Does Roll-Center height have much effect?

[911st]

Originally Posted by jluetjen

...

Camber gain is good when the car is leaning while cornering, but a bad thing under heavy breaking when the compression may be reducing the contact patch. This is especially so on bumpy tracks.

Those are a couple of guesses on my part.

Makes sense to me.

On the back, if we set the neg camber at something like -2.5 and hit the brakes, even with stiff suspension, the rear is going to raise up and take the camber out some which should help there. However, what is the toe change going to do?

As to the front, most of us we need more neg camber there bad to get the front tires to work in the corners. I have seen many pictures of racing 911's that have more static neg camber in the front wheels than the rear. I suspect there is a clue there.

The fronts compression under braking as noted should hurt front wheel braking potential due to camber gain. However, with the weight transfer, I would guess the contact patches would still increase.

However, on the turbo Porsche tilts the A arm by playing with the attachment points up front. Further at the same time Porsche raised the inner rear arm attachment to gain Anti-squat. I wonder if setting the rear a bit more on the not so low side in the back and low in the front (more rake) and spacing the front of the A arms attachment down would help any to reduce squat and help any with braking potental.

Of course stiffer springs would help. Also, increasing spring rates and if needed then opting for lower rate sway bars might help reduce front dip and camber gain.

[911st]

Ok, this has all helped me accept that I am in pretty good shape out back. My tire ware from DE/TT's is acceptable as are temps using a probe.

Up front however is another thing. I desperately need more neg camber (have -1.2). I am mostly sold on raising the spindle (& add bumper steer adjustment). This should help my camber curve a bunch and I suspect get more around -.5 or more effective camber.

I still however will need more static neg camber and not sure how I should best achieve this.

My options as I see it are:

a) have the spindles decambered. Gives more tire to lip clearance, tighter fit toward strut, not sure what this dose to "scrub radius" what ever that means.

b) go to Smart Racing Camber King that offsets the strut in further and allows me to pull the towers a bit closer together. Then add ER offset ball joints. Should get me in the -2 to -2.5 range I crave.

c) lengthen the front A arms to push the bottom of the wheel out further. This will increase the front wheel rate with my torsion bars to some degree. Kind of the same as "b" I guess.

I think the least cost and easiest is to just decamber when raising the spindle.

Again, my car's is mostly full weight 86 that will stay with softer than full race suspension. Anything that creates camber gain with travel is preferred over just adding static neg camber as this will help my tire wear to a small degree during non race driving.

Anyone see a best approach from a handling dynamics point of view?

I guess in reality, anything more that just trying to achieve close to proper neg camber at the track so I can get decent hold and better tire ware may be overkill and over analysis. Still...

[Homegrown]

a) have the spindles decambered. Gives more tire to lip clearance, tighter fit toward strut, not sure what this dose to "scrub radius" what ever that means.


Anyone who knows how to decamber the spindles, I will raise my spindles but can`t see how to alter camber.

Best regards Stefan

[ted]

Originally Posted by 911st

more static neg camber and not sure how I should best achieve this.

My options as I see it are:

a) have the spindles decambered. Gives more tire to lip clearance, tighter fit toward strut, not sure what this dose to "scrub radius" what ever that means.

...

Just a thought and my example might be extreme..
The spindles were raised and the camber on the strut set to zero.
We were told it would have improved the scrub if we had put a little postive camber into the strut and then lenghten the lower a arm for negative camber.
The bump steer spacer was rather long so it was gusseted to the steering arm.

[burgermeister]

Originally Posted by petevb

It happens.

So here's the question- you've got people spending big bucks looking for more camber gain with turbo trailing arms, etc. But as the as tire tech moved on and 17/ 18" wheels became the standard, Porsche went to less camber gain with the 993. The old 911 (or the 964) gives you more than one extra degree of camber gain over an 80mm compression vs the 993.

Which means...?

I don't know, but I'd be willing to bet that the 993 has higher roll stiffness.

As far as why the 993 is more stable in the rear - rearward and side loads no longer produce compliance toe-out, which probably goes a long way to calming lift-oversteer. Also it has more and lower aspect ratio rubber - always a bonus.

[911st]

Originally Posted by ted

Just a thought and my example might be extreme..
The spindles were raised and the camber on the strut set to zero.
We were told it would have improved the scrub if we had put a little positive camber into the strut and then lenghten the lower a arm for negative camber.
The bump steer spacer was rather long so it was gusseted to the steering arm.

Way cool.

Do you recall how much the spindle was moved?

What dose the improvement in "scrub" do for dynamics or handling of the car?

I am after neg camber and improving the camber curve, getting an improvement in my roll center sounds like a good thing, and I like getting some of my suspension travel back so I do not destroy my shocks or my car -- now if I just understood scrub.

Thank you much for the info.

[ted]

I searched scrub on this forum and lots of helpful old threads came up.
Hope this helps.
http://forums.pelicanparts.com/showthread.php?t=316895&highlight=front+suspension

[911st]

ted, the pic and link was helpful.

The minute I saw the picture I think I starting to get Scrub radius.

I think it is the leverage between around which the front wheel turns and where the tire is centered on the ground.

On an SC/Carrera it is determined by drawing a line down through the center of the strut (hinge point) to the ground and comparing that to where the center of the tire patch is. The longer the difference between the center of the wheel and the point about which the strut rotates to higher the scrub. This leverage has an effect and the larger that distance (or lever) the larger the effect.

Anyone that has driven a factory 930 or Turbo Look as felt the front wheels pulling at the steering wheel under braking as each front wheel's traction changes.

On a narrow body car the ball joint overlaps the rim so the scrub radius is probably not very large. In a 930 with stock wheel that are pushed out about 2" the scrub radius is significant.

Thus, Doing this mod on a narrow body is probably not that big of a deal.

However, this mod on a 911, as it makes the A arm longer will increase the wheel spring rate on a torsion bar car.

I am however wondering if it flattens the camber curve as the A arm as it rotates from a downward angle toward being level it will not push the strut as far with the same amount of compression.

I am after camber gain with wheel travel and I am a narrow body so I do not think lengthen the A arm is my best approach though is has benefits like less bump steer and less steering effort.

Though I do feel like I have to use a fair amount of mussel to keep my car on a long sweeper.

Make any sense?

[911st]

Sorry, lengthing the A arms will soften the front wheel spring rate.

[Bill Verburg]

Originally Posted by petevb

.......I'd love to see numbers like roll center height on a race setup 993 (they lock out the collapsing link) ....

Me too, all I can add is that since I put the solid mounts on my993 to lock out the sliding links the handling has been transformed immensely to the good

[Bill Verburg]

You don't want to change the scrub radius too much, the main effect is on steering wheel feedback, early 911(through '89) have a small amount of (+) steering scrub radius if that is increased steering wheel kickback forces increase dramaticly and the car will become very darty, late cars from 964 up have a small amount of (-) steering scrub radius which damps out steering wheel kickback, (-) is necessary on ABS equipped cars

[Bill Verburg]

Scrub Radius discussion

[petevb]

Originally Posted by Bill Verburg

Me too, all I can add is that since I put the solid mounts on my993 to lock out the sliding links the handling has been transformed immensely to the good

So that suggests to me that roll center height is likely the big difference between a set-up 911 vs a set-up 993.
The 911 trailing arm suspension with raised pickups, the inner pickup roughly 20 mm higher than the outer, heim joints instead of rubber, coil-overs, stiffer chassis, wider track, etc looks like it would move very similarly to the race set-up 993 suspension. But I've never experienced an early 911 with the rear-end stability of a GT3 (I've never driven a race moded 993, so I can only imagine it's similar).
One conclusion is that the difference is in the one big variable we're not plotting vs the 993, namely the roll center height...

For those that don't know what I'm talking about, corner entry speed in a 996/ GT3 blows away the average early car- there is absolutely no comparison. Somewhere in these graphs/ variables is the reason why...

[911st]

Thanks for the info and links. I now understand scrub and it seems not worth the effort to modify on a 911 narrow body front end.

Slip Angle?

Are there ways to reduce this on the back of a 911? I am guessing wider tires, less weight, favorable front rear suspension balance. Basically things the increase rear traction.

Is there any alignment considerations like toe that can effect or reduce tire slip angle at the same G force?


911 Anti Dive.

Can one copy what was done to a 930 suspension to impart some anti dive into a normal 911.

On the front, they changed the front A arm orientation by raising the back and lowering the front. Can one just put spacers under the front A arm attachment points to lower the front of the torsion bar tubes?

At the back of the tubes, was the sheet metal changed on a 930 to raise it there? or was the alloy steering rack beam modified or different?

I wonder if dialing in "wedge" on a 911 with a nose down attitude aids in reducing squat.

On the rear, one might be able to raise the inner arm attachment. However, would that reduce Camber Gain?

[burgermeister]

You can, to some extent, lower the front of the front A-arm with washers / spacers (eventually the back attachment to the crossmember won't play along). You will get a pretty small change in anti-dive. You will also add a forward motion of the wheel to jounce travel, which will adversely affect ride. Modern cars have much better ride because they have fore-aft compliance, which means the wheel can travel rearward with jounce when it hits something.

[ferdsgt3rs]

I race with people that have Extremely modified chassis 1967 -1992 only alittle slower than the 996 or the 997 Cup. What I've seen is a very light chassis a RSR suspension of that era, 13'' rear bias ply slicks 11'' inch front tires stuffed under a Getty FRP RSR body. Tha camber is not steep maybe 2.25 neg rear 1.75 front.
The motors are 3.4 -3.8 making any where from 310-400 hp N.A. Here are some pics a 1976 RSR GT3 class and a 1992 GTL RSR clone race car. They have copied Porsche's ideas in the garage.

[ferdsgt3rs]

Nice car!!!
It's the second car, 993 look

[petevb]

Originally Posted by ferdsgt3rs

I race with people that have Extremely modified chassis 1967 -1992 only alittle slower than the 996 or the 997 Cup.

Well we've seen Matt Lowrance (sp?) and similar either normally aspirated or turbo early cars wipe the floor with 996/7 cups. The issue is they are blazing by in the straights but do loose in corner entry. So it seems if power to weight is similar the early car is at a big disadvantage. My thought was that the suspension geometry was primarily responsible, and I still suspect that's mostly the case, but with the data we have now I can't yet point to the actual reason.

[ferdsgt3rs]

There is your answer call Mat. The racers in the older cars have great skills, but the 996/997 are far superior in design and development. Power to weight for the older cars is what keeps them up front. Cup Cars are endurance racers the ones above do sprint races, both designed for different racing. If you think it's geometry I agree. Since Porsche came out with the "LSA" light ,stableand agile. It's been in a constant state of development.

[Mahler9th]

There is some interesting discussion here. I had dinner at Mat's house last night...

We again discussed why there are so few water-cooled cars that have raced frequently with PRC... tough to figure out.

I can say that many of the 911-based cars that are PRC frequent entrants were built and set up by Mat, most running up front. And his customer base is growing.

People have long ago figured out how to make these cars go fast... which combination of parts, which set-ups and so on. We are lucky to have so many of them in businesses here in our area.

By the way, don't forget that many of the cars in the PRC GT classes are running bias ply slicks, and these run 0 to 1 degree of negative camber.

Also don't forget that none of these cars drive themselves... there are some pretty good shoes in the PRC!