Geo

I've got 32mm for mine. I was debating 32 vs 33. Vaughan is running 34 in his 924, but he hasn't had time to sort the suspension once he put them in. I'm just not sure and may even end up with a set of 33 or 34 as well. Since I have a 80% LSD I'm concerned about the effect on understeer, but that will all shake out once the car is done and in testing. I'd like to end up around 600-700lbs for front springs. Of course the downside is with such heavy springs you just have to pray it doesn't rain since it makes changing to a wet set-up neigh on impossible.

Bill L Seifert

Geo

I don't know about where you live, but I have owned my rains (Hoosier Dirt Stockers) for 3 years, and the only time they have been on the ground, was when I loaned them to a friend to take to Barber last year. I obviously don't worry too much about rain. (wait and see, I will use them next time out.)

What do you think about only lowering the car 3/4 to 1 inch, to help keep bump steer at bay?

Bill

Geo

Hehe. In Texas it either doesn't rain or we get an Act of God.

I haven't measured bump steer, but I'd imagine limiting the lowering would help in that regard. The real question is would you give up more in other places? I don't know the answer to that, but Jon Milledge recommends running at the legal minimum ride height. In fact, he says it's IMPORTANT to run at that height. I don't know why.

All I could recommend is to test it out. I think the rear will accomodate a 1" ride height change, so if you figure out how much to adjust toe when changing from one ride height to the other (or better yet, get toe plates), it would be interesting to know the trade-offs.

Flat Top

No matter how 'beefy' I do not like the single shear bump steer systems. Attached is a photo of the system we use. Note that our spindle has been lowered by 30 mm hence the large adjustment that we have.

If I am not mistaken manufacturers design road cars with bump understeer - not IMO ideal for racing.

Regards


Johan Vermeulen

Larry Herman

Sweet. I understand that just lowering the tie rod end would produce a lot of torque in a single shear setup. That is why I raised the rack and bent the tie rod ends down. That left me with a pretty strong setup, but not as nice as yours (but not as obvious either). From the type of spindle/rotor, I assume that this setup is on a 911/SC/Carrera.

Flat Top

Larry

The car is a 73 RSR replica. More pics here: http://www.almost.co.za/

The original chassis was a '72 which was stripped down to the absolute bare shell. Stripped, resprayed and the rebuild with the best equiptment we could afford. Big reds, Quiffe lsd, 10" inch and 13" Hoosiers, etc.

Our motor popped (flywheel came loose and cracked the block). Now busy building a 3.5 lt race motor. Weight 860 kg (wet) or 1900 lb.



Johan Vermeulen

sjanes

Johan,

"spindle lowered by 30mm"? Do you mean the spindle was "raised" by 30mm (like RSR shocks have)? Either way, that's a pretty cool solution. Do you have a better picture of how the bottom support is mounted? I looked at your web site, but the suspension pic was too small.

Currently, I use the standard spindle height and rack spacers to reduce the bump steer, but I've considered moving to RSR shocks (with the raised spindle).
It looks like you have a good solution to the single sheer tie rod drop link.

Bill L Seifert

Geo, I have a toe guage, so re-adjusting toe is no big deal. I do remember Millidge saying that, so I guess I will experiment with it.

Here's one thing. I run both 16" and 15" wheels, to change the gearing for different tracks. I set my ride height with 225-50-16 tires on 7x16 wheels. When I go a different track, and use 225-45-15's on 7x15's, my car is a little low on ride height. (I also squashed my exhaust pipe backing into my garage.) That could be a problem if I won, but seeing as I finished 6th at my last race, very few people protest 6th place. But if I ever get good,(Slim chance) I will have to watch that.

To Larry Herman,

You didn't do that bending of cast parts on your 2004 GT3 did you?

Bill

Larry Herman

Nooo My GT3 is absolutely unmolested. I did that on the cast iron spindles/steering arms (like the ones in Johan's picture) on my old Carrera.

Flat Top

sjanes

Sorry, I meant raised by 30 mm.

Larry,

Here is a picture showing how the support is attached to the shock.

Bear in mind that raising the spindle by 30 mm on my '72 911 brings the ball joint very close to the rim (16"). I think the factory originally moved the spindle 18 mm. The larger the rim size used the less of a problem, obviously. When moving the spindle you also change the roll centre height.

Because I raised mine so much it becomes problematic to use heat to bend the control arm - not only does it need to be bent downward but the ball joint receiving end has to be curved in the opposite direction so that the hole retains some of its original angle.

911pcars

FYI, bump steer theory and corrections are further explained in Fred Puhn's, "How to Make Your Car Handle" and Paul Van Valkenburg's "Race Car Engineering and Mechanics" (both published by HP Books), among others.

Sherwood

Flat Top

BUMP, BUMP, BUMP STEER.

I have found that using a laser pointer, bolted to the hub as shown, that projects a laser dot onto a sheet a graph paper works best.

Of course all the appropriate precautions to ensure repeatability such as applying the brakes, locking the steering wheel, removing springs and anti roll bar and IMPORTANTLY ensuring that your graph paper and spirit level is horizontal, have been taken.

Innovative and different. For example you do not need to take any measurements at all. Just plot your graph as the dots moves through its arc of travel. Just make sure you cleary mark the stardard ride height and use that as your reference.

When pumping the trolley jack a clear line is shown by the laser which I position about 2 metres in front of the car. The arc instantly reveals itself.

Johan

kurt M

Flat top. Good stuff! Thanks for the post and the inovative idea.