Could someone explain to me what the relationship between caster and camber is? I thought I understood the concepts pretty decently until this past weekend when a buddy said something like "our cars don't have enough caster to take advantage of a lot of negative camber." ??

He ony had about 2 degrees of camber in the fronts and I was saying that maybe he should up that to 3 or more. And he said that it wouldn't make a difference. ????

BTW, this is on the macstrut Si.

Thoroughly confused... please help.

 

To maximize contact patch and grip, you really need the negative camber on the outside tire when you turn. If you're not turning (or sliding) its not really all that helpful if at all.

Here's my oversimplified understanding of caster:

Lets say you have a car with -1 degree of static negative camber when the wheels are straight.

The same car also has 1 degree of caster...the struts are leaning back 1 degree to the top mounts.

Theoretically if you turn the wheels 90 degrees, the outside wheel will then have -2 degrees of camber (-1 static minus 1 for caster), and the inside will have 0 (-1 static plus 1 for caster) degrees of camber.

That's simplifying it, but in a nutshell caster will modify your static camber settings when the wheels are turned. If I'm wrong, someone please elaborate.

I dont know what your friend was getting at, but -2.5 or -3 is noticeably better than -2 when autocrossing the EP3 in my experience.

 

Functionally, caster is "dynamic camber" in that the camber where the rubber meets the road changes (typically, goes more negative) as the wheels are steered. More caster angle also increases the wheels' self-straightening tendency.

It's nice to have the tires flat on the road to maximize fore/aft grip but you want negative camber in corners. Caster can achieve this, where "static camber" would not.

K

 

Caster...

 

Exactly. But only to a point. You have to find the happy medium. B/c when caster helps the outside wheel gain negative camber it also helps the inside wheel gain positive camber.

 

I believe the exact measurement of caster is the 2x difference in camber between turning the wheel (tire) 14.5* left from center, and turning it 14.5* right from center...

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Ponyboy &raquo;</TD></TR><TR><TD CLASS="quote">Exactly. But only to a point. You have to find the happy medium. B/c when caster helps the outside wheel gain negative camber it also helps the inside wheel gain positive camber.</TD></TR></TABLE>

Isn't that what you want? Since the outside wheel likes negative camber, wouldn't the inside want to be positive? I don't suppose it really makes much of a difference though, since the inside doesn't have much load on it.

 

well, it does make a *slight* difference. Circle track racers that go CCW always like to have positive camber on the inside left wheel. When I hit a curb and had +1.3* camber on my left wheel, making left turns seemed a little nicer (it could have been me)...

 

caster is just a stability angle.

Having negative caster (top of strut mount is forward of lower strut mount) will give you a "shopping cart" effect. I'm sure you've pushed a shopping cart really fast and the front wheels wobble. This is because the mounting points in relation to the center of the wheel is forward, causing the wheels to follow and become wobbly.

Take positive caster for example.... a bicycle. Why do you think you can ride with no hands on a bike? Its because of the caster- from the fork. THe load is actually pushing the wheel forward, causing the wheel to become stable.

With severe positive caster, your car will turn back straight MUCH easier than one with not as much negative caster. Look at a 240SX. Stock caster is somewhere aound 6.5*, whereas a Honda is around 3 give or take a little. Drive a 240 without PS and you will be fighting that steering wheel. I think caster isn't understood by a lot of people


Modified by Emerika at 5:24 PM 11/24/2004

 

I think you have it backwards. A shopping cart has negative caster, and a bike has positive caster

 

Thanks for the input guys. I guess I understand what caster is but hadn't really thought about how it acts like camber when the wheels are turned. So from reading everyone's input it seems like there's no reason to limit the amount of static negative camber irrespective of what the caster angle is. -3* static camber is always better than -2* camber no matter what the caster angle?

Oh and I guess I should've mentioned that this was speaking from an autox perspective, so yes, handling is to be prioritized over everything else.

 

Since camber is among the easier adjustments to change (next to tire pressure), it is typical practice to adjust it based on the clock and tire termperatures. It just isn't accurate to say that "more is better" or "3 is better than 2."

I occurs to me we should do a reality check on precisely what caster IS on a car - which has nothing to do with how it is sometimes measured...

Looking from the side, caster is the angle between vertical and the axis around which the steering pivots. On a dual A-arm suspension, this would be a line between the upper and lower joints on the hub carrier/steering knuckle. On strut cars like the VW, the general rule is to push the top of the strut backward to achieve a more positive caster angle, since the suspension rotates around that upper mounting point.

There is a similar measurement when you look at the suspension from the front - the inclination of the steering axis. This makes at least two interactive settings that influence how alignment changes as the wheels are steered - on top of static camber. This also explains why, if the rules allow, engineers will gladly change suspension geometry relative to stock designs.

Things get really fun if you consider the Ackerman effect on top of all of this, wherein the inside and outside wheels steer at different rates, relative to one another. This results in dynamic toe changes, differing left to right.

K

 

Heh.. I recently whipped up a quick OpenOffice spreadsheet that showed camber as a function of static camber, caster, inclination, and steering angle. If this thread is still alive (and people are interested) on Sunday, or if I have a PM on Sunday, I could dig it up.

 

that would b great

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Burgh &raquo;</TD></TR><TR><TD CLASS="quote">Thanks for the input guys. I guess I understand what caster is but hadn't really thought about how it acts like camber when the wheels are turned. So from reading everyone's input it seems like there's no reason to limit the amount of static negative camber irrespective of what the caster angle is. -3* static camber is always better than -2* camber no matter what the caster angle?</TD></TR></TABLE>

again, it's positive, not negative! And there is always the possibility of having too much camber/caster, but 3* of caster is not too much...

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by LX4CYL &raquo;</TD></TR><TR><TD CLASS="quote">I think you have it backwards. A shopping cart has negative caster, and a bike has positive caster</TD></TR></TABLE>

I did....lol. Long day yesterday. I went to bed right after I posted, and reflected on what I said and how wrong it was, but was too lazy to get up