Im about to do the initial alignment on the challenge car.
Camber and toe are easy to set with my tools and ability, but caster is not.
I know caster effects steering wheel effort and return, but what else does it do aside from these?
Im ASSUMING theres a minimum amount of caster already engineered into the miata suspension, but we all know how assuming works.
So, what does more caster do, and how does it benifit motorsports?
Caster changes camber gain when turning the steering wheel. More positive caster = more negative camber gain on the outside wheel.
It also changes steering effort as you mentioned due to changing the self centering torque of the wheels. More positive caster = increased self centering torque.
More positive caster can also help with feedback due to the increased torque so you may feel more in the steering wheel.
On my phone so not posting pics right now.
In reply to Dusterbd13-michael :
For autox caster isn’t nearly as important as camber. Max out the front camber & let caster be whatever it ends up being.
Pete Gossett said:In reply to Dusterbd13-michael :
For autox caster isn’t nearly as important as camber. Max out the front camber & let caster be whatever it ends up being.
Not necessarily. On the FSP ZX2 (when it was still in STS/C) we moved the camber plates 90* and made them into "caster plates", We slotted the struts at the knuckle and put some static caster in there. The result was the car had better forward bite (no limited slip in ST) off corners and had more lively steering feel in transition. When the car went to FSP (with the LSD) we kept the setup. The setup yielded static camber in the mid-upper 2's and caster in the 5's
German cars especially are known for having high caster settings . My understanding is that they are known to handle well, at least generally speaking.
In reply to Dusterbd13-michael :
The faster a car goes ( top speed ) the more caster is needed. ( in general )
Race tracks like Elkhart Lake ( 4 miles long with three long straights) , Brainerd, (3 miles long with one long straight). Need more caster Blackhawk farms needs less.
You can trade caster for camber depending on factors like length of straights, radius of critical turns, abrasiveness of the pavement.
In other words there is no absolute right number for any car. It’s all a series of compromises, trade offs if you will, of a loss of exit speed here for a better exit speed there. Better braking in turn••••• in exchange for •••••••
The trick is knowing what those trade offs are and under what conditions. Trust me, the best get it wrong often. But when it’s right, you’ll get your personal best lap times and it will feel so good you’ll never want the race to end.
Caster also provides weight jacking, so as you turn tighter it'll pull weight off the inside rear. Remember this when increasing caster on an open diff RWD car. But that same effect also helps the car rotate and feel lively, almost a little bit kart-like.
In reply to FSP_ZX2 :
You’re correct about some Mac strut cars. Sorry, I should have included “for a Miata” in my comment.
I honestly don't even know where the caster is on my Miata. Our alignment rack uses heads that clamp to the outside of the tire, and my Miata's tiny wheel wells are chock full o' tire, so when I try to do a caster sweep, the sensor heads crash into the fender and you can't get a reading. So I just set camber and toe and let it go.
Keep it stock.
It's a different game for strut cars because the camber curve is not as favorable. You can certainly add too much caster to a double A arm car to the point where you're gaining too much camber. That doesn't mean I know the answer and I look forward to those with more knowledge on actual settings giving us the answer.
So generally speaking in Motorsports more Caster is better to the point that it starts slowing down. Correct? But correct Caster setting isn't something to chase until such point as I find I need to. Correct?
Michael, if your question is with regard to your Challenge car, set the camber to the max. If you happen to get more than 3* (not likely) then dial it back to 3 and get what caster you can. Otherwise just take all the camber you can.
FWIW, there's an in-depth piece on alignments headed to your mailboxes. JG wrote it.
In reply to DeadSkunk :
It is!
Apparently i continue to overthink it. What you said was my original plan, and then i couldn't sleep last night.....
I noticed a huge difference on my street car(e36/5), jumping from 4° to 6°. It became way more stable on the highway but I didn't notice much low speed steering effort, likely because it's already pretty high on the car.
On the racecar(W116) I adjusted the caster and camber to max because the wonkey suspension only allows about -1.5° camber so we hoped for as much camber gain from caster as possible. Becasue I adjusted both at the same time it's hard to tell how much the caster helped, but I can say that the camber gain is noticeable looking at the turned tire from outside the car.
PMRacing said:Caster changes camber gain when turning the steering wheel. More positive caster = more negative camber gain on the outside wheel.
It also changes steering effort as you mentioned due to changing the self centering torque of the wheels. More positive caster = increased self centering torque.
More positive caster can also help with feedback due to the increased torque so you may feel more in the steering wheel.
On my phone so not posting pics right now.
That's only half the story though. Without caster, the effects of kingpin inclination will cause both wheels to go into positive camber as the steering is turned. That's ok for the inside wheel (actually it's beneficial) but really bad for the outer. You can see this on almost all production cars at fill lock. This is because they have KPI in excess of their caster angle, and also to some extent weight jacking. With caster, your inside wheel can remain in positive camber (or at least less than its static negative camber) but your outside tire also gains negative camber!
If you were starting from a clean sheet you could design a front suspension and steering system with zero KPI and zero static camber and then tune the caster angle to get the amount of camber gain required for the tires used. But packaging makes this very difficult for normal production cars.
Also, caster does help steering self centering but it actually has little impact on vehicle stability. The amount of trail built in to the steering system is the primary driver for vehicle stability. With predefined steering knuckles, increasing the caster also increases the trail and making the car even more stable. There have been cars in the past that have had reverse caster that worked just fine!
Caster is faster!
Caster is like free dynamic camber.
I added about 6 degrees to the front of my 2nd gen RX7 in order to get the tire contact patch on the ground through the corners.
I was able to then have slightly less static camber. This helped stability, turn in, tire wear, and braking!
I dropped 3 seconds a lap at Watkins Glen by just adding caster and dialing back to about 2.5 degree of front camber.
In reply to FSP_ZX2 :
Agree. when I nearly doubled the stock caster it really improved the overall performance/feel of my ZX2SR
My settings were close to yours.
My Jeep benefited greatly from cranking the caster up a good bit as well. More nimble in a rallycross, feels better on the street, etc. Not really sure what it's set to, but probably in the 7 - 9* ballpark. A few little suspension tweaks and a rebuild for the rear LSD fixed any concerns with unweighting the inside rear tire and it was good to go. Only downside is that on slick surfaces it tends to push in very tight turns as it ends up gaining too much camber.
Don't mean to HiJack, but how do you adjust the caster on a ZJ? Doesn't that throw off your pinion angle?
buzzboy said:Don't mean to HiJack, but how do you adjust the caster on a ZJ? Doesn't that throw off your pinion angle?
It does affect pinion angle. I'm running offset ball joints for an extra 1/2 degree of caster and 1/2 degree of camber up front. And I've got a double cardan at one end, CV at the other front driveshaft (CV is at the axle end) so there's some ability to adjust caster and have somewhat imperfect pinion angle before the angles become significant enough to cause vibrations.
Most of the main points have been hit on already. If you want some interesting reading look up Dick Shine's setup for his VW race cars. He thought caster was evil, and ran the front suspension at stock or raised ride height, and lowered the rear a bunch. Note this is for a strut front, twist beam rear suspension FWD car.
I'm not entirely sure he was "right" but this post by RR98ITR certainly is informative as to why he might be on to something:
https://www.f1technical.net/forum/viewtopic.php?t=6092&start=30
I'm hoping to experiment a lot more with caster this season and use data to figure out if more or less is better on my car on track.
Raise the front and lower the back, you say? An ironic way to make caster more positive! But only by a little, so mostly useful as a joke..
I did read that and I think I get the general thrust of it. Someone else already alluded to the fact that part of what is being accomplished by adding positive caster is increasing trail. Aside from caster adjustment it is difficult to do that. I have considered the possibility of welding and redrilling the lower ball joint hole on spindles but never considered myself enough of a handling-phile to justify turning it from a thought experiment into a highly tedious real experiment.
One thing that hasn't been mentioned much is the effect on ride quality. I've been working on a lifted Tundra recently and have done a couple of alignments to it already and will probably do one more. Mostly this is because I'm volunteering it as practice for my students. In the process I've run different caster settings and noticed the slight difference in ride quality from a less positive setting. Overall I prefer the more positive setting and will probably return it to that. With adjustable upper control arms and stock eccentric bolts on bottom I'm able to almost double the stock setting from ~2 to ~5 degrees. Although, another thing is when you increase tire diameter, the amount by which your contact patch trails your steering axis line grows as well.. but in a sports car thread I can see why noone is talking about amplifying your caster by putting on some mudders. 
Vigo said:Raise the front and lower the back, you say? An ironic way to make caster more positive! But only by a little, so mostly useful as a joke..
I did read that and I think I get the general thrust of it. Someone else already alluded to the fact that part of what is being accomplished by adding positive caster is increasing trail. Aside from caster adjustment it is difficult to do that. I have considered the possibility of welding and redrilling the lower ball joint hole on spindles but never considered myself enough of a handling-phile to justify turning it from a thought experiment into a highly tedious real experiment.
One thing that hasn't been mentioned much is the effect on ride quality. I've been working on a lifted Tundra recently and have done a couple of alignments to it already and will probably do one more. Mostly this is because I'm volunteering it as practice for my students. In the process I've run different caster settings and noticed the slight difference in ride quality from a less positive setting. Overall I prefer the more positive setting and will probably return it to that. With adjustable upper control arms and stock eccentric bolts on bottom I'm able to almost double the stock setting from ~2 to ~5 degrees. Although, another thing is when you increase tire diameter, the amount by which your contact patch trails your steering axis line grows as well.. but in a sports car thread I can see why noone is talking about amplifying your caster by putting on some mudders.
Yeah, I kind of get the gist of what that forum seemed to be talking about. It kind of sounded like they were trading ultimate grip for more driver feedback and a more gentle falloff in steering wheel feel? I guess I can kind of understand that keeping the front tires flat on the pavement is crucial to the overall speed of FWD cars. But I think for other cars I would always try to aim for a setup that maximizes grip at all times, and as far as I know that requires caster.
I guess I can kind of understand that keeping the front tires flat on the pavement is crucial to the overall speed of FWD cars
Well, I think most of the people trying to get the last little bit out of a FWD are in some kind of racing class that limits their ability to cut up the body to add bigger tires or other lower hanging fruit. The last 5-8% mentioned in that thread could be pretty significant to some of those people. I think running almost no positive caster on an autocross-only FWD is a pretty narrow niche that probably wouldn't apply to a higher speed form of racing, or really any other kind of car. One of the other things trail and positive caster do is 'self countersteer' when a car is oversteering. When you look at in-car drifting vids where the driver more or less just lets go of the wheel and then catches it when they want it to stop spinning, that won't happen with 0 caster or trail. Some people want to go fast, have the option to increase front traction through more conventional means, and would prefer the vehicle to be more recoverable vs 5% faster but a LOT% harder to catch when you're not racing in an empty parking lot.
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