Caster Questions!!

[RSTPerformance]

WEll, I have finally decided to do something about my Negative Caster issues! My car is designed with several degrees of Negative Caster. I am wondering first of all why any car would be designed this way? Is it becasue it is easier to turn the wheel with negative caster? Or maybe it's just lazy design work? Or is it better in any circumstances? I am under the impression that Positive caster is better. Is this true and if so how much?? What are peoples ideas and how much are people running for *degrees of caster? Thanks for your help and your input!

Stephen

Edit... fixed my error on calling caster camber!! I am talking about caster not Camber!

[This message has been edited by RSTPerformance (edited May 05, 2005).]

[Bryan Watts]

Camber or castor? You seemed to use them interchangeably. http://ITForum.ImprovedTouring.com/wink.gif

I'm assuming you are talking about castor, and since I don't know much about it, I'll share some words that Barry Battle from Road Race Technologies posted over on Bimmerforums.com regarding positive castor:


The concept behind caster is that it adds camber during steering. As you stated it also helps return the wheel to center which helps with high speed stability. But there are negatives as well. As you steer caster also induces weight jacking as one tire will tend to move up and the other down.

But the biggest thing that is overlooked IN MY OPINION is the added mechanical trail that you get from a lot of caster. Extend a line from the upper strut pivot through the lower control arm ball joint to the ground. The distance between that point and the tire contact patch is the mechanical trail. More mechanical trail will increase the amount of force returning the wheel back to straight. BUT... added force on the steering wheel REDUCES steering feel. This is because tires have what is called pneumatic trail. This is a very similar concept to mechanical trail. As the tire goes into its slip angle the rubber wants to return to zero. As the tire starts to slide this force goes away to an extent. This is what gives the driver "feel". So, If you have a lot of caster, the mechanical force returning the steering to center masks the force of the pneumatic trail returning the wheel to center. IN MY OPINION I think that is why you will see a lot of people say how great caster is on our BMWs. Added caster "feels" great from the extra force but in reality it masks steering feel.

So you have to weigh the added camber gain vs steering feel for your needs.

[This message has been edited by Bryan Watts (edited May 05, 2005).]

[joeg]

Stephan--I would not mess much with the Caster. You are FWD with a big lump in front...right?

[RSTPerformance]

Originally posted by joeg:
Stephan--I would not mess much with the Caster. You are FWD with a big lump in front...right?

Yes most of the weight of the car is in the front. About 800 per corner compaired to 4oo per corner in the rear. Currently I have lots of negative caster. I measured the camber while the wheel is static and I had about 4 degrees then when I turn the wheel in I loose up to almost 2 degrees of camber! I know that I will never turn the wheel to full lock while driving on the track but I wnated to get the full extreme to understand how it works. Obviously loosing camber where I need the camber is a bad thing. I have probably added to much camber to the car to get my tempatures even due to the change in actual camber while turning.

Do you really think negative caster is good on any car? The things I don't understand is why they built the car this way in the first place? I was thinking maybe tire wear or easier to physically turn the wheel? I have read that with positive caster it is more difficult to turn the steering wheel and that the steering wheel will center itself faster with the more caster you add.

I still have no idea what to do! Everyone says something different and that's why I posted here to see what others are doing http://ITForum.ImprovedTouring.com/smile.gif
Stephen

[Bryan Watts]

Based on your camber problems...I would definitely go with 0 to positive castor to see how the car handles. It would probably be nice to not have to run so much static camber.

[pfcs]

Audi coupes have positive (one degree?) castor. Check your manual-it's in there.
Positive castor causes the inner wheel in a turn to gain camber (go more +) and the outer to loose camber (go more -) like a road grader or some Mercedes which had like 10* castor. All cars I know of have pos castor.
castor related camber change is arguably a good thing. As the car rolls in a turn, the dynamic cambers change in ways that the castor related camber changes tend to mitigate. In the 80s? Penske produced a high castor kit for his cars to use on slow road courses which allegedly was helpful.
I ran +3.5* castor on my Volvo; steering was relatively light-unexpected! I don't know what the castor was on my Golf.
I would expect that large +castor on a FWD would cause large steering feedbacks accerating/turning-what you'd expect of a shopping cart with front wheel drive, but if I've learned anything from preparing cars, the only way to REALLY find out is to try it yourself. If your strut top/"camber plates" also allow castor adjustment, then try it out at the track. The disturbance it would cause to toe/camber/corner weights woud be negligible. And if you really have negative castor-something is wrong. I expect you're confused. Of couse, it's the first time.


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phil hunt

[jc836]

Our experience with the Prelude showed that when we lowered the car the Caster went from the stock setting of 2 degrees to over 4. The car was only tracked for HPDE, btw and was otherwise street stock. The "feel" in the handling became firmer and more precise. Turn in was also better. My tech at the dealership advised me that it is more important to have both sides as close to identical as possible. Just my observations http://ITForum.ImprovedTouring.com/smile.gif

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Grandpa's toys-modded suspensions and a few other tweaks
'89 CRX Si-SCCA ITA #99
'03 Dodge Dakota Club Cab V8-Patriot Blue gonna tow

[Bill Miller]

Originally posted by RSTPerformance:
Yes most of the weight of the car is in the front. About 800 per corner compaired to 4oo per corner in the rear. Currently I have lots of negative caster. I measured the camber while the wheel is static and I had about 4 degrees then when I turn the wheel in I loose up to almost 2 degrees of camber! I know that I will never turn the wheel to full lock while driving on the track but I wnated to get the full extreme to understand how it works. Obviously loosing camber where I need the camber is a bad thing. I have probably added to much camber to the car to get my tempatures even due to the change in actual camber while turning.

Do you really think negative caster is good on any car? The things I don't understand is why they built the car this way in the first place? I was thinking maybe tire wear or easier to physically turn the wheel? I have read that with positive caster it is more difficult to turn the steering wheel and that the steering wheel will center itself faster with the more caster you add.

I still have no idea what to do! Everyone says something different and that's why I posted here to see what others are doing http://ITForum.ImprovedTouring.com/smile.gif
Stephen


Stephen,

When you checked the camber change w/ steering angle, was that going 'both ways'? In other words, did you check it when it would be the inside wheel and when it would be the outside wheel, in a turn (both lock extremes)?


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MARRS #25 ITB Rabbit GTI (sold) | MARRS #25 HProd Rabbit
SCCA 279608

[RSTPerformance]

I am heading to the shop today and will get you guys some numbers. I will try to take some pics as well so you can see what I am up against. I had my camber plates installed by someone else and if my car is suppossed to have 1 degrees of + caster then I would have an issue on the installation of the camber plates. Which is entirly possible! I definetly have negative caster (strut top is farther from the driver than the ball joint on the bottom)

Thanks for the help so far guys!
Stephen

PS: Hijack... Phil e-mail me with whats new and your plans... what are you building or are you rebuilding, ect. ect.



[This message has been edited by RSTPerformance (edited May 06, 2005).]

[JohnRW]

Mr. Wizard chimes in here: If you have a camber gauge, you can measure caster (even at the track, on the dirt, in the rain...after sunset) -

Caster = 1.4 X Camber Change for +/- 20 deg. steering

So...from 'straight', turn the wheels 20 deg. one way & measure the camber. Turn the wheels 20 deg. the other way (40 deg. total) and measure camber again. Multiply the total change in camber by 1.4 to determine caster.

Find that protractor that you lost in the 8th grade, and throw it in your tool box.

[RSTPerformance]

Originally posted by JohnRW:
Mr. Wizard chimes in here: If you have a camber gauge, you can measure caster (even at the track, on the dirt, in the rain...after sunset) -

Caster = 1.4 X Camber Change for +/- 20 deg. steering

So...from 'straight', turn the wheels 20 deg. one way & measure the camber. Turn the wheels 20 deg. the other way (40 deg. total) and measure camber again. Multiply the total change in camber by 1.4 to determine caster.

Find that protractor that you lost in the 8th grade, and throw it in your tool box.


That sounds great I will give more feedback when I take those measurements.

Here is what I did today for measurements tell me what you think... I took all measurements while car was on jack stands so every is relevent to that. It was also all taken from the Right Front (Had to take off the fender to do this! which also meant the bumper and grill.. arrrgggg)

What I currently have:
Camber with the wheels straight = -5.5
Camber with wheel turned all the way left -2.4 Camber adjustment is greater and greater the more you turn the wheel. (It changes faster the more you turn).
Camber with the wheel turned all the way Right equals Positive camber yes that is correct Positive 1.4 As you start to turn it it starts to go towards zero again the more you turn the more camber changes at a faster and faster rate.

If I add caster (moved top of strut back about 1.5 inches I get the following readings:
Camber while wheel is straight -7.5
camber as I turn left increases to a max of -8.7 at about 3/4 to lock. It then starts to go back after I turn the wheel from 3/4 to lock. It ends up going back to -7.4 at lock
Camber as I turn left changed towards zero and changed at a faster rate the more I turned until I came to -2.0 at lock.


So this is all confusing and none of you probably understand what I am saying http://ITForum.ImprovedTouring.com/smile.gif It certainly looks confusing to me because I always thought caster measured the strut angle but it doesn't since the nuckle sticks out. If I draw an imaginary line down form the strut it would be about 2 inches back from the ball joint. Suspension is interesting http://ITForum.ImprovedTouring.com/smile.gif

Stephen

[pfcs]

AHAH! you ARE confused! The castor angle is the angle of a line from the upper strut bearing to the ball joint, measured in the fore/aft plane. (the angle expressed in the lateral plane is kingpin inclination).
When this line intersects the ground ahead of the tire contact patch you have positive castor. In other words, if the upper pivot is behind the lower one, pos castor.
Hold your hand straight (like you're gonna slap your brother) Pretend its a tire and put tall man as the contact patch. If tall man is straight up/down and you steer the tire, camber stays at 0. Using right hand (RF tire), lean it back from perpendicular a bunch (45 deg). Now ring finger is contact patch-you have huge +castor. Steer right and camber goes way pos/left: neg.
Above info and your first measurements jive. Your second measurements are suspect.
Possibly something is moving around, causing the camber to go screwy. Why the big straight ahead camber change? Was someone trying to hold the strut top in the new position by hand while you tried to measure? Was the suspension unloaded? In any case, if you look at your results, they show you did increase the pos castor; in your first test, you had a 3.8* change, and in the second, 6.7*. Now if you don't get it, slap yourself!
PS: car is absolute writeoff. even the ecu got broken as well as trans case 3/4 suspension, headers, shell etc. I was still hurting when I got back and went for X rays-had 3 cracked ribs. Medical didn't even take my pulse before declaring me fit while I still sat in the RF of ambulance! (this doesn't reflect on club-it was Road Atl open practice)
At this point I'm pretty bummed out and unable to imagine starting over. Things were looking very promising before the wreck. In hindsight, I might have even won the ARRC!

ps: if someone cme up with a very rust-free 2 door digifant shell, I ight reconsider!
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phil hunt

[This message has been edited by pfcs (edited May 06, 2005).]

[mowog]

I'm not really going to say anything different, but maybe differently. For starters, you should NOT be measuring camber, castor, toe, bump steer, or anything else with the car on jack stands and your wheels at full droop. Put the car on the ground, and put something close to your weight in the seat (tires, tools, spare head and blocks, anything). Roll the car around, bounce it up and down, and whatever it takes to get things to settle some. Point the wheels straight ahead and mark either the top of the steering wheel or the bottom of the steering wheel with a ring of adhesive tape or similar (I'm sure you've seen steering wheels marked with a white ring at the top).

Now on to castor: as stated several times, the top should be more rearward than the bottom. I'm used to cars with kingpins or two ball joints. If you look down from above, the top of the kingpin, or the top ball joint, is further rearward than the bottom. I'm not very familiar with strut cars, but the same principal applies. Now to start, don't worry about how much castor you have, start with making both sides the same. Turn the wheels 3/4 of a turn (that white mark should be at 3:00 or 9:00). Measure camber, noting if it's positive or negative. Turn the wheel the other way so the white mark is at 3:00 or 9:00 (opposite of what it was). Measure camber. Using fingers and toes if necessary, figure out how much the change was. Adjust castor till the camber change amount is the same on both sides.

Note: assuming you are starting with positive castor, by turning the wheel the same amount (I've always found 3/4 turn is good), and setting castor the same on both sides, you can now fine tune and things should come out OK. This eliminates the necessity to make sure you are turning the wheels exactly 20 degrees, or whatever, and it ensures you can repeat the settings.

Now you can adjust the castor to suit your taste, but make sure both side produce the same total change amount. Now set camber to suit. (Someone who's done this more recently - castor, then camber, right? Of course toe is last.) If you don't know what you should start with, check with someone who runs your same type of car. If no one is available, and you have no idea where to begin, start with 3.5 degrees of castor (now you have to use the math noted above to determine actual castor), and 3 degrees negative camber, and 1/16" toe out. Then you will need to test, use pyrometers, learn to read tire wear, and use the seat of your pants to get the car set up the way you want, with the minimum of wear and tear on the equipment.

Good luck!

[This message has been edited by mowog (edited May 12, 2005).] (maybe some day I'll learn to type, spell, or proof read before I hit submit - yeah, right)

[This message has been edited by mowog (edited May 12, 2005).]