Question about scrub radius

[PcarPhil]

Hey guys,

I've got a question about scrub radius. Take the following front setups (911 based front suspension components, not 930, no spacers):

-7x16 911 Fuchs with 205/55 tires (+23.3mm)
-8x16 951 Fuchs with 225/50 tires (+23.3mm)
-9x16 Fuchs with 245/45 tires (+15mm)

Looking at Bill's chart it seems that the scrub radius for all 3 of these setups is about the same? Is that correct?

Here's Bill's chart:

Link to chart:
http://members.rennlist.com/1976c38/

Thanks for the help!

[PcarPhil]

Here is the same chart but with a few lines added:



It looks like scrub radius with the 9" Fuchs is the worst of the 3 combos...but...still not too bad. Better than 8" Fuchs with 1" spacers anyways.

[Bill Verburg]

that's correct, the factory made a real effortto keep scrub radius close to the smae no matter wwhat wheels were used. The front is particularly critical as that is where the steering feel comes from

[PcarPhil]

Originally Posted by Bill Verburg

that's correct, the factory made a real effortto keep scrub radius close to the smae no matter wwhat wheels were used. The front is particularly critical as that is where the steering feel comes from

Another idea with using 9x16 Fuchs up front:

If I swapped the 911 front hubs for 912 hubs, and removed the strut dust covers, that would bring the wheel centerline in another 1/4" or so. That almost puts the centerline of the 9" wheel where the 7" wheel is on your chart.

I guess I need to check my caliper clearances with the 911 hubs and 9" wheels to see if I could move the wheels in 1/4". I sort of doubt it.

[PcarPhil]

Originally Posted by Kaefer

Another idea with using 9x16 Fuchs up front:

If I swapped the 911 front hubs for 912 hubs, and removed the strut dust covers, that would bring the wheel centerline in another 1/4" or so. That almost puts the centerline of the 9" wheel where the 7" wheel is on your chart.

I guess I need to check my caliper clearances with the 911 hubs and 9" wheels to see if I could move the wheels in 1/4". I sort of doubt it.

I went out and took a look at the front setup. It does appear that 9x16 Fuchs could work with Carrera calipers and 912 hubs. 4x4 off road shock covers could replace the OEM strut dust covers (or run no covers).

So 912 hubs + 9x16 Fuchs appear to approach (if not equal) the scrub radius of a stock 911 front end with 7x16 Fuchs. That's pretty cool!

I guess the next question would be what year 912 hubs could be used to replace 911 hubs?

[Bill Verburg]

Most owners are more concerned w/ filling the wheel well than scrub radius

anyhoo
there was a thread on Early S registry about different hubs

This pic is from that thread

[Tim K]

Originally Posted by Bill Verburg

This pic is from that thread

Hmmm. No luck finding the thread. Do you have the link?

Tim K

[Jack Olsen]

There are a few different ways to skin a cat. On my car, Tyson Schmidt extended the front control arms by 35mm and tweaked the suspension geometry so that my 9.5-inch front wheels would have better scrub radius than an early car with 6-inch wheels.

Here's how he explains it:

On Jack's car, I changed the front control arm length. (longer)

Raised the spindles beyond what was possible on the RSR, due to utilizing 17" wheels versus the RSR's 15's.

Changed the steering axis inclination with custom geometry front struts for better scrub-radius in conjunction with deeper offset front wheels. (The altered geometry allowed deeper offset while retaining the wider track.)

These changes result in improved steering balance, particulary under hard braking. (from the reduced scrub-radius)

Higher front roll-center height, for more built-in resistance to body roll- also keeps camber curve in the proper range when car is set very low. (raised spindles)

Less roll-center height change as the suspension moves through it's travel, for more consistent/predictable handling. (longer control arms)

Reduced scrub-radius also benefits tire clearance during turns, since the wheel pivots closer it's center-line instead of swinging forward and rearward in the wheel-well. (Notice how oblong the front flares are on an original RSR, which were necessary for tire clearance with it's very positive scrub radius, combined with it's 9" wide wheels)

[Bill Verburg]

Originally Posted by Jack Olsen

There are a few different ways to skin a cat. On my car, Tyson Schmidt extended the front control arms by 35mm and tweaked the suspension geometry so that my 9.5-inch front wheels would have better scrub radius than an early car with 6-inch wheels.

that's the correct way to do it. All 911 have a little (+) scrub radius, which gives them their lively steering wheels. All ABS equipped(964 up) have a little bit of negative to keep them going straight w/ ABS functioning. 993 has a little more than 964 for sportier feel, never looked into what they did w/ 996 or 997.

So when wider wheels w/ less o/s are used the A arm and king pin angle need to change to compensate.

[EarlySport]

I think the most difficult part of extending the front control arms ( for me anyway ) is how to correct the amount of negative camber gain.

I don't want to end up with more than about 3 degrees, and I'm pretty sure I'll have to do something with the strut/spindle. That said I think this is a very worthwhile mod and I'll definitely be doing it.

Some discussion from my thread here

Extended Front Control Arms ?

[Jack Olsen]

We had to build the strut with positive camber added, knowing that its new angle would skew things in the other direction.

[PcarPhil]

Originally Posted by EarlySport

I think the most difficult part of extending the front control arms ( for me anyway ) is how to correct the amount of negative camber gain.

I don't want to end up with more than about 3 degrees, and I'm pretty sure I'll have to do something with the strut/spindle. That said I think this is a very worthwhile mod and I'll definitely be doing it.

Some discussion from my thread here

Extended Front Control Arms ?

Craig, Bill, Jack, and others,

I agree - extending the control arms and repositioning the strut/spindle is a more proper way of dealing with scrub radius on a widebody car. Me being me though needs to check to see what I can come up with for various approaches to reducing widebody scrub radius. Hence the question about 912 hubs.

I really appreciate the knowledge being passed on here. I'm not one to bolt 2" spacers and 7" Fuchs to the front of a 911 widebody and call it a day.

[J P Stein]

I have a problem with increasing the length of the A-arm......tho it will indeed improve the scrub radius. My Bilstein shock inserts showed serious wear where the rod passes thru the shock body.....took off the chrome & got into the base metal for an area nearly an inch long on the outer side. This is with a stock A-arm. Increasing the angle of the strut would only exacerbate this, I figure.

BTW, I was running 10 inch wide wheels with slicks. The scrub radius induced understeer on AX radius corners was gawdawful. We live & learn.
8 inch wide wheels & a slightly narrower slick cured this.

Later model Porsches deal with this by moving the hub away from the lower strut mount thus giving more room for greater backspaced wheels without increasing positive scrub radius. The strut may also be less inclined, inboard to outboard.

[Dantilla]

Tyson Schmidt is WAY smarter than me.

[EarlySport]

Originally Posted by J P Stein

I have a problem with increasing the length of the A-arm......tho it will indeed improve the scrub radius. My Bilstein shock inserts showed serious wear where the rod passes thru the shock body.....took off the chrome & got into the base metal for an area nearly an inch long on the outer side. This is with a stock A-arm. Increasing the angle of the strut would only exacerbate this, I figure.
.

I'm not 100%, but I think the extended A arm will REDUCE the side load on the strut. I think the most load comes from a long 'wheel centre to spindle join on strut'. Reducing this by lengthening the arm and then removing spacers ( or changing wheel offset ) will reduce the side load on the strut.

I could be wrong..

[Chuck Moreland]

Originally Posted by J P Stein

I have a problem with increasing the length of the A-arm......tho it will indeed improve the scrub radius. My Bilstein shock inserts showed serious wear where the rod passes thru the shock body.....took off the chrome & got into the base metal for an area nearly an inch long on the outer side. This is with a stock A-arm. Increasing the angle of the strut would only exacerbate this, I figure.

BTW, I was running 10 inch wide wheels with slicks. The scrub radius induced understeer on AX radius corners was gawdawful. We live & learn.
8 inch wide wheels & a slightly narrower slick cured this.

Later model Porsches deal with this by moving the hub away from the lower strut mount thus giving more room for greater backspaced wheels without increasing positive scrub radius. The strut may also be less inclined, inboard to outboard.

I don't think lengthening the control arm will exacerbate the wear you see, which is not normal.

Bilsteins strut bodies have what amounts to a linear bearing near the top which rides on the insert in the area you saw wear. Immediately above the linear bearing is supposed to be grease packed, retained by a seal.

I suspect your grease/seal was inadequate.

[J P Stein]

Originally Posted by Chuck Moreland

I don't think lengthening the control arm will exacerbate the wear you see, which is not normal.

Bilsteins strut bodies have what amounts to a linear bearing near the top which rides on the insert in the area you saw wear. Immediately above the linear bearing is supposed to be grease packed, retained by a seal.

I suspect your grease/seal was inadequate.

I'm not talking about the upper part of the shock insert that passes thru the strut housing, but the inner rod that passes into the shock body. This rod is approx. .400 diameter. The wear may well have been caused by the wide wheels increases mechanical advantage over the upper "linear bearing"....which "feel" tight. At any rate, there was an angular distortion at work here. Raising the spindle also is contributory.

[rfloz]

Originally Posted by Bill Verburg

that's correct, the factory made a real effortto keep scrub radius close to the smae no matter wwhat wheels were used. The front is particularly critical as that is where the steering feel comes from

Is any of this data available for 930 fitments?

As I understand it, the early 930s had an approximately 1" spacer in front while later ones had none, but apparently the hub itself was extended the same distance. In the rear, I am informed all 930s (and Factory Turbo-Looks) had a 28mm spacer in the rear (there was one mention of a 2" spacer in the rear of older 930s).

There are also differences in the rear trailing arms and possibly the hubs compared to Carreras.

My own car is a 1984 Factory Turbo-Look with no spacers in front and the 28mm spacer in the rear (identical, I am told, to 1984 through 1989 930s). I have ordered 9 & 11 1/2 CCWs and would like to at least be able to figure out what effect the new wheels will have on scrub radius.

Of course, as mentioned, for auto-cross use filling the wheel well with the largest available wheel/tire combination is the priority. But if, e.g., a small spacer might improve things geometric, that would be good to know.

[PcarPhil]

Bob,

Do you have the offset and backspacing specs of your new wheels?

Originally Posted by rfloz

Is any of this data available for 930 fitments?

As I understand it, the early 930s had an approximately 1" spacer in front while later ones had none, but apparently the hub itself was extended the same distance. In the rear, I am informed all 930s (and Factory Turbo-Looks) had a 28mm spacer in the rear (there was one mention of a 2" spacer in the rear of older 930s).

There are also differences in the rear trailing arms and possibly the hubs compared to Carreras.

My own car is a 1984 Factory Turbo-Look with no spacers in front and the 28mm spacer in the rear (identical, I am told, to 1984 through 1989 930s). I have ordered 9 & 11 1/2 CCWs and would like to at least be able to figure out what effect the new wheels will have on scrub radius.

Of course, as mentioned, for auto-cross use filling the wheel well with the largest available wheel/tire combination is the priority. But if, e.g., a small spacer might improve things geometric, that would be good to know.