Making the a-arms "wider"...besides camber issues,, what other "gotchas" will I hit?? Options

[J P Stein]

QUOTE (Bigbohr @ Mar 29 2006, 06:08 AM)

This is what a 964/993 front looks like: the strut is more inclined towards the middle of the car, plus the strut center at hub level is way inward from where the balljoint is. Both of these features allow you to run higher offset rims and larger tires, plus this reduces scrub radius. I think a 993 actually has a slightly negative scrub rad. If you look at a front hub, you may be surprised to see where the strut is connected to the hub, it's behind the hub center. It looks like Porsche did that to allow passage for the front axles on a Carrera 4. The steering arm is also at a different elevation. Anyways, the geometry is very different.

I need pics, damnit. (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

[groot]

Ummmm, drugs are bad, umkay.

QUOTE (Brett W @ Mar 29 2006, 05:50 AM)

Scrub radius is bad.  This is one of the major problems with the strut suspension.  When you increase the tire width you have to increase the scrub radius.  The more scrub radius you have the harder the car will be to drive and the more effort the driver willl have to put in.  Many race cars are going to a zero scrub radius design, but that has its draw backs as well.

Brett,

Hook me up with some facts, bro. Steering effort... okay, increased loads into the knuckle... okay. I'll buy that, those things are bad. What else makes it bad?

JP,
I'm not sure I buy "skidding through the pits" due to scrub radius (maybe ackerman), but I'm willing to hear arguments.

[DNHunt]

This technical stuff is really hard to picture. We need Miles to do a drawing of this stuff. He just has a way of being able to clear things up.

Ok go easy on me guys. This is my first try at understanding this. As I see it and as JP explained it to me, a line drawn through the centerline of the damper in a strut meets the ground under the tire. A line through the center line of the wheel meets the ground and the distance between them is the scrub radius. Oh shit, I'm all screwed up already. Maybe the line goes through the king pin. Help Miles!! (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

Dave

[groot]

Picture this.... as the tire patch creates grip, that force acts through the suspension and since your suspension does not directly control that point, a moment is created (offset force creating twist).

The scrub radius is the difference between the center of the tire force and the point at which the force acts in the suspension. Viewed from head on you can imagine a line projected on the ground from the strut.

[DNHunt]

This isn't as good as a drawing from Miles but, it helped me. Read about Steering axis inclanation, Included angle and Scrub radius

Dave

[groot]

That's where I stole the drawing... sweet.

BTW... in real life it's rarely the center of the tire due to static camber. The center of pressure of the tire with any camber will be inboard of the center of the tire.

[Joe Ricard]

Dave you need this upgrade

[DNHunt]

Hey Joe

Remember Mile's drawing of a cam. That's some funny shit. I miss him.

Dave

[Joe Ricard]

at the risk of going "off topic" yup I remember. I was just looking for that pic. I think it is on my home PC.

[Bigbohr]

I'm a lill bored at work ... so I made this illustration of what a 993 strut looks like. The strut and hub are in black, the A-arm red. The green line connects the top of the strut and the balljoint center. That's the (virtual) pivot when you turn the wheel. This green line shows what the scrub radius is. Looks like I exaggerated that part a bit. The green line shouldn't touch the ground that far outboard.

[J P Stein]

Kevin:
You may be wise not to agree, but.....this wasn't a problem before
the 10 inch wide tires & wheels. I guessing it is a scrub radius issue.
Nothing else was changed.

[DNHunt]

So, the choices to reduce scrub radius are change 1)the steering axis inclanation 2) reduce camber 3) use a narrower tire 4) use a wheel with more backspacing. I'm guessing most of you fast guys wouldn't want to do 2 or 3 and 4 is limited by the physical contraints of the wheel well and strut. Mueller talked about lengthening the A arm which changes the steering axis inclanation so it changes scrub radius but it gives the strut more inclanation. JP has mentioned that he put divots in the damper shaft already and suspects it's side loading if I remember right. So I suspect that change isn't too great either at least for a car that is being driven hard. It seems like a dilemma to me. I think you guys are exceeding the limits of the front suspension.

I realize this is a dangerous thing to suggest but, Mike I think you need to broaden your focus and move the damper inboard, add an upper A arm, a new steering knuckle and spindle assembly and some bracing through the trunk. Oh and try to make it bolt right in, well maybe a little cut and weld. I think you can do it. Remember, you are my inspirational leader.

Dave

[groot]

Gottcha, JP. Scrub radius will f@#$ you for static parking efforts (i.e. steering the car when it's not moving and you're the tire would have to actually displace in addition to twisting on its patch), but rolling efforts shouldn't change that much at low speeds because the tire patch is constantly refreshing.

I still contend that scrub radius ain't the evil thing being suggested if there isn't a lot of compliance in the suspension and your knuckle and arms (human arms, that is) are up the increased stresses.

[Mueller]

JP needs powersteering (IMG:http://www.914world.com/bbs2/html/emoticons/driving.gif)

hmmm....for some "odd" reason, I just happen to have this cut down strut sitting in a box at home...now what to do with it??? (IMG:http://www.914world.com/bbs2/html/emoticons/idea.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/smash.gif) (IMG:http://www.914world.com/bbs2/html/emoticons/welder.gif)

[J P Stein]

OK, here's the way I see the problem...no pics (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)
"Normal" positive scrub radius adds steering feed back to the steering wheel. Let's figure (WAG) .50 inches with a 5.5 inch wide 914 wheel. As the tire turns from straight ahead, it describes an arc of .5 radius. The center of forward motion comes from the rear wheels to the center of the ball joint......pushng the front wheels foreward. If the tire is turned
45 deg, the center of the tire is leading the ball joint by about .25 in.....easily overcome as there is little mechanical advantage.

My 10 inch wide tire/wheel sets the scrub radius to around 2.25 inches futher out ....all the offset is outboard..for a total
scrub radius of positive 2.75 inches. At 45 deg, that would mean the center of the tire is 1.37 inches foreward & an equal amount outboard of the balljoint.....the force is pushing into that causing the tire to skip sideways.
Granted, max lock is maybe 20 deg deflection. I used 45 to simplify the calcs and the actuals would be half that.
Then again, I could well be fulla shit (IMG:http://www.914world.com/bbs2/html/emoticons/wacko.gif)
Either way, I'm getting that radius down to about 1.25 with my "new improved" set-up.....we'll see.

[groot]

JP, good illustration (figuratively). I agree with you that the same sweep angle creates a larger arc with a larger scrub radius.... which is why I believe scrub radius can increase the required steering wheel torque/efforts.

Where I disagree is in how is effects the car. Let's discuss.... As you change the steering wheel angle scrub radius matters (increased efforts, larger arc by the wheel for a given steering wheel angle), no question. But, if you're at a fixed steering wheel angle, does the scrub radius matter? It shouldn't, unless compliances are in the equation.

Hmmmmm, I think I've talked myself into believing that positive scrub radius can cause the front tires to skid, but only as your changing the steering wheel angle. I do think it makes sense that the faster you go the smaller this problem because the tire patch refreshes more quickly.

[J P Stein]

QUOTE (groot @ Mar 30 2006, 05:00 AM)

Hmmmmm, I think I've talked myself into believing that positive scrub radius can cause the front tires to skid, but only as your changing the steering wheel angle. I do think it makes sense that the faster you go the smaller this problem because the tire patch refreshes more quickly.

(IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif) Yeah, it made my head hurt also.
My highschool physics didn't cover this or I've forgoten in the past 40 years.
On the track, normal steering angles are fairly small. At AX, sometimes all the angle you can get is just enuff and you dialing it in at entry by the handfull....once it starts to push, it's a bitch to make it stop. On slaloms & sweepers you don't use much...then I'm loose....thus the quandry. How do you get the car to behave in the same manner in all circumstances

[andys]

Your increased scrub radius accentuates the changes in geometry when you turn the wheel, so it makes managing these changes more of a challenge. Two effects that some tend overlook, is the effective change in wheelbase and weight jacking.

Andys

[eeyore]

QUOTE (andys @ Mar 30 2006, 09:12 AM)

Two effects that some tend overlook, is the effective change in wheelbase and weight jacking. Andys

AHA!

So when the front wheels are turned, kingpin inclination, caster, spindle angle, blah, blah, and blah and cause the INSIDE wheel to lift the car. Correspondingly, the outside wheel comes off the ground. Toss in some Ackerman differences and the outside wheel slips.

[Mueller]

pics for JP.....'91 C2 (sorry for being so dirty, I know, I'm a bad 911 owner, hahaha)