Author Topic: why it is important to prevent rear wheel tuck-under  (Read 6516 times)

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Offline Pantdino

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why it is important to prevent rear wheel tuck-under
« on: March 20, 2007, 05:22:55 AM »
Here is some video of swing-axle cars rolling over when they get sideways on the Nurburgring.  The red 850 Coupe looks like a really nice car.  I also saw a white 850 Coupe and a green 600, both of which did OK.

http://www.liveleak.com/view?i=53a_1174161054

Jim

Offline ChrisD

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Re: why it is important to prevent rear wheel tuck-under
« Reply #1 on: March 20, 2007, 07:55:44 AM »
So we use low profile tyres and lower the rear as and get a positive camber angle, right?
Fiat 600 Abarth

Offline Pantdino

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Re: why it is important to prevent rear wheel tuck-under
« Reply #2 on: March 21, 2007, 04:05:58 AM »
Chris,
I think you mean negative camber-- so the top of the tires are closer together than the bottoms.

I suspect the only way to prevent this jacking is with the limiting strap or cable. so the suspension cannot go into positive camber no matter what happens.

I think what happens here is that the driver lifts off the throttle or brakes, which causes the rear of the car to rise. The rear suspension goes into positive camber and that plus the weight transfer away from the rear makes the car start to slide / spin.  The sideways force on the tire pushes it inward, rotating the suspension  arm into even more positive camber until the rear of the car is way up high.  The tire is then so far inward it "trips" the car and it rolls.

If the suspension arm can't rotate down into positive camber the whole process will not start.

Anyone have a different explanation?

Jim

Offline Pantdino

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Re: why it is important to prevent rear wheel tuck-under
« Reply #3 on: March 21, 2007, 04:21:35 AM »
Some more information about this video-- I learned about it from the Pantera club forum, where it is mentioned that this corner comes after several much faster ones and is blind.  So these drivers were caught going way too fast into a corner that drops downhill, which of course means you have a lot less grip.
They didn't have much choice but to brake or at least lift off the throttle, which put them into a spin.

Rear-engined cars don't do well in a corner without power on, so it's best to approach any blind corner slowly--brake in a straight line then apply power as you enter the curve.

Offline ChrisD

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Re: why it is important to prevent rear wheel tuck-under
« Reply #4 on: March 21, 2007, 08:05:28 AM »
Yes, I mean negative camber Jim (I always confuse it).
You explaind it well. Plus the downhill corner is a good reason. I do I little drifting with my car but never in corners looking like that.

Do you have any instuctions of how to install a limiting strap? I guess some wellding is required in the swing arm, right?

Chris
Fiat 600 Abarth

Offline Pantdino

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Re: why it is important to prevent rear wheel tuck-under
« Reply #5 on: March 22, 2007, 04:55:47 AM »
Hi, Chris,

I haven't seen a diagram or photo of a limiting strap.  Usually the shock absorber is what limits the lower position of the suspension, so my guess is that the BEST way to do it would be to use a shock and spring that are shorter than stock and is at maximum length when the wheel is at 0 degrees camber. 

A steel cable or strong leather could be used as a strap with stock springs, but then it would have to be strong enough to hold the spring in compression.  Using a shorter spring would be better, as then the strap would just have to be strong enough to hold the actual weight of the suspension arm, brake, wheel, and tire, which isn't  that much.

I don't know the details of how it's done and what material will be used

Maybe when my car gets here it will have a strap and I can photograph it.

Jim




Offline ChrisD

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Re: why it is important to prevent rear wheel tuck-under
« Reply #6 on: March 22, 2007, 07:56:09 AM »
I have seen in a video a 600 raised in the rear with a jac and the swing arm did not drop like it usually does, but it was in 0 camber. I bet the owner had something like that installed but the camera was not close enaught to take a look. I have also seen a covertion to the rear swing arm with the shock absorber located outside the original mounting place and closer to the wheel on the outside of the swing arm. Has anyone seen anything like that? Is it something that Abarth did for better handling?
Fiat 600 Abarth

Offline ChrisD

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Re: why it is important to prevent rear wheel tuck-under
« Reply #7 on: March 25, 2007, 06:29:13 PM »
I managed to find this picture of a swing arm with limit wirecable. There where no pictures of the location that the cable is bolted to the chasis.
Fiat 600 Abarth

Offline Pantdino

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Re: why it is important to prevent rear wheel tuck-under
« Reply #8 on: March 26, 2007, 03:28:17 AM »
Thanks, Chris!

It looks to me like in this case the cable is long enough that the suspension gets some positive camber.  I suppose there's a trade-off between ride quality / suspension travel and safety.

Jim

Offline ChrisD

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Re: why it is important to prevent rear wheel tuck-under
« Reply #9 on: March 26, 2007, 02:59:05 PM »
Maybe with shorter springs when having the swing arm in 0 camber you can do the same thing. With a long spring like that in the picture even if you install the wirecable holding the swing arm in 0 camber I don't know how much it will hold because of the spring pushing it to negative camber. So I guess it is all in the spring first and then to the wirecable.

Look at this picture with the relocated shock absorber. Have you seen anything like that before? You think there is any benefit having the absorber there?
Fiat 600 Abarth

Offline Pantdino

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Re: why it is important to prevent rear wheel tuck-under
« Reply #10 on: March 26, 2007, 06:10:13 PM »
Chris,

You are right.  It will take a very strong cable to hold a long spring in compression, so the proper way is to use a shorter, stronger spring.
The different shock is a good way to limit the bottom position of the suspension arm, and you can choose the stiffness and kind of shock you  want as a bonus.

The only problem I see is that the shock and mount seem to stick out from the sheet metal further than the stock shock, so it may hit the tire if you mount a wide wheel and tire.

Jim

Offline ChrisD

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Re: why it is important to prevent rear wheel tuck-under
« Reply #11 on: March 27, 2007, 09:08:55 AM »
I will go for the cable myself. This shock absorber seems to be the standard Fiat 600 with removed top cup. I just don't like the idea of mounting custom mounting brackets and prefare the stock ones that are defenatly strong and can take the preasure.

By the way, what is the best rear spring lenght for that job: Keeping the swing arm in 0 camber when the car is parked? 200mm???
Fiat 600 Abarth

Offline Paul vander Heyden

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Re: why it is important to prevent rear wheel tuck-under
« Reply #12 on: April 08, 2007, 04:31:43 AM »
Chris,

The photo of the car with the cable arrangement is mine.  This works quite OK, and in this case it did have about 2 degrees of positive camber when fully extended.

The problem with the Fiat type suspension is very similar to what got the Corvair in trouble with Mr. Nader.  The suspension can have a "jacking effect" particularly when subjected to large right-left-right- left motions.  One wheel goes positive, the the car attitude changes and the other side does the same.  In the meantime the suspension on the other side has not settled and the rear roll center keeps getting higher off the ground.  If the car now "catches" on an irregularity in the road or track, over it goes.

One method of limiting swing arm movement is to drill a small hole in the chassis next to the spring mount. (This would end up in the rear seat area next to the coil/shock tower.  Put a straight steel rod with a rubber shock bushing on it  and poke it trough the hole and attach the other end to the trailing arm.  Lengthen or shorten the rod as needed.  As the arm goes up and down the rod simply pushes up and down.

Good luck

Paul Vanderheijden
www.scuderiatopolino.com

Offline Pantdino

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Re: why it is important to prevent rear wheel tuck-under
« Reply #13 on: April 08, 2007, 06:56:24 PM »
Paul,

How long do you make the cable?  I'm thinking it should be just long enough to hold the swing arm in contact with the base of the spring, but not compress the spring. So only the "dead" weight of the parts is being held by the cable.

Is that what you do?  That means you have to have the spring the correct length and not try to compensate with the cable.

Thanks,

Jim

Offline Paul vander Heyden

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Re: why it is important to prevent rear wheel tuck-under
« Reply #14 on: April 15, 2007, 04:38:14 AM »
Jim,

Unfortunately it does not always work out as you would like.  Many times on order to get the ride height that you want, the spring is actually slighly compressed and tensioning the cable.  I saw what I consider now to be a better way of controlling the arm movement.  Attack a rod (1/4 steel) with spherical rod end to the arm.  Then drill a small hole next to the shock housing and put a rubber bush with a chassis washer and a lock nut.  Now as the suspension moves the rod goes up and down and limits when the rubber bush contacts the chassis.

Alternatively, as I have done since the photos were taken is to replace the standard spring/schock assembly with a "coilover assembly that fits in the standard mounts and itself limits the total travel available.  It also allows infinite adjustment of ride height and spring stiffness.