The Top Mitsubishi Galant VR-4 Resource

Join the best E39A 1991-1992 Mitsubishi Galant VR-4 community and document your GVR4 journey.

  • Software Upgraded - Reset Your Password to Login
    In order to log in after the forum software change, you need to reset your password. If you don't have access to the email address you used to register your GVR4.org account, you won't be able to reset your password. In that case, follow the instructions here to regain access to the forum.

Discussion on caster

Whoodoo

Well-known member
Joined
Jul 11, 2009
Messages
927
Location
Binghamton, NY
One thing I have been wrestling with is exactly how caster is defined and why it is defined that way. Of all my searching and reading, caster has been defined as the tilt of the axis created by the top and bottom steering pivots.

Caster primarily affects straight line stability because it moves the contact patch (and drag force) either in front of (like on an office chair) or behind (like on a car) the steering axis. Changing caster will also change dynamic camber. Dynamic camber is the amount of change in camber you get when you increase steering angle. Caster and camber are related because they are both measures of the angle of the steering axis, just looking at the car from a different perspective. This means that they are related and as you turn the wheels from center, the caster angle begins to affect the camber. Defining caster by the angle of the axis created by the top and bottom steering pivots seems like it doesn't quite encompass all of whats going on.

Let me explain. In the evo suspension chapter 2 thread I'm sure you all have read lately, the idea of adding caster by putting the strut ear spacers either in front of or behind the knuckle's strut mount has been discussed. Because the steering axis will remain unchanged, essentially what you are doing is rotating the knuckle about the lower ball joint. If you rotate the knuckle backwards by putting all your spacers in the front, then you are actually increasing caster because you are moving the tire's contact patch even farther back. However, you are not changing caster because the steering axis hasn't changed, and there will be no affect on dynamic camber.

Also, another point to make is that by rotating the knuckle backwards, you have moved the steering arm upward, which will affect bump steer.

So first, how exactly is caster defined? Is it merely the angle of the steering axis? If so, then what would you call moving the contact patch either forwards or backwards independently of the steering axis?

Second, how beneficial is an increase in caster for our cars? According to Dennis Grant at Far North Racing, caster isn't all too important. Is the point of an increase in caster to get the resulting increase in dynamic camber? Is the drawback of increased steering effort a reason to not increase caster? By rotating the knuckle in the fashion described above, you could either increase or decrease steering effort (and feedback) at the expense of only changing steering arm height (hence affecting bump steer).

Thirdly, is it even worth the effort of discussion? From my measurements, you only need to make up a few millimeters between the strut ears, so is it really going to make any difference at all? I kind of doubt it, but discussion is good and if someone really has a conviction to move the contact patch back, perhaps this discussion is worthwhile.

I tried to make this as concise as possible, but suspension is very complicated indeed /ubbthreads/images/graemlins/hsugh.gif
 

belize1334

Well-known member
Joined
Nov 18, 2003
Messages
3,316
Location
Bozeman, MT
Quote:

One thing I have been wrestling with is exactly how caster is defined and why it is defined that way. Of all my searching and reading, caster has been defined as the tilt of the axis created by the top and bottom steering pivots.



This, IMHO doesn't capture the whole effect... You can get more "effective" caster by tilting the steering knuckle back as we were discussing in the Evo suspension thread. This doesn't move the pivot points but it does increase the angle between the lower pivot and center of rotation of the wheel. This is "effective" caster since it looks and acts like caster for low steering angles but has less effect as the wheel is turned farther. Now that I'm thinking about it, the main point of caster is to get more camber when the wheels are cranked... maybe this "effective" caster is useless and only serves to make the car squirlier in a straight line... /ubbthreads/images/graemlins/dunno.gif
 

Whoodoo

Well-known member
Joined
Jul 11, 2009
Messages
927
Location
Binghamton, NY
Increasing caster, by which I mean moving the contact patch farther back, will decrease squirliness (it will make the car want to go in a straight line) because the drag from the tire has a larger moment arm acting on the steering axis. Whether you get that from actually moving the top or bottom points, or by tilting the knuckle, it doesn't make a difference. Moving the contact patch back the same amount by either method will still get you the same result in this area. What's different is that by moving either the top or bottom mount, you are changing the steering axis angle and you will get more dynamic camber.

Another thing to note is that when you increase caster, it becomes harder to turn the steering wheel, but you will also get that much more feedback through the wheel.
 

4thStroke

Well-known member
Joined
Oct 22, 2007
Messages
1,864
Location
Vancouver, WA
One thing that I have wondered is, if they say negative caster improves low speed stability and positive caster improves high speed stability, where do you draw the line between low speed and high speed? Under what conditions will we see beneficial or adverse affects due to caster (low speed and high speed may be too broad).
 

Whoodoo

Well-known member
Joined
Jul 11, 2009
Messages
927
Location
Binghamton, NY
It is completely dependent on exactly how much caster you have. At low speeds, the resistive drag force acting on the tire patch of the wheel is going to be small, so having lots of negative caster would not be as noticeable. The faster you go, the larger that force will be, and in order to minimize the unstableness you will feel, you must proportionally decrease the moment arm that force has acting on the steering axis, i.e. move caster closer to zero.

Also, I thought negative caster was bad and no one really uses it. With negative caster, you get negative dynamic camber, meaning as you turn the wheel they lose camber.
 

toybreaker

iconoclast
Joined
Apr 30, 2006
Messages
3,581
Whoodoo, I think you've got an excellent grasp on the subject, and you and Spencer are asking damn good questions! /ubbthreads/images/graemlins/applause.gif

I took a stab at tring to visualise what happens when moving the shims around, and the answers aren't that easy to come by. (in fact, I think I might have been wrong in my reply to the how-to (:gasp:)


In a side view, visualising moving the knuckle backwards and forwards between the strut ears would suggest that having the knuckle forward would be the right choice. That puts the balljoint as far forward as possible and *should* tip the strut more backwards.

That logic survives as long as it takes to make a three dimensional model with the upper and lower pivot points fixed in space ... then it falls apart with a quickness in the face of reality ...

In fact, Whoodoo, I think you've got it figured out exactly right.

The castor angle itself doesn't change, because the realtionship of the centerline between the upper strut mount and lower balljoint remains the same

... but you will gain some additional castor "trail" ...


On a side note, I asked a couple of my buddies in the business about this, and we all had it wrong. It wasn't untill I made a three dimensional model that I figured it out for myself.


Well done, Whoodoo, well done!
 

belize1334

Well-known member
Joined
Nov 18, 2003
Messages
3,316
Location
Bozeman, MT
Quote:

In a side view, visualising moving the knuckle backwards and forwards between the strut ears would suggest that having the knuckle forward would be the right choice. That puts the balljoint as far forward as possible and *should* tip the strut more backwards.



I'm going to disagree here. If you put the spacer on the front of the knuckle then the bottom ears of the strut swing forward. Swinging the bottom forward is akin to leaning the top BACKWARD. Shimming the knuckle will have no effect on the location of the balljoint. The balljoint only moves if you move the control arm (as with caster bushings). Still, the more I think about it the more I feel like the "pseudo caster" you pick up from shimming the strut serves only to destabalize the wheel without the benefit of dynamic caster while cornering. Moving the balljoint forward with a bushing, on the other hand, has the desired affect of actually offseting the two pivot points which means negative dynamic outside camber in corners.
 

Whoodoo

Well-known member
Joined
Jul 11, 2009
Messages
927
Location
Binghamton, NY
I would argue that using the shims to lean the knuckle backwards would actually serve to stabilize the wheel more. Positive caster increases straight line stability because it makes the wheels want to point forward.

One thing that just came to mind is the affect of having the strut assembly loaded off-axis. We are talking about changing the angle of the knuckle a few fractions of a degree here, so the actual affect of moving shims forward or backwards is more than likely so small it won't make a difference. However, by moving the knuckle forward or backwards between the strut ears, you are also moving the strut forward or backward in the opposite direction, which causes the strut to have some element of off-axis load. Struts aren't meant to take huge side loads, and while this will only introduce a small amount of radial load, it will diminish the life of the strut seals.

There was a company a while back called ShockTek that modified bilstien struts for use in dsm's. One of the issues they had was inadequate hardening of the strut rod. Because the steel wasn't hard enough, the rod would become scored in the area of the seal and start leaking. I'm sure the evo struts are much better made, but adding more radial loading to the strut will only serve to decrease it's lifespan.

But like I mentioned before, is fractions of a degree going to make a noticeable difference? Probably not. It is, however, very beneficial to discuss the subject because the idea of increasing the primary effect of caster (straight line stability) without affecting other suspension geometry or dynamics may be quite beneficial.
 

misterfixit

Well-known member
Joined
Aug 4, 2004
Messages
1,596
Location
Midlands, UK
All you do is shift the axis of the wheel forwards and backwards. (yes you do affect the trail) but not by changing the caster or "effective caster" but by changing the offset to the steering axis.

Caster is the angle the steering axis prescribes with the vertical in a purely side on plane.
Camber is the angle the angle the steering axis prescribes to the vertical in the perpendicular plane (that across the car)

The trail is the distance from where the steering axis hits the ground to where a line dropped from the wheel axis is dropped vertically. (more trail = more stable steering to a degree).

More caster gives more bite at turn in.

The other factor not discussed is by moving the angle of the knuckle the outer tierod joint is raised/lowered this will also serve to change the bumpsteer effect of the steering as body weight is shifted and the suspension is compressed/extended.

Rich
 
Last edited:

iceman69510

Turn Right Racing
Staff member
Joined
Mar 5, 2001
Messages
10,964
Location
Michigan
Quoting Whoodoo:
However, by moving the knuckle forward or backwards between the strut ears, you are also moving the strut forward or backward in the opposite direction, which causes the strut to have some element of off-axis load. Struts aren't meant to take huge side loads, and while this will only introduce a small amount of radial load, it will diminish the life of the strut seals.




I don't believe so. With a stock upper mount, that load is now in the rubber bushing. Camber plates will allow the strut piston to move to that new position.
 

Whoodoo

Well-known member
Joined
Jul 11, 2009
Messages
927
Location
Binghamton, NY
What I'm saying is by moving the strut off the centerline of the steering axis, you are going to be giving it a side load. Yes, the rubber bushing in the top mount will take some of that, but it will be transmitted through the strut first. Once again, it's such a small change it may not make a difference but should still be considered.

Thanks Rich for the input. Now, I'll ask if you have any information on our suspension specifically and if an increase in trail is beneficial and also what the stock bumpsteer characteristics are and whether they can be improved from a performance standpoint?
 

slugsgomoo

Well-known member
Joined
Oct 16, 2003
Messages
3,776
Location
Tacoma, WA
I've had the whiteline caster bushings in my old talon (now my dad's car) since 2005. Just a few notes from my anecdotal experience, and keep in mind that car has a manual rack, so some of it may be a bit more exaggerated than in a power steering car.

1) it makes initiating turn in noticeably more effort
2) stability in turns seems to have improved slightly
3) really, really wants to snap the wheel back to the centerline

It's supposed to add some dynamic negative camber, which may contribute to the better feel mid corner, though the increased caster may account for that as well, I couldn't tell you. I will say that with a power rack it may not be as pronounced, but with the manual rack, having the bushings makes it snap back to centerline pretty well, and in some odd situations it'll actually snap back pretty violently (hit a bump coming out of a turn with a very loose grip once, ouch).

I think it's a good mod if you can get 'em.
 

misterfixit

Well-known member
Joined
Aug 4, 2004
Messages
1,596
Location
Midlands, UK
To answer more information on our cars specifically:

Wheel camber (angle the wheel is set at from the vertical across the car 0°20'±30' for 2wd and 0°30'±30' for 4wd (top in towards the centre of the car. Note: Most likely a differeence in the angle the wishbones sit at rest on the stock car, changing the width across the lower ball joints.

Steering caster (angle between steering axis and the vertical along the car) 1°±30' for a manual rack car, and 2°±30' for a power rack car. ***slugsgomoo: here is probably where you are picking up the extra feel of snapback as the factory backed off the caster for manual rack cars (this is done by flipping the top mount round to fill the front three holes in the strut tower to move the top mount forward)

Steering camber (angle between the steering axis and the vertical across the car) 13°55' for 2wd and 13°35'

Toe in from the factory is 0°±0°18' so parallel near as damn it.

Shock side load:
The shock is bult to take quite a bit of side load anyway, as it doesnt prescribe a line between the top mount and bottom ball. However as this is only in the across the car plane, i would suppose the magnitude of force input to the unit is less than front to back as the car is rolling. but hey I'm no expert and I don't have a dynamic model to analyse. (Would be cool to build on in adams or something then you could see forces in and forces out!).

Other:
The wishbone pivot axis is not parallel to the floor anyway, it is kicked up at the rear to provide anti-dive to mitigate against pitching when you heave on the anchors. Again, this specific geometry I do not know or have ever needed to measure.

Generally as a rule you set up suspension to have the arms as flat as you can so any deflection away from rest then causes the wheel angle to change all in the same sense. If you set the arms say 15° down as the bump is hit the track widens then gets narrower assuming you move the arm past horizontal. If you set the steering like this to (tie rods horizontal at rest) then the same is true.

How the galant is set up I'll not pretend to have view of, but I'll have a look and measure some stuff when I get onto my DD rebuild. It's the transition from directions that can give the wierdness as the suspension is compressed and extended.

Set up, My take on it:
As for my cars, they do not have adjustable suspension, but they are running Hatch springs on the back, this lifts the rear of the car and sharpens the steering by lessening the caster. It also increases the angle of the wishbone axis to the floor and makes the stock suspension less wallowy (increases the anti dive of the front). The tail end of my sill sits about 10mm higher than the front.

When racing model cars I used to set up the suspension to be level at rest, and the damping to allow the suspension to travel to within a mm or two of the end of stroke under race conditions. I also used to like the setup to be as neutral as possible (0° camber, parallel steering axis, longest upper link) you could then let the tyres work.

Dirt bikes are the same, you set them to have a ride height that suits the rider (touch the floor with one foot) and then sdamping to allow control over the stroke you need where you are running.

For a road car you can stiffen everything up purely because the terrain is smoother and the wheel travel is less, but you have to make sure the damping controls the movement of the wheel.

I like the geometry more neutral and the suspension to work, purely because my car is battered up and down motorways with my family in it so it must be comfortable, but when hustling it quickly I like the fact I can weigh up the suspension with the mass of the car and it turns and stops well. That said a good strut bar and a solid body keep everything where it should be and pivoting about what it should.

Hope this helps.

Rich
 

Whoodoo

Well-known member
Joined
Jul 11, 2009
Messages
927
Location
Binghamton, NY
I think it's now safe to say that despite all this great info on caster, toe, stock setups and how changes will affect it, the difference one would make to the overall suspension geometry by leaning the knuckle one way or the other is not going to be significant or really noticeably. We're talking about moving the strut mount forward or backward only a few millimeters, which is not going to make any difference in the real world as far as how the car feels to drive.

I may get bored tomorrow and turn my camber plates 180, hence decreasing caster and see what kind of a difference in feel i get.
 

656of1000

Well-known member
Joined
Jan 18, 2005
Messages
292
Location
Phoenixville, PA
Theoretically, if you used a DSM camber plate on a GVR4 (ie. turned 90*), caster would be adjustable. Has anyone tried this?
 

Whoodoo

Well-known member
Joined
Jul 11, 2009
Messages
927
Location
Binghamton, NY
I'd rather have adjustability in camber rather than caster. I thought about doing this too, but the benefits of caster adjustability don't merit the loss of camber adjustability.
 

656of1000

Well-known member
Joined
Jan 18, 2005
Messages
292
Location
Phoenixville, PA
You could still adjust camber with an eccentric bolt between the strut and the spindle. I don't know if there's an advantage to using eccentric bolts over a camber plate. If the difference is negligable, a combo of eccentric bolt for camber and DSM camber plate used for caster would be a pretty nice setup.
 

Whoodoo

Well-known member
Joined
Jul 11, 2009
Messages
927
Location
Binghamton, NY
Camber adjustment via eccentric bolts is limited by the amount of space you have between the inside of your wheel and the strut body. It's good to have the vertical wheel axis (looking from the front of the car) as close as possible to the steering axis inclination (the angle of the strut looking from the front), but the wider your wheel is, the larger the included angle has to be. Everything is a trade off, but in terms of ease of adjustment, camber plates > eccentric bolts.

Ideally, you would use eccentric bolts to angle the knuckle so that the inside of your tire doesn't rub on the shock body, then use camber plates to adjust camber from there. This will minimize the included angle and give easy camber adjustment at the same time.

The reason why you want to minimize included angle is because as the wheel turns, it changes camber. For example: you have a steering axis inclination of 12* and camber of -2*. This gives you an included angle of 10*. Now, turn the wheel 90* (not possible, I know, but for the sake of explanation). Now, because the knuckle angles the wheel 10* away from the steering axis at all times, the outside wheel will have 10* positive camber and the inside will have 10* negative. As you can see, you get more and more camber opposite of what you want the further you steer.

EDIT: also, caster is something that should be set and not really adjusted much. Ideal caster will be as large as possible without creating too much steering effort. Ideal camber on the other hand will change based on the kind of driving you are doing, tire behavior, and many other similar variables.
 
Last edited:

misterfixit

Well-known member
Joined
Aug 4, 2004
Messages
1,596
Location
Midlands, UK
Quoting 656of1000:
You could still adjust camber with an eccentric bolt between the strut and the spindle. I don't know if there's an advantage to using eccentric bolts over a camber plate. If the difference is negligable, a combo of eccentric bolt for camber and DSM camber plate used for caster would be a pretty nice setup.



Two different adjustments. The eccentric bolt adjusts the wheel camber only. It doesnt affect the steering axis camber. Camber plates adjust the steering axis camber, which takes the wheel camber with it.

Rich
 

gtluke

Well-known member
Joined
Sep 16, 2001
Messages
4,210
Location
dirty jersey
I love caster. I put almost 8deg of caster in my car by using the evo top hats and drilling out the shock towers.
Think of it this way, you can leave a "normal" 1.5 or 2deg of camer when the wheel is straight, but when you start turning the wheel you get a lot more. Its the benefits of increased camber, but only when turning.
I might be wrong here, but if you have sick camber, the inside wheel is cambered the wrong way and has less traction. With increased caster it keeps the inside tire flatter to the ground.
I added a ton of caster and the car feels awesome.
 
Support Vendors who Support the GVR-4 Community
Boosted Fabrication ECM Tuning ExtremePSI Fuel Injector Clinic Jacks Transmissions JNZ Tuning Kiggly Racing Morrison Fabrications RixRacing RockAuto RTM Racing STM Tuned

Recent Forum Posts

Top