is not looked at as hp is a measure of work and when you are going 220 mph your motor has to work, and pretty damn hard.

Rex

[jimmy six]

Rex. I like your explanation and then your calculations on how you got there. The part that interests me is where you say you need more horspwer but don't have it or has it but can't apply it.. For more of a non-aero car that makes complete sense as "when" a roadster wants to spin.

A 34 grille shell may go thru the air better than a 29 or 32 but none wants to stop accerlerating. A Camaro/Firebird style vehicle many never feel his car wander like a standup roadster or at least not to the same extent.

John W has learned a lot in a short time and will continue to as all of us with roadsters. I'm glad this doesn't happen much at slower speeds.................Good Luck to all

[jww36]

Thanks to everyone for all the input. Now my head is spinning!

Weight distribution question - In balancing the weight in the car, do we want a liitle more weight on the left rear to compensate for the torque wanting to "roll" the car to the right rear?
John

[akk]

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

[SPARKY]

"this shows 1.4% wedge turning the car left.

It is not much, but still there.

Just another opinion, J

I personally like some wege on the Lf-Rr so that the drive line torque is somewhat nullified under way---but I am a "no Suspension" Torsen loving , "lakester guy" lol

[maguromic]

Just to throw another wrench in the mix, how are your shocks (valved) set up?   If you have adjust ability, here are some things you can do.

Low speed rebound will help with driver feel and keep the platform stable. Going too aggressive will make the car feel harsh and choppy.

Low speed compression (bump) will help with getting the tires up to temperature quickly. This basically helps to control how the unsprung weight of the car is controlled.  Ideally, you want to run as much as possible to where the car is not unsettled on the larger bumps on the salt.

High speed rebound helps control the spring and how the body is moving after hitting the larger bumps.  You want run as little as you have to, so the car doesn't "jack down" while going down the track.

High speed compression if set too aggressive can launch your car when hitting the larger bumps on the salt.  You want to try and run much as you can get away with.  But be very careful when going firm.  Tony