19mm raised front spindles - adjustable bump steer or no?

[donbecker1234]

Getting my front suspension done, I have the KW adjustable with the 19mm raised spindles.

For those with more experience, is going to adjustable bumpsteer a good idea at 19mm?

Or is this typically done at larger than 19mm?

I have the Rebel Racing Adjustable kit

...but just noticed they say something about "28mm" so I wasn't sure if this is not needed or a bad idea over just running stock tierod ends.

I'm replacing struts, bushings, torsionbars, tierods, etc so everything is all over the place settings wise, so that's likely contributing to why it looks like I don't need them.

[Jonny042]

Yes, you do, badly. I have the same suspension and measured the bumpsteer. 28mm isn't enough, I needed to use 42mm of spacers to get my bumpsteer to measure zero. See below:

PS - as long as you are using the proper bump steer kits like the Rebel ones, you don't need the bottom reinforcement brackets for the amount of spacer you'll need.

Originally Posted by Jonny042

Just finished doing bumpsteer measurement and adjustment on the front end - been putting this off for a while but finally dug in.

What actually triggered the whole exercise is that I did a slight ride height adjustment in front, raising the ride height maybe 3/4". I measured the toe before and after and couldn't believe my eyes - I gained 3/16" of toe-in just raising the front that little bit!!

After finally settling on a front ride height of 151mm (measured the factory way but accounting for the 19mm raised spindles) I marked a line on the front bumper and backed the ride height adjustment so I could raise and lower the car around ride hieght and measure the toe changes.

I played with some pretty coarse adjustments and finally honed into doing 2mm of shim changes at a time. All I can say is WOW. A much smaller increment than I thought can make a measurable difference!!!











The picture above shows all the shims in place during one of the "extreme" tests. Ultimately I ended up with 42mm being the magic number for total spacer height. My final check was to run the suspension through 2-1/2" of travel (1-1/4 above and below ride height) and have no measurable difference in toe.

[r lane]

I have no engineering data to support my thoughts on this, but having your steering link not in double sheer is frightening to me esp with that much lever. I am thinking racecar, might not worry me much if it is for the street.

[Bill Verburg]

Yes, you do, badly. I have the same suspension and measured the bumpsteer. 28mm isn't enough, I needed to use 42mm of spacers to get my bumpsteer to measure zero. See below:

Such drama, he's raised the outer ends of the a arm and steering arm 19mm nowhere near the 40mm that you did,

gotta agree w/ rlane, there is a large increase in twist on the steering arm when o/s rod ends are used

It's still in a decent range and if you want to further correct use rack spacers to raise the inner ends of the tie rod

see this discussion

[winders]

Originally Posted by r lane

I have no engineering data to support my thoughts on this, but having your steering link not in double sheer is frightening to me esp with that much lever. I am thinking racecar, might not worry me much if it is for the street.

I ran a race car with really wide front tire for 8 years with as much spacing in single shear as Jonny042 shows in his photo with zero issues. I know of three other racers that did the same with no issues.

[donbecker1234]

Originally Posted by winders

I ran a race car with really wide front tire for 8 years with as much spacing in single shear as Jonny042 shows in his photo with zero issues. I know of three other racers that did the same with no issues.

Thank you for sharing first hand experience, greatly appreciated.

[donbecker1234]

Originally Posted by Bill Verburg

Such drama, he's raised the outer ends of the a arm and steering arm 19mm nowhere near the 40mm that you did,

gotta agree w/ rlane, there is a large increase in twist on the steering arm when o/s rod ends are used

It's still in a decent range and if you want to further correct use rack spacers to raise the inner ends of the tie rod

see this discussion

I have an M491/930 so I believe the rack spacers are "built in" on those as opposed to standard 911's.

[Jonny042]

Originally Posted by Bill Verburg

Such drama, he's raised the outer ends of the a arm and steering arm 19mm nowhere near the 40mm that you did,

I have the exact same suspension as the OP on my white car. KW V3 with 19mm raised spindles.

[Bill Verburg]

Originally Posted by Jonny042

I have the exact same suspension as the OP on my white car. KW V3 with 19mm raised spindles.

Then the better solution for you on that car is the rack spacers

I hate to beat a dead horse but once again

suspension curves for a stock 911 at std 108/16 ride height w/ stock suspension
The relevant curve is vorspurverlauf, toe change


The red line shows where you are w/ a 40mm lowered chassis(yes up on the y axis is suspension compression or chassis lowering), at 40 mm compression there is -34' of toe added over the static setting( yes that is toe in). Each of the y axis ticks is 10mm , so for 19mm drop look up just under 2 ticks then go left to the curve, the slope of the curve is an indication of roll steer at that height, the magnitude is given by the x axis value corresponding to where you are on the curve( it can also be represented by the ordered pair( -34', 40mm), notice that the slope at 19 is the same as at 0, same bump characteristic rate. W/ a stock suspension you may wish to correct that back down, use a 12mm rack spacer and now you are just up 7mm from where it was stock, again since the slope is the same the rate at that point is the same as it was originally

[Mahler9th]

I think of bump steer as a characteristic, not a "number you can measure."

Plotting a bump steer curve is a project.

But then once you have a curve, what do you do?

Of course you can use shims to get a different curve.

But what curve is desirable?

The target might be best informed after considering a number of variables.

For a car like mine, I'd seek advice on desirable curve shape(s) from professionals.

My car's builder (also Winders' builder) has informed advice on tried and true shapes.

I imagine that Craig Watkins also has informed advice.

I imagine that Cary Eisenlohr has informed advice.

These people have experience at the highest levels of racing.

There are many products like adjustable tie rod ends, and struts with options to raise the spindles. And rack spacers. Some of these things are designed for simple "sweet spots."

I don't think there is a simple answer to the question "what bump steer curve shape is best."

And I don't think there is a simple single number for "bump steer."

[Superman]

I had Rebel raise my spindles and weld a bracket below the steering knuckle. Tie rod attaches to a shaft that runs between the two. Very strong, easily adjustable. It was a very good improvement, even on this street car. Drives like it has power steering now.

[Mahler9th]

I also see no reason to "reinvent the wheel" on bump steer.

It can be fun to fool around.

But if optimal handling is what is desired, and one's general configuration is in the "fat part of the curve," then there are likely many folks around that can give advice on optimization.

Trailing arm 911's have been around for a long time and have been raced for a long time. Plenty of experience out there if one seeks out the expertise.

[donbecker1234]

Originally Posted by Mahler9th

I also see no reason to "reinvent the wheel" on bump steer.

It can be fun to fool around.

But if optimal handling is what is desired, and one's general configuration is in the "fat part of the curve," then there are likely many folks around that can give advice on optimization.

Trailing arm 911's have been around for a long time and have been raced for a long time. Plenty of experience out there if one seeks out the expertise.

I'm in the Atlanta area. If you (or anyone else) has a suggestion on a shop I'd be happy to hear it.

I purchased the Rebel kit as I had read about others doing it. Now that I have the suspension on, I'd rather just get it to a shop for proper alignment and setup.

[Bill Verburg]

Originally Posted by Mahler9th

I think of bump steer as a characteristic, not a "number you can measure."

Plotting a bump steer curve is a project.

But then once you have a curve, what do you do?

Of course you can use shims to get a different curve.

But what curve is desirable?

The target might be best informed after considering a number of variables.

For a car like mine, I'd seek advice on desirable curve shape(s) from professionals.

My car's builder (also Winders' builder) has informed advice on tried and true shapes.

I imagine that Craig Watkins also has informed advice.

I imagine that Cary Eisenlohr has informed advice.

These people have experience at the highest levels of racing.

There are many products like adjustable tie rod ends, and struts with options to raise the spindles. And rack spacers. Some of these things are designed for simple "sweet spots."

I don't think there is a simple answer to the question "what bump steer curve shape is best."

And I don't think there is a simple single number for "bump steer."

You are entitled to your opinion, and I guess if you can't or don't want to think about it then by all means go to a shop that you trust, but the general consensus is that you want to minimize bump or roll steer, and that you would rather have toe in on compression than toe out, though either can be dealt w/

measuring is a tedious time consuming project but one can learn a lot from those that have done it

above are the factory curves, I trust them to have done it right

They are provided for each reader to do their own thinking on the subject

[Bill Verburg]

Originally Posted by donbecker1234

I have an M491/930 so I believe the rack spacers are "built in" on those as opposed to standard 911's.

not as far as I know, but I don't generally work w/ 930s

the 930s do have the cross member raised by 13mm and if I remember correctly a bit more anti-dive geometry, this does raise the inner end of the a-arm by 13mm which does move you down by 13mm on the above curves

they also got the mono-ball steering connection

[donbecker1234]

Originally Posted by Bill Verburg

not as far as I know, but I don't generally work w/ 930s

the 930s do have the cross member raised by 13mm and if I remember correctly a bit more anti-dive geometry, this does raise the inner end of the a-arm by 13mm which does move you down by 13mm on the above curves

they also got the mono-ball steering connection

Hi Bill, yes that's it, the anti-dive. I recall reading about it when researching awhile back.

The mono-ball steering, I think you are referring to turbo tie rods. I just put those on, the M491's came with the non-turbo tie rods originally.

[Mahler9th]

Sorry to perhaps be misunderstood.

The factory curve shape(s) are for a set of variables, including the specific model and parts.

PAG have been shipping cars for a long time, but the factory settings are based on a set of criteria that are factory criteria. Their criteria may not be optimal for some folks seeking better handling.

Take ride height for example. Some models have been shipped with ride heights set to meet prevailing bumper height laws. Often better handling can be achieved with lower ride heights. We know that for example because some models have shipped with lower ride heights ex-US.
For years folks have sought after and achieved improved handling with factory parts and/or a few aftermarket parts, and this has often involved deviations from factory settings.

Seeking greater negative camber than stock settings is an example.

In my experience with "stockish" trailing arm 911's, the vast majority of improvements are achievable with a small range of parts and settings.

During my 30 years of performance driving and racing, I don't know of many shops that have supported AX'ers and racers that have recommended and/or performed bump steer curve measurements and optimization on a per car basis, since the fat part of the improvement curve can usually be achieved with known recipes.

I have been doing my own set up (CB and alignment) for 25+ years. I have sought advice from folks with experience with my type of car. Advice on parts and set up parameters.

In my experience, finding folks that can give good advice on set up may best be accomplished via networking, for example via PCA connections and/or shops that have and/or do support clients that engage in performance driving activities like AX, DE and racing.

In my experience corresponding about Porsches on the internet since about 1992 or 1993, Forums can be helpful, but over time there are fewer and fewer experts participating with the type of experience to which I refer.

[Jonny042]

There physically isn't room for enough rack spacer to correct for the raised spindles.

Even if all you wanted to do is get back to the stock curve, you'd have to use 19mm of rack spacer. This due to the fact that raising the spindle effectively worsens the relationship of the tie rod to lower control arm. If you want to go further with a slightly lower ride height and add 15mm more rack spacer, to be equivalent to the Bruce Anderson recommendation for lowered cars (which you cite as the solution you are happy with) you're up to 34mm of rack spacer just to get to the equivalent point where you are at with your car. Since there isn't room for that sort of nonsense above the front crossmember, the bump steer kits that replace the outer tie rods come into play.

Originally Posted by Bill Verburg

Then the better solution for you on that car is the rack spacers

I hate to beat a dead horse but once again

suspension curves for a stock 911 at std 108/16 ride height w/ stock suspension
The relevant curve is vorspurverlauf, toe change


The red line shows where you are w/ a 40mm lowered chassis(yes up on the y axis is suspension compression or chassis lowering), at 40 mm compression there is -34' of toe added over the static setting( yes that is toe in). Each of the y axis ticks is 10mm , so for 19mm drop look up just under 2 ticks then go left to the curve, the slope of the curve is an indication of roll steer at that height, the magnitude is given by the x axis value corresponding to where you are on the curve( it can also be represented by the ordered pair( -34', 40mm), notice that the slope at 19 is the same as at 0, same bump characteristic rate. W/ a stock suspension you may wish to correct that back down, use a 12mm rack spacer and now you are just up 7mm from where it was stock, again since the slope is the same the rate at that point is the same as it was originally

[Jonny042]

Originally Posted by Mahler9th

During my 30 years of performance driving and racing, I don't know of many shops that have supported AX'ers and racers that have recommended and/or performed bump steer curve measurements and optimization on a per car basis, since the fat part of the improvement curve can usually be achieved with known recipes.

It's a VERY time consuming task. Which probably accounts for the rarity of it. You also make the point that we're all driving a car with suspension geometry was carved in stone in the early 60's and didn't change till 1989.....

Along the way someone came up with the bright idea of rack spacers, found that they had room for 15mm, and said "good enough". For many cars, yes.

The point I'm trying to make is that if you have raised spindles you can do much better. I don't know why that's such a revolutionary idea. Not like I came up with it on my own. For crying out loud isn't there a reason you can buy them off the shelf?

(EDIT - for those following along, and to the OP with his 19mm raised spindles, if you don't find a shop willing to take on the task of actually measuring the bumpsteer curves, If you use in the range of 35-40mm of spacer with the rebel/tarret/elephant/rennline style kits, you will be in a safe range, equivalent to running 15mm of rack spacer on a car without the raised spindles.)

[smadsen]

Why does anyone care about bumpsteer in droop? If you've identified a three-inch sweetspot (of nominal zero bump), I'd adjust so its all located above ride height. Otherwise you're just adjusting your droopsteer, a relatively meaningless component after the wheel eventually leaves the ground.