Excessive Caster?
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Excessive Caster?
My caster is out 1/8" on my 86 911.
Is this excessive? Should it be perfect?
Does anyone know the specs for wheel to wheel?
I'm getting a bit of a wobble feeling under pressure.
The shopping cart thing.
Thanks
Is this excessive? Should it be perfect?
Does anyone know the specs for wheel to wheel?
I'm getting a bit of a wobble feeling under pressure.
The shopping cart thing.
Thanks
Last edited by 911 Rod; May 24, 2006 at 12:39 PM.
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Shopping cart? You might have something more than a camber problem! Check everything, and then get to an alignment shop that has a tech who knows 911s. Your car (avatar?) looks like it's about Euro height, so you can run the following specs (primarily for the street):
F camber: .50 to .75 degree negative (same on both sides)
F toe: zero
R camber: 1.0 - 1.25 negative (same on both sides)
R Toe: zero
Of course, if you do a lot of track events you will have to take along a pyrometer, start a log, and check tire temps (in three places along the contact patch) after every run. Having an accurate baseline to start with is crucial. You will probably find that introducing a small amout of + toe at each end will give you better turn in, and a little more neg. camber might help your control at the apex. Everybody drives different, so you will have to use that log to find the suspension settings and tire pressures that work for you. If you have a tire that gets 25 - 30 degrees hotter on the inside shoulder (but still on the contact patch) than on the outside shoulder you're probably in pretty good shape. If the inside is about the same temp as the outside you can dial in more neg. camber, etc., but every change no matter how minor should be logged. Let the fun begin!
Pete
F camber: .50 to .75 degree negative (same on both sides)
F toe: zero
R camber: 1.0 - 1.25 negative (same on both sides)
R Toe: zero
Of course, if you do a lot of track events you will have to take along a pyrometer, start a log, and check tire temps (in three places along the contact patch) after every run. Having an accurate baseline to start with is crucial. You will probably find that introducing a small amout of + toe at each end will give you better turn in, and a little more neg. camber might help your control at the apex. Everybody drives different, so you will have to use that log to find the suspension settings and tire pressures that work for you. If you have a tire that gets 25 - 30 degrees hotter on the inside shoulder (but still on the contact patch) than on the outside shoulder you're probably in pretty good shape. If the inside is about the same temp as the outside you can dial in more neg. camber, etc., but every change no matter how minor should be logged. Let the fun begin!
Pete
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Ever want to slap yourself?
I meant caster.
Thanks for the specs Pete.
My car is within your specs. as far as toe and camber goes.
Rod
(We just had a 3 day weekend here so things are a little fuzzy)
I meant caster.
Thanks for the specs Pete.
My car is within your specs. as far as toe and camber goes.
Rod
(We just had a 3 day weekend here so things are a little fuzzy)
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Hi Rod,
Get max caster, but both sides should be identical or you will have problems with pulling to one side (especially under braking), despite proper toe and camber. What I don't know is whether 1/8" is within spec. Does it drive and brake straight?
Edward
Get max caster, but both sides should be identical or you will have problems with pulling to one side (especially under braking), despite proper toe and camber. What I don't know is whether 1/8" is within spec. Does it drive and brake straight?
Edward
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It drives straight and brakes straight as well.
Actually you can slam on the brakes and it holds it's line.
Max caster? I would think you would want min. No?
If the wheels are ahead of the suspension does it not want to dart? (shopping cart before the wheels fall back theory)
Actually you can slam on the brakes and it holds it's line.
Max caster? I would think you would want min. No?
If the wheels are ahead of the suspension does it not want to dart? (shopping cart before the wheels fall back theory)
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Rod: The usual number for caster is about 6 degrees. A bit of difference isn't as important as, say, a camber difference, but you should still try to get it close. Positive caster is created by locating the top of the suspension strut so that a line drawn through its center will touch the ground forward of the center of the tire's contact patch. Ride height affects caster, my Euro-height car has both sides adjusted to a max of 5 degrees and some minutes, and I was not able to get them the same (about 20'). They're close enough that the car does not wander (my slight wander comes from a slight toe out setting), and tire wear is not affected so the small difference (my car does not see track use) is OK. What is your actual caster number? Too high will produce heavy steering, too low and your steering will get light and a bit unstable feeling.
Pete
Pete
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Hi Rod,
Of course, Pete is the guy to listen to. But in answer to your question about caster, it's not that the wheels are ahead of the suspension. Caster on cars is analagous to rake on motorcycles. Think of the forks and draw a line down the axis to where it touches the ground ...it is ahead of the wheel's contact patch. THAT is what you want, and it is, in effect, the same as the shopping cart model: the axis of steering rotation is vertical and ahead of the wheel's contact patch.
As Pete said, the more caster you have, the greater the steering effort as more of the car's weight rests on this angle of inclination (the struts' angle) as opposed to a vertical axis (the shopping cart), but it also offers greater tactile feedback of steering input and greater self-straightening when in a turn. So in practical terms, too much caster is bad, but I do not think that "too much" is possible given the limited travel the strut's top mount has in our cars.
Edward
Of course, Pete is the guy to listen to. But in answer to your question about caster, it's not that the wheels are ahead of the suspension. Caster on cars is analagous to rake on motorcycles. Think of the forks and draw a line down the axis to where it touches the ground ...it is ahead of the wheel's contact patch. THAT is what you want, and it is, in effect, the same as the shopping cart model: the axis of steering rotation is vertical and ahead of the wheel's contact patch.
As Pete said, the more caster you have, the greater the steering effort as more of the car's weight rests on this angle of inclination (the struts' angle) as opposed to a vertical axis (the shopping cart), but it also offers greater tactile feedback of steering input and greater self-straightening when in a turn. So in practical terms, too much caster is bad, but I do not think that "too much" is possible given the limited travel the strut's top mount has in our cars.
Edward
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Thanks guys.
You really know your stuff.
Not sure about the degrees, I'm measuring wheel to wheel.
My symptoms are instability at speed.
There does not seam to be much steering feel.
Peter you hit it in the head!
I am getting "steering will get light and a bit unstable feeling".
And this is on the track!
The top of the struts are set with one all of the wayback and the other all of the way forward.
Bring the struts closer will result in the caster being closer.
Thanks again
Rod
You really know your stuff.
Not sure about the degrees, I'm measuring wheel to wheel.
My symptoms are instability at speed.
There does not seam to be much steering feel.
Peter you hit it in the head!
I am getting "steering will get light and a bit unstable feeling".
And this is on the track!
The top of the struts are set with one all of the wayback and the other all of the way forward.
Bring the struts closer will result in the caster being closer.
Thanks again
Rod
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There is a little bit of a happy medium you have to achieve on these earlier cars between the caster and the camber. On my car, if the alignment shop gets max caster, I limit the amount of negative camber I can achieve (at Euro ride height). If you max out the camber first, your caster is a bit more limited...
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The area in which the top strut mount resides is like a box with a range in which you can lock down the strut. If you like, you just max out the camber/caster to its appropriate corner (the inside/rear "corner" of the box, so to speak) and let your skilled, Porsche-knowlegable alignment guy work out the fine adjustment. Alternatively, if you want more than max can give you, grind down about 1/4" off the upper shock plate on the inside edge, then slot the holes/area in the body where this plate bolts onto, and you can get more camber without resorting to spherical bushings ...it's a cheap neg-camber mod. I was able to get -2 degrees in the front end doing this ...a lot for street, but just the beginning for dedicated track work, but I'm limited on funds. 
Edward
Edward
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Currently.
1.3 neg camber back
.7 neg camber front
0 toe front
1/16" toe back
I'm pretty happy with these but the car does not feel planted at speed.
This is why I'm thinking caster.
I would really like to figure this one out in my own garage.
If my caster is out 1/16", should I move the side with the longer wheel base back?
1.3 neg camber back
.7 neg camber front
0 toe front
1/16" toe back
I'm pretty happy with these but the car does not feel planted at speed.
This is why I'm thinking caster.
I would really like to figure this one out in my own garage.
If my caster is out 1/16", should I move the side with the longer wheel base back?
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Usually on lowered cars you want to move the top of the strut to the rear to increase your caster angle, but still you will probably not get 6 degrees. Remember, if you draw a straight line, looking from the side, down through the middle of your strut to the ground that gives you one of two caster "pieces." The other is to draw a vertical line through the center of the wheel down to the ground. The difference between the two lines is your caster. Because you said that one of your struts is moved all the way forward I would guess that it's the cause of your instability (ie; insufficient caster). Longitudinal movement gives you caster changes, lateral movement gives you camber changes. Have fun with it!
Pete
Pete
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I'm having fun for sure Pete.
Will more caster not give me more of the wobbling feeling I am experiencing?
This is my goal.
Turn 2 at Mosport is a full throttle off camber double apex.
I trying to feel full grip on my outside front tire.
Firm without the slight wobble I'm feeling now.
Thanks
Rod
Will more caster not give me more of the wobbling feeling I am experiencing?
This is my goal.
Turn 2 at Mosport is a full throttle off camber double apex.
I trying to feel full grip on my outside front tire.
Firm without the slight wobble I'm feeling now.
Thanks
Rod
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I think that the easiest way to explain what I like my 911 to do on the track is to make it work the best where time is to be made. In other words, the car needs to work in the highest speed corners, and the corners that lead onto straights. If that's accomplished, and the car bites well on turn in, all the other corners involve adjustments to lines, and corrections from entry to exit - you have to adjust to the car! I can't comment on your "wobble" without driving your car, but a proper 911 with between 5.5 and 6 degrees of caster (same side to side for the track) will do the job. Yes, steering corrections are necessary, the front end will seem light depending on speed, and the level of performance you can expect from the outside front tire is dictated by the overall balance of the car. If the front works too well you will begin to approach understeer, which can transition into oversteer in a sudden and ugly way. I think that you're ready to get your car on some great optical equipment - believe it or not, some machines can't measure caster. Make sure that you find someone who knows how to use his alignment rack, you really need a baseline from which to work.
Pete
Pete