changing front roll center height worth it with spindle change?

[fredmeister]

Hi guys,
I have a question for any of you track junkies whether you really noticed any increase in cornering by raising the spindle height on the front struts in a 911?

I have done preliminary suspension work to the car like lowering to Euro ride height and stiffening T-bars and sway bars along with Elephant racing polybronze bushings all around.

I have a 79 930 turbo and am sending the front struts in for a rebushing to eliminate free play between the strut insert and the housing. At the same time I was thinking of spending the money on raising the spindles 19mm to put the car at Euro height but allow me to leave the lower control arm in a more favorable camber gain position when the car rolls.

Is it worth "the coin" to make the 19mm height increase? I am looking for a subjective answer from those of you that have done this.

This will raise the front roll center correct? and how does this effect the amount of low speed understeer that I am seeing right now. I would like the car to understeer less in low speed corners and just turn better anyway. Right now you have to throw the car around the corner and rely on getting the rear around and get on the throttle early to compensate for the turbo lag.

For a frame of reference: I have 23 front, 31 rear T-bars installed and SMART racing adjustable anti roll bars set approx. mid range in the car so its still streetable but about 85% stiffer than the stock T-bars.

I do a lot of coaching at DE events and get my butt handed to me often-times in the corners by the normal instructor crowd of GT3's and other track only cars.

Any insight would be great from you guys.

Thanks,
Fred

[Flieger]

Raising the roll center will make the front more stiff in roll. You will need to back off on your sway bar to get back to the same balance.

[winders]

Yes, it is worth it. While raising the front roll center will increase load transfer at the front, it will reduce load transfer due to body roll. In other words it will reduce body roll at the same lateral loads.

There is a reason just about every well setup 911 on the planet has raised spindles. It gets the suspension on lowered cars closer to the ideal operating range. If the car is lowered a lot, raising the spindles more than 19mm may be warranted. Check with someone that has experience with lowering 911s and competing with them about this.

[stownsen914]

Raising the spindles is a great mod for a lowered 911 because it puts the suspension closer to its intended range of motion and keeps the shocks from bottoming.

Determining roll center depends on all the pickup points, not just the spindle height, so don't assume you're actually raising roll center. You'd have to map it out to see if you're really raising roll center.

Scott

[winders]

Raising the spindles certainly raises the front roll center for a given ride height. There is no doubt about that...no mapping required! I would say that 99.9% of the modified 911 cars out there have not made a change to the front suspension pickup points in a way that would affect roll center.

[stownsen914]

If not changing other pickup points, then yes that's true

[winders]

Originally Posted by stownsen914

If not changing other pickup points, then yes that's true

The reality is that no one changes the 911 front chassis pickup points in a way that would affect roll center so why even mention it? All it does is unnecessarily complicate the discussion.

[stownsen914]

Originally Posted by winders

The reality is that no one changes the 911 front chassis pickup points in a way that would affect roll center

Well now that's not true. I did, others who are serious about setting up their suspension do to. Even if only by bolting on ERP suspension or similar. But granted that's not what the OP is asking so I won't digress.

[winders]

Bolting on the ERP suspension doesn't change the static roll center much. My race car has the ERP 935 front setup too so I know a little about that. If you use the ERP setup to increase track width, that certainly has an effect on roll center. But that is not the chassis pickup points. Some people will raise the rear pickup points but that affects anti-dive more than it does the roll center.

Regardless, no one make changes to the front suspension pickup points with the ERP stuff, or anything else I am aware of, that lowers the roll center. The only way raising the spindles would not raise the roll center would be if you made other changes that lowered the roll center. What changes does anyone make that purposely lowers the roll center AND improves performance?

[David Borden]

So where does the roll center end up on a lowered 911 with stock and raised spindles?

Is the roll center going below ground when the cars are lowered with stock spindle location?

David

[panzerfaust]

Lots of racers here, I'm surprised nobody mention the camber gain motion lost on an excessively lowered car without rises spindles?

[KTL]

Actually the 935 style front suspension does change the pickup points horizontally a considerable amount and vertically a small amount. Rebel Racing's 935 setup moves 30mm horizontally inward and 8mm vertically upward. But let's get away from that point?

Raising the spindles has various side effects on a lowered car, like the guys have mentioned. Let's try to list them:

Positives (P)

P1. Restores static geometry to keep suspension in better range of operation. This has a twofold effect.

P1a. Your A-arms can stay nearer the optimum slightly-angled-downward (inner pivot point higher than outer ball joint) orientation

P1b. Picking up the spindle assists in placing the steering tie rods in a near-level position to reduce bump steer when the suspension travels. As the suspension travels to extreme positions, the tie rods effectively change length and this changes your toe. This is where the tie rod/steering arm drop links come in handy.

P2. Reduces the amount camber gain (in other words, lost negative camber) as the suspension travels upward/compresses. You don't necessarily gain negative camber by raising the spindles. You just reduce the amount of negative camber being lost as the suspension compresses.

P3. Reduces likelihood of shocks bottoming-out. As the suspension compresses, the shock shaft is traveling toward its stop. By raising the spindles you're actually lowering the car w/out any suspension and shock absorber travel.

Negative side effects of raising the spindles:

N1. It restricts the size of wheels you can use. The more you raise your spindles, the larger the wheel diameter must be. Because as the spindle goes up, you're effectively reducing the clearance between the bottom of the A-arm (actually the ball joint) and the inner diameter of the wheel. Look how tight this clearance is with raised spindles & 16 in. wheels





N2. Changes the scrub radius, which is a measurement made at the pavement surface between the projected angle of the strut tube/steering axis and the centerline of the wheel. Apparently if you don't de-camber the spindles as they are raised, scrub radius grows and so does your steering effort. We had some discussion about this here http://forums.pelicanparts.com/porsche-autocross-track-racing/861401-more-front-negative-camber.html#post8588364

N3. Reduces front track width ever so slightly.

What else?

[winders]

Originally Posted by KTL

Actually the 935 style front suspension does change the pickup points horizontally a considerable amount and vertically a small amount. Rebel Racing's 935 setup moves 30mm horizontally inward and 8mm vertically upward.

All true. But does that have a large affect on the static front roll center if the spindles are not also raised? I don't think it does.

[KTL]

I don't think it has a big effect on roll center elevation since the largest pickup point change is in the horizontal direction. But it does benefit the camber curve and that's said to offset the need for raising the spindles a large amount.

[stownsen914]

Originally Posted by KTL

I don't think it has a big effect on roll center elevation since the largest pickup point change is in the horizontal direction. But it does benefit the camber curve and that's said to offset the need for raising the spindles a large amount.

Actually, moving the A arm pickup points laterally does affect roll center, especially if the A arms are at an angle relative to ground.

[Flieger]

Originally Posted by stownsen914

Actually, moving the A arm pickup points laterally does affect roll center, especially if the A arms are at an angle relative to ground.

The key is the last part of that statement. If the A-arm is horizontal then you can move the pickup point laterally as much as you want and get the same static roll center. The longer arm will reduce the amount that the roll center moves when bump/rebound travel happens, however.