How to get more caster?

[David]

I recently had my alignment done and the mechanic could only get 5.3 degrees of caster on both sides but I'd like to get around 6.5. I see camber kits to get more (or less) camber but nothing on caster. Is there an off the shelf item to get more caster adjustment? Is there any other trick?

[Driven97]

Caster is the lean-back angle of the steering axis when viewed from the side. So you can slide the upper mounts rearward, or move the ball joint forward. Or both.

If your camber plate is maxed out rearward, the next step is to get a plate with a smaller diameter bushing /bearing. Are you sure the plates you have are all the way back and you're at 5°? Caster isn't super fun to measure, so there's also a chance the measurement is off.

Elephant sells offset ball joints that are typically used to get more camber, but if you install them offset forward instead, they'll give you more caster.

[T77911S]

i would be curious why you cant meet factory spec.

if both sides dont meet it i would be suspicious of the mechanic or the spec he is looking at.

[David]

I was standing right there while alignment was being done and I feel pretty good that he knew what he was doing but I've ordered a Longacre AccuLevel so I can check camber and caster before I mess with anything and so I can compare to the alignment readings from last week.

Is the 6.1 that Porsche specs call for in the middle of the adjustment or at the very limit?

[Trackrash]

First question is why do you want more caster? Are you having tire wear or handling issues? Does the steering wheel not want to self center?

There are a couple of things you can do to increase your caster. First you need to find out why your upper strut mounts won't move further to the rear.

In my case, I removed all the black tar crap from the strut mount and slightly enlarged the holes for the three mounting bolts. This allowed me to move the upper strut mount further to the rear.

BTW, what are your other alignment specs? And to be sure, is this a stock Turbo?

[T77911S]

my other question is
why is it so far out of spec. why are both sides so far out of spec. why the same amount
too many odd things going on here.

it would be nice to know which car you are working on.

i was thinking as trackrash was, tar build up but then the chances both sides will only go the exact same amount is odd.


is he putting the car on a machine
if not how is he doing it

i found a guy that does my 930 for $80!!
has the latest and greatest alignment machine.
now i take ALL my cars to him.
some guys here think you need a "porsche" shop to do your alignment.
i think a good accurate machine and a guy that knows what he is doing works great,

[Bill Verburg]

Originally Posted by David

I recently had my alignment done and the mechanic could only get 5.3 degrees of caster on both sides but I'd like to get around 6.5. I see camber kits to get more (or less) camber but nothing on caster. Is there an off the shelf item to get more caster adjustment? Is there any other trick?

There's nothing wrong or bad about 5.3° of positive caster, many if not most 911s will be closer to that than the factory 6°5'+/-15' spec. largely because it's usually difficult or impossible to get to or beyond 6°


you have to understand what caster does and how it interacts w/ other specs

more caster slows the turning response, less caster speeds it up, more caster also increases aligning torque, the self centering of the wheel.

the aligning torque of a 911 also comes from the large scrub radius that 911s run, caster and scrub radius and a very direct rack and pinion setup are what give the 911 such great steering feel.

most track cars will run ~5°5' to speed the steering up a bit especially so for A/X, the small loss of self centering isn't an issue

Additionally more positive caster also reduces tire clearance where the fender and bumper meet, w/ some setups this can be an issue often requiring the flange at that point to be trimmed.

[David]

The handling is fine but I'm still fighting an instability above 85-90 mph. Not sure if it's alignment, looseness in the suspension/steering, or aero.

My recent changes are raising the spindles 30mm, lowering the tie rods to about 1.5" below the steering arm, and going coilover. Along with getting it all aligned and corner balanced after all this, the handling, bump steer, and at speed feel is much better. But something is still not right.

I had a formula racer buddy to tell me to try zero toe versus the .12 degrees total toe in I have now which seems counter intuitive.

So current alignment is:

Total front toe in .12 degrees
Caster 5.3 (most I could get)
Front camber -.5 degrees (least I could get)
Total rear toe in .2 degrees
Rear camber -1.6 degrees (least I could get)
Ride height about 25"

[manbridge 74]

Track car? Most of the latest track tires specify shooting for as much caster as possible.

Street car, older 911s had around 5-5.5 and did fine. Think chopper (lazy tuning) vs sport bike (quick turning) fork angle for a better visual.

[Trackrash]

What tire and wheel combination are you running?

Also with raised spindles, you MUST actually measure and adjust the bump steer. Just dropping the tie rods about 1.5" may not be an optimal setting. AHIK

Also describe your instability. On the track, on the highway, when transitioning into turns, on uneven surfaces, with cross winds, when braking?

[Bill Verburg]

caster spec is 6°5'+/-15', ie from 5° 50' to 6° 20' (5.83° to 6.33°)

25" ride height isn't all that low, I'm not sure why you need raised spindles at that height. I'm not sure what that front end geometry is doing, Did you do a bump sweep to check?

If it's only an instability issue at high speeds I'd suspect an aero component to the problem, all the alignment specs are right where a typical sporty street 911 might want to be. Though 0° toe pressed is the spec for front toe, your static toe isn't far off, forward motion tends to lower the toe which is why pressed is spec'd

[David]

The instability is mainly on back roads with bumps, crown and probably a bit of cross wind. It's not jumping out of the lane or actually darting side to side, it just feels like it wants to which doesn't give confidence to go fast. On smooth highway, I took it up to 120 and it felt good but light on the front making me think aero issue. But I figured a little more caster would help too.

It's a '79 930 (but I'm posting on 911 forum since more traffic) so big whale tail and front spoiler/bumper is a fiberglass 965. Originally I figured since a 965 was stable, I should be too, but now I'm thinking more front downforce or at least less lift is needed.

Something else I did was change the steering wheel from 350mm to 380mm and that feels much better.

[Trackrash]

I had the same issue. It was a combination of tires/wheels, and bumpsteer not being optimal.

Are you still running the same tire and wheel combo you had before this started happening?

[David]

I did not do a bump sweep check but after a track day and back road driving last week, it feels better than it ever has.

With the raised spindles, I expected the A-arms to be angled down more but here's how the look:

[David]

Originally Posted by Trackrash

I had the same issue. It was a combination of tires/wheels, and bumpsteer not being optimal.

Are you still running the same tire and wheel combo you had before this started happening?

I switched from Toyo R888 in 255/315 to Toyo R888R in 225/275 sizes and that seems to have helped. I made a bunch of changes tho so I can't tell what actually helped and what didn't. I've driven 911's of the same age and they feel stable at high speed so something is not right.

[Mahler9th]

I have not read every post in detail.

It seems like you have knowledge and vocabulary, but perhaps do not have a lot of experience and/or confidence with 911/930 set up. If true, perhaps this is in general, or relative to the configuration of your car.

I have been doing my own set up on my 911 for more than a decade. One of the people that helped me when I started created the SmartCamber and SmartStrings tools and was an engineer for the Lizards for a long time. Others include quite a few folks that have run these cars in full professional anger since the high (maryjane days) of IMSA.

I have also helped numerous friends with set up during the last 20 years.

Some of the advice I have seen in this thread is not consistent with any of the advice I have been given nor the experiences I have had.

Just so you know, my car is a '75, running 12 and 14 inch wide Hoosier slicks with 16 inch diameter. It has RSR coilovers in all four corners and weighs about 2000 pounds. Some of my set up parameters may relate to your car and use case, many may not.

In any case, I would seek out local experts familiar with 911 handling and set up, for in-person conversations, as opposed to just appealing to friends with generic knowledge, and "experts" in these forums.

And to optimize the latter, it might be good to post a complete description of the car and your goals for its use.

I think I can kind of piece that information together from the various posts, but it might be more effective if you summarize.

[Mahler9th]

By the way, very few of the racers and shops I know bother to plot and shim for bump steer with raised spindles. Of course, part of the reason why is that there are so many known recipes in terms of parts and specs. But another is likely the general law of diminishing returns.

I would never say that it isn't a good idea, but I would never use the term "must."

For fun I plan to do my car this year. When I do I will seek out folks with pro racing experience with my type of car to help determine targeted specs for the curve. None of those people participate in these forums.

[Bill Verburg]

Originally Posted by David

I did not do a bump sweep check but after a track day and back road driving last week, it feels better than it ever has.

With the raised spindles, I expected the A-arms to be angled down more but here's how the look:

The A arm looks good, a slight down angle as you go out board, It's too low when the A-arm is parallel to the ground or worse slopes up, but it's difference in the arcs of the A arm and the steering arm that causes bump steer, ideally they would move in parallel arcs but in practice that's not possible, The object of the bump steer correction kits is to return the relationship of the arcs back to where it was from the factory after the car has been lowered, The rack kits move the steering rack and hence the inner pivot of the steering arm down, raising the spindles moves the outer end of the steering arm up. Both have the same goal to restore the original steering arm geometry wrt the A arm which has the inner end of it's arc dropped when the car is lowered.

930s have a much bigger issue w/ scrub radius because the tires are pushed out compared to a 911, this increased scrub radius amplifies any geometry related issue, what would be unnoticeable in a 911 becomes a much bigger issue in a 930.

It sounds like the car is suffering from a bump steer issue amplified by the large scrub radius

I'd suggests a bump sweep to see what is actually going on as the wheel moves up and down



On the left is a stock 911 bump(vorspur) curve, the more horizontal the worse it is, the more vertical the better it is, lowering the car moves the X axis down, rack spacers or raised spindles moves the axis up

[Trackrash]

You can try adjusting your tire pressure. Sounds silly, but if lowering the pressure say 4 lbs improves your stability, it is worth a try. In my case stiff tire walls combined with the tread not being allowed to evenly contact the road caused the car to move around unnervingly on uneven roads.

I also found that a small change (around 2mm) in my tierod location made a noticeable difference. If your bump steer is an issue you will most notice it as the car enters gentle turns, as on the freeway.

[T77911S]

when i had new tires put on my 930 they inflated them to i think it is factory spec of around 45psi.
i thought it was the tires since i had some nice MICHs on there before but the car was REALLY squirrelly especially under braking. the car would move all over the place.
i did not check the pressures until i took it to the track.

as bill was saying, ideal tie rod angle goes back to the instantaneous center so that everything is moving on the same arc.

if it is just unstable also check the rear toe.

BTW, my 930 is very stable at 160...
and i am running factory suspension settings. dont know what the caster was set to.