offset versus scrub radius Options

[r_towle]

Does it make a difference what the offset is on a wheel with regards to scrub radius?

It does not seem that it would, given the stock suspension, no custom struts etc.

Aside from that, what is the reason (aside from cost and availability) for using large spacers versus getting wheels that mount up with no spacer, yet are wide for flared cars?

rich

[Matt Romanowski]

Does it make a difference what the offset is on a wheel with regards to scrub radius?

It does not seem that it would, given the stock suspension, no custom struts etc.

Aside from that, what is the reason (aside from cost and availability) for using large spacers versus getting wheels that mount up with no spacer, yet are wide for flared cars?

rich

I think you answered your own questions. Scrub radius is from the king pin axis at road level to the center of the contact patch at road level. Any wheel spacing, offset, etc change to the rim and it changes the scrub radius (assuming all else equal).

People don't buy new wheels because they are expensive and spacers are cheap.

I personally believe that people put far too fine of a point on scrub radius. People understand it so they think they should go crazy trying to manage it.

[r_towle]

Still confused, sorry.
Just wondering if a wheel built with no spacer, which I happen to like the look of, would make th car handle worse or better....

So, deep dish wheels, front and rear.

[J P Stein]

The following sketch does not represent all of the aspects of SR vs its effects....but the best I got.
It does show the method of measuring SR and one fella's attempt to fix it thru camber changes

An increase in SR does indeed effect the car & handling, basically one increases the leverarm of the axle against rotation....leading to increased steering effort....that's the easy part to figure out.

The increase also puts the CL of the tire/wheel farther fom the center of rotation.This gets this center "out ahead"of the rotation when turning....not a problem for minor (say 5-10 deg) steering inputs. Major inputs , such as found at AX, are another story. These will cause the tire to "shuffle" sideways rather than roll. ....been down this road with 10 inch wide front wheels...giving me about 2.5+ inches of SR.....resulting in an increable PUSH.

Fixed it with 8 inch wide wheels but with only a .5 inch loss in tire width.

[r_towle]

Seems to me, given my original quest, that anything larger than stock wheels makes the scrub radius worsen.

It does not seem to matter what the offset is at the end of the day.
I say that with the knowledge that I will keep the inside dimension the same, roughly 4.5 inches of backspacing....
I cant go more than that or it rubs as we know.

So, the wider the tire gets, or if I use a spacer I just move the thinner tire out farther...either one makes the scrub radius increase and become less than optimal.

Not intending to chang the suspension geometry, at least on this go -around...

Got it.
I think.

Rich

[J P Stein]

One is limited by the 914s front suspension. The most backspace I could phisically get on the car was 5 inches. That does not prevent an increase of wheel width.
I ran an odd set up of 8 wide (Porsche stock backspace) inch with cantis & 10 inch straight side silcks/wheels (5 inch backspace) in back. Not an ideal set-up but good enough for SCCA NTs in EM or XP.....twice as many wins as losses & free tires are gud.(IMG:style_emoticons/default/biggrin.gif)

The 8 inch fronts increased the SR about 1.25 inches over stock. Apparently that was not too much as I could still dial out push in the standard way. I do know that another inch will get ya. Oversteer is good at AX.

The front Hoosier R35 cantis were about 9.5 inch tread width and it is tough to get heat into the tire at AX with a light car (about 350 lbs per front wheel). Them first couple runs were ineresting tho.....I think tire tread width is kinda self-limiting.

All the BS that Brit is going thru with the car started because of the limitations of the 914s front suspension......there's a clue there. (IMG:style_emoticons/default/smile.gif)

I have seen other makes & models cars go thru this same problem.Those that cured it went pretty much the same way. Those that never did finish somewhere behind them.

[andys]

And increased scrub radius creates a cross jacking effect, though that's not necessarily a bad thing depending on your handling goals. Racing Karts run huge amounts of scrub radius just for that reason.

BTW, what is the stock 914 wheel backspacing, 4.5 inches?

Andys

[zig-n-zag]

In an effort to eliminate front spacers, I found some late 911 turbo hubs.

They are 22mm wider over standard 911 hubs.

I don't know what effect they will have on the scrub radius.

[SirAndy]

In an effort to eliminate front spacers, I found some late 911 turbo hubs.
They are 22mm wider over standard 911 hubs.
I don't know what effect they will have on the scrub radius.

Neither spacers nor wider hubs or even backspace or wheel offsets are really relevant.

They all can affect the scrub radius, but the real answer to take away from all of this is:
Whatever you use, make sure your front wheels are as close to the strut as possible without rubbing on the strut or fender.

(IMG:style_emoticons/default/driving.gif)

[r_towle]

Lots of great knowledge.
Seems that my initial question of basically "can I run deep dish 8 inch wheels on the front, with no spacers is a yes.
I am not a fan of spacers for a lot of reasons, but I also love the look of deep dish wheels....so that is today's plan.
Rich

[Randal]

The following sketch does not represent all of the aspects of SR vs its effects....but the best I got.
It does show the method of measuring SR and one fella's attempt to fix it thru camber changes

An increase in SR does indeed effect the car & handling, basically one increases the leverarm of the axle against rotation....leading to increased steering effort....that's the easy part to figure out.

The increase also puts the CL of the tire/wheel farther fom the center of rotation.This gets this center "out ahead"of the rotation when turning....not a problem for minor (say 5-10 deg) steering inputs. Major inputs , such as found at AX, are another story. These will cause the tire to "shuffle" sideways rather than roll. ....been down this road with 10 inch wide front wheels...giving me about 2.5+ inches of SR.....resulting in an increable PUSH.

Fixed it with 8 inch wide wheels but with only a .5 inch loss in tire width.

I listened to what you said JP and reduced my tire size to a little over 8" with 5" offset. Hard to believe but I've experienced no under steer since. The Avon tires could have helped, but honestly I haven't found the limit on corners yet.

Probably should go out and just push the hell out of the car until I find the limit, but those Avons are expensive; meat grinding them just isn't on.

[J P Stein]

For anyone that has the chance........
Take a look at Frank Stagrano's (sp?) SCCA CP class dominating Mustang.
Note the front wheels......now look at those that are .....er....not dominating.

I had a brief conversation with him. IIRC, he said that was the 4th or 5th front end set-up on his car. One thinks someone else would notice....unless they're too busy building V-8 horsepower. (IMG:style_emoticons/default/confused24.gif)


Randal:
You went a bit over my front tire widths and it sounds like it worked. If so and you over cook it, you should hit the wall backwards. (IMG:style_emoticons/default/biggrin.gif)

[Matt Romanowski]

The following sketch does not represent all of the aspects of SR vs its effects....but the best I got.
It does show the method of measuring SR and one fella's attempt to fix it thru camber changes

An increase in SR does indeed effect the car & handling, basically one increases the leverarm of the axle against rotation....leading to increased steering effort....that's the easy part to figure out.

The increase also puts the CL of the tire/wheel farther fom the center of rotation.This gets this center "out ahead"of the rotation when turning....not a problem for minor (say 5-10 deg) steering inputs. Major inputs , such as found at AX, are another story. These will cause the tire to "shuffle" sideways rather than roll. ....been down this road with 10 inch wide front wheels...giving me about 2.5+ inches of SR.....resulting in an increable PUSH.

Fixed it with 8 inch wide wheels but with only a .5 inch loss in tire width.

That diagram has nothing to do with camber. The guy is changing the strut to spindle angle, which changes the scrub radius. Changing just the camber of the wheel has no affect on scrub radius.

[Randal]

For anyone that has the chance........
Take a look at Frank Stagrano's (sp?) SCCA CP class dominating Mustang.
Note the front wheels......now look at those that are .....er....not dominating.

I had a brief conversation with him. IIRC, he said that was the 4th or 5th front end set-up on his car. One thinks someone else would notice....unless they're too busy building V-8 horsepower. (IMG:style_emoticons/default/confused24.gif)


Randal:
You went a bit over my front tire widths and it sounds like it worked. If so and you over cook it, you should hit the wall backwards. (IMG:style_emoticons/default/biggrin.gif)

Your right.

Going to dial in a bit more front bar for the Cascade Lakes road course. Last year the car was loose on the third corner, which is a flat to off camber left.

[J P Stein]

That diagram has nothing to do with camber. The guy is changing the strut to spindle angle, which changes the scrub radius. Changing just the camber of the wheel has no affect on scrub radius.

Gee, I left out some context on the sketch. That will almost always get a rise from some expurt.

The author(?) of the sketch changed his strut angle to increase the *obtainable* negative camber on his 911. The mod had the effect on SR also....two good reasons to do the mod. The downside is the slug (or extra seam) in the A arm...I'm agin' it..do I have to explain the term "slug" + the whys & wherefores?

I too modded my A arms but only to increase strength in the area I'd seen them break.

[J P Stein]

Going to dial in a bit more front bar for the Cascade Lakes road course. Last year the car was loose on the third corner, which is a flat to off camber left.

Your choice, of course, but I wouldn't change the set up to suit one corner.
I've been caught out in nasty "off camber" situations also.....but just once per event.
One is well served by easing off there...... (IMG:style_emoticons/default/biggrin.gif)

[Matt Romanowski]

That diagram has nothing to do with camber. The guy is changing the strut to spindle angle, which changes the scrub radius. Changing just the camber of the wheel has no affect on scrub radius.

Gee, I left out some context on the sketch. That will almost always get a rise from some expurt.

The author(?) of the sketch changed his strut angle to increase the *obtainable* negative camber on his 911. The mod had the effect on SR also....two good reasons to do the mod. The downside is the slug (or extra seam) in the A arm...I'm agin' it..do I have to explain the term "slug" + the whys & wherefores?

I too modded my A arms but only to increase strength in the area I'd seen them break.

If we want to be rude about it, look at the drawing again, expurt. He increased the kingpin inclination angle, which would reduce the amount of camber he could get provided he didn't move his top strut mount inward.

We could say he is making the strut to spindle angle more obtuse.

Edit: And to help you out, I did notice he lengthened the A-arm. Without the exact numbers of what was done, you don't know the changes that were made. Most likely he ended up in the same boat he was in before. If you want to do these things right, you have to run the numbers through a suspension design program to find out what you are really getting.

Also note, real race teams will change roll centers for different tracks, but don't mess with scrub radius. It's defined by the uprights which are the design of is rarely changed.

[andys]

Slight hijack, but since we're talking scrub raduis I though it might be best to ask here while everyone is still motivated :-) On my LS1 conversion car, I'm running 205-50-17 fronts with about 1/4" - 5/16" scrub; I think, if memory serves me correctly, I have 4 degrees caster. Since, IMO, my car steers like a dang truck, will taking out that scrub radius and going to zero have any significant effect on steering effort?

Thanks,

Andys

[Randal]

Going to dial in a bit more front bar for the Cascade Lakes road course. Last year the car was loose on the third corner, which is a flat to off camber left.

Your choice, of course, but I wouldn't change the set up to suit one corner.
I've been caught out in nasty "off camber" situations also.....but just once per event.
One is well served by easing off there...... (IMG:style_emoticons/default/biggrin.gif)

Think I'll sneak up on that corner and leave the car alone. If the weather is warm, and it looks to be that way given the forecast, I'll just let the Avon's do their thing.

To be honest changing anything on the car before running that hill gives me a certain level of nervous apprehension, i.e., the willies!

[andys]

You know, I have to appologize for throwing out some numbers before really thinking enough about what I typed.

The wider than stock tire has a centerline offset that I didn't consider (I'm going to assume stock is zero scrub radius). The wider tire has an additional 1/2" differential at the centerline to a stock 914 tire/rim. If I were to run the same backspacing as the stock tire/rim, then I would indeed have 1/2' scrub radius. Now, add to that the original 1/4" - 5/16" in my above post, the total scrub radius would be 3/4" - 7/8". Tire diameter plays a roll too, as the 17" tire is 2 1/2" larger in diameter than what was on the car in its stock configuration. As a former racer I understand this is just a bunch of words since what really needs to happen is to perform actual measurements, but I'm not that energetic any more :-(

Since I was intending to put the car on the track to tune and balance the chassis/suspension, I adjusted in a bit of camber; 1.2 degrees. I won't run that much on the street, but that amount of camber will probably result in increased steering effort as well.

Andys