911 Camber and Caster Settings...

[Fly911]

This is a question for alignment gurus!

I'm planning to increase the camber setting on my '87 Carrera. I have currently 2.6 degrees negative camber, but would like 0.5 50 1.0 degree more negative. Due to the coil over suspension, I can't move the camber plates more inward. So I would need to move the control arms out, or lengthen the control arms.

I'm currently running 7.5"x17"ET36 w/225/50-17 and 21mm spacers. Again, due to the coil over suspension, I need minimum 6mm (i.e. ET30) for the wheel to clear the coil overs.

By extending the lower control arms by 20mm, and at the same time move the camber plates outward by 15mm, I will go from -2.6 to -3.1 degrees. At the same time I will change the spacers from 21mm to 7mm. This will give a slight increase in front track, and a significant reduction of the scrub radius.
So more negative camber, increased track and reduced scrub radius will improve turn in and steering response.

However, the control arms are slightly angeled towards the back, and an increase in the control arm length will change the caster angle towards negative. More negative caster will have a negative effect on the steering and car balance. The caster will be around 0.1 degree more negative.

So my question is: How much will the change of caster effect the steering and car balance? And, should I move the control arms a bit more forward to increase the caster instead? And if so, by how much?

I hope some of the alignment experts can elaborate around this topic...

[Trackrash]

I'm guessing this is a track car? You might ask in the AX / racing forum, but I can't imagine that you will notice .1 degree difference in caster. I think that once you finish your mods, there will be some work needed on alignment settings to get things the way you want anyway.

What tires are you running? They need more camber than what you have?

There was a recent thread that went through some of the though processes and mods you mention. Yet another tire/wheel choice thread...

That said it would be cool for you to document everything you do in this thread.

[chris_seven]

Early cars had around 6.5 degrees of +ve caster and this has a very positive impact on the steering particularly of a SWB car but I would agree that a 0.1 degree change would be undetectable.

[PcarPhil]

Originally Posted by Fly911

So I would need to move the control arms out, or lengthen the control arms.

This is something I've been wanting to do for quite some time, mainly to further reduce scrub radius. My plans also include raising spindles and decambering the struts (I don't need lots of negative camber).

I agree with the others that a caster change of 0.1 degree won't be noticeable. That 0.1 degree could be dialed back in with your caster/camber plates. I also suspect you could shim the control arms forward a wee bit by manipulating slop in the control arm mounts. Or of course you could change the angle of the arm when performing the control arm modifications.

Speaking of the control arm mods - what plans do you have for the actual lengthening of the control arms?

Please keep us updated with your progress!

[Fly911]

Kaefer, I'm planning to cut the main control arm and the angled strut at the pivot point on the longitudinal part of the arm. Since I have a coil over suspension, there are no torsional forces on the control arm, unlike the original torsion bar set up. I would not have done this with a torsion bar suspension! Also, I will reinforce the diagonal strut with a small gusset plate in each corner.
While I'm at it, I might increase the caster angle.
I already have raised and de-cambered spindles and de-cambered ball joints. De-cambering the spindles was not such a great idea, with this modification in mind... The de-cambering of the spindles is bringing the wheel closer to the strut and coil spring, making it necessary to use spacers. With a neutral spindle, I would have to lengthen the control arm more to achieve the same negative camber, but that would not have been a problem. However, the most important part would be, that with a neutral spindle angle and no spacers required, I would have significantly reduced the scrub radius, and hence improved the steering response. But that is too late now...

[KTL]

You can get camber boxes installed in the strut towers and that will give you all the negative camber you want. Then no need to futz with sectioning and lengthening the A-arm. The camber boxes are indeed box-shaped and the purpose behind that, besides ease of fabrication compared to round-ish shape, is that it provides plenty of clearance for the coilover. If you contact Roger Hamlin, he'll probably make you a set. He's the one who originally fabricated them for SmartRacing Products. WTB: Front Camber Boxes

Also noteworthy is that when you lengthen the A-arm, it pushes the wheel rearward a bit. That reduces your tire clearance at the back of the wheel well. So if you run tall-ish and wide tires, you can easily get some rubbing on the wall of the chassis. I have noticed this problem on my racecar. On other 911 cars i've had with wider front wheels and tires, I would see rubbing on the straight area of the rearward area of the inboard chassis wall. On the racecar I see the tire can contact the curved transitional area in the chassis tub.


I can't comment on the feel all that well for your projected plans because my situation also has raised strut spindles and bent steering arms for bump steer correction. But I can say that my setup (~3/4" lengthened A-arm, slightly raised spindles (greater than typical RSR 19mm spec but not so much that 16" wheels don't clear), bent steering arms) provides a lot of front grip and the steering effort is a lot higher than my street cars have been. I go thru front tires as fast as rear tires on the racecar.

Back when I did DE days with PCA in the street cars with R comps, I could do sessions all day long and not really notice the steering effort being a lot of work. However, the first time I did a DE event with the racecar, my shoulders were quite sore the next day from the added steering effort. Take that for whatever its worth.

Here's a few threads I recall participating in and sharing my thoughts on the modified A-arms

How much camber...

More Front Negative Camber?

Analyzing Tire Temps

Stretched Control Arms

[Fly911]

KTL, camber boxes are MUCH more work than stretching the control arms. My car is sitting on 4 jack stands now, without any drivetrain. Taking the control arms off and take them to a weld shop is way easier. And cheaper... If money was no subject, I would have gone for the Elephant Racing GT3 style suspension. Or the 935 style for half of the Elephant price. Or my own design for 1/10 of the Elephant arms...
And it's not only camber, but also scrub radius. With camber boxes I would get the wheels further inwards, needing even thicker spacers to widen the track and fill out the fenders. That is the opposite of what I want. A shorter scrub radius will also give better wheel well clearance on steering inputs.

Your links are interesting, I have seen some of them before. Camber settings are not absolute science, and would vary with car weight, tire and rim sizes, driving style, track, etc, etc. With my current R888 tires I have pretty even wear in the back (-3.5 deg. camber) and wear more on the outside on the fronts (-2.6 deg.). I believe an even wear on all 4's would be ideal. Now my rears are shot before the fronts, but I will change all 4 regardless. So more front wear doesn't bother me.

I'm curious about the increased steering effort that you describe. That is something that nobody wants, unless you are some kind of macho freak... What causes the increased steering effort? Is it the camber or scrub radius? I would assume that a smaller scrub radius would decrease the steering effort (?). More positive caster would probably increase steering effort, and of course wider and more sticky tires.

[Tom '74 911]

Here's a photo I've saved of an interesting a-arm mod to accomplish what you are describing. I don't know how I feel about the durability of what's essentially an eye bolt in that configuration. I think you'd be bending them often.

I've also heard of club racers (illegally) chopping off the end of a stock a-arm and welding in a piece grafted from a scrap a-arm to lengthen them slightly... once the weld is dressed and the a-arm is re-powder coated, it's really hard to see the modification w/out measuring.

[Fly911]

Tom, I have seen this one before, but I'm sceptic to the design where all the brake forces are directly on that one bolt.
By extending the A-arm at the pivot point (i.e. all the way in), there are no torsional forces on the new weld, and the triangulation with the diagonal rod, would take up longitudinal forces (like braking). You will however lose the adjustability for more or less camber, but that can be accomplished by adjusting on top of the strut with camber plates. But I must admit that it looks pretty cool...

[KTL]

I know how much work goes into the camber boxes. I chatted a lot with my local friend Chris when he cut them into his chassis. Measure 20 times, cut once...........

Installing front camber boxes - any gotchas?

It's not as hard as it appears and it solves a lot of problems people run into when trying to get more camber and more coilover clearance. Or you can just notch the tub where the coil fouls, and widen out the holes in your chassis strut tops

Let's also not ignore the fact that your wheels have less than optimum widths and offsets, thus the need for a spacer in the first place? However, using a small spacer is certainly not a bad thing when you're fine tuning the scrub radius.

Hey, i'm not touting my increased steering effort as some kinda benefit or testament to my strength. I'm 6 ft. tall, 170 lbs soaking wet, so I certainly am not an Incredible Hulk bad a$$. I'm just sharing what i've experienced on my car based on the modifications I inherited with it. BTW I didn't specifically select all the racy pieces that make up the car. It came mostly already modified as I bought it.

I would attribute that increased steering effort to the bending of steering arms. When the arm is bent to make up for the raised spindle, bending it obviously shortens the length of the arm. Shorter arm = less leverage = whoever's turning the steering wheel has to apply more effort. The bent steering arm also affects Ackerman. The Ackerman angle has been changed so that I no longer benefit from the unmodfied steering arm's "toe out in turning" design. A stock steering arm will open up the inner turn in wheel more than the outer so they can roll along a different radius. It basically relies on the dog leg curve that the steering arm has, and being re-shaped took away most of this feature. That's what Clint Smith at Rebel Racing told me when I asked his opinion about applying bump steer adjustment to my steering arms.

Also noteworthy is my car is a widebody. So I don't know how far one can go with the A-arm lengthening before we start to run into fender clearance issues. I've still got quite a bit of room at the outer fender area when running 8.5 or 9 inch front wheels, offsets of +25mm and +15mm respectively.

I wholly realize camber settings are vehicle and track and vehicle setup dependent. Thanks. But just my opinion, i'd ditch those R888 tires and you'll probably have a better time honing in on your camber settings and tire pressures. Those tires are a thorn in a lot of people's sides, trying to figure out their sweet spot. There's plenty of other tires to choose from that will behave more agreeably to traditional tuning adjustments. Plenty of 911 drivers here have used them and a lot of the astute drivers have not had great things to say about them.


Yup that mod that Tom shows is typically called the Dawe solution since Peter Dawe is the one who often does that type of A-arm adaptation. Dawe is very experienced in racing these old Porsches for much longer than a lot of us. So I would think by now that he would have stopped doing that approach if problems were found with the single shear design.. That said, i'm not endorsing it. I prefer to see a traditional ball joint that has a much higher tolerance to wear than a spherical rod end. Especially in a dirty environment like this location.