10 replies to this topic

[J.L.Racing]

Hi Guys,

 

New to the forum here. I've been looking into doing my own set up recently and have a few questions regarding the measurement of caster. See reference article below:

http://www.disco3.co...surement[1].pdf

 

1. Based on this article, am I correct to understand that the turn angle is based on the actual toe angle and not the steering angle. In other words if I start with toe out on my RF wheel, then my right turn would technically be more than left turn in order to achieve symmetry? 

2. Figure 2 in the article also showed dynamic camber being affected by caster, can anyone explain how?

 

Also, when my car is on hub stands, do you guys have any general suggestion on avoiding suspension binding? I know the design of the hub stand are supposed to accommodate that, but would I not introduce binding when I first jack up the car and put it onto the hub stands?

 

Thanks,

Jonathan

[davew]

I was a front end/suspension tech for 12 years. Did truely. thousands of allignments. Had every car and light truck certification available from ASE, Hunter, Goodyear, etc. Been doing racecar allignments since 1993. As someone who was asked to teach at the Hunter training facility, that is way too much math and theory for me.

 

Caster is the line formed by the ball joint pivot points. Which we can not get at to measure so we have this complicated system of turning the wheel one way, then the other way, measure camber and subtract. Then to get TRUE caster reading you multiply by 1.444444 and then you have an answer. Or you can use a big expensive machine that does the math for you. Or you can come up with a method that is repeatable and works for you. Just remember to always use the same system. Then only compare your system to your system on identicle cars.

 

My method:

Set steering wheel and rack to center position, adjust if they don't match

Turn wheel 1/2 turn in one direction, measure camber. 

Turn wheel opposite direction 1 turn (1/2 turn the other way). Measure camber and record difference. Wether you turn out, then in or vice versa only tells you if you have positive or negative caster. Since no new car has been built with negative caster since the early 1970's (bias ply tires) our number is always positive.

Now do the other side and compare.

 

Keep in mind a few things;

different cars have different ratios between the steering wheel angle and the amount of turning accomplished. Even our manual racks have a different ratio than our power racks. If you want to compare car types, you need to use a turn angle rather than a steering wheel angle. But again, if you always do it the same, your numbers will be repeatable and accurate for your own use.

 

Those big expensive machines actually use a 20* toe reading to make their measurements from. So rack ratio is not important. Measuring that way without pretty sophisticated equipment or measurements is difficult. That is why I use the 1/2 turn method. Turn tables are only accurate if started with no tire bind and from zero. Used that method back in the 80's.

 

Caster effects camber as the tire turns. Ever seen an old Ford van or 2 wheel drive truck with the wheel turned? It appears that the wheel is going to fall off, there is so much dynamic camber. This is from the HUGE amounts of caster those vehicles used.

 

Suspension bind has nothing to do with the use of hub stands or not. It is the ability to flex of the rubber bushings. Our ball joints are actual joints, they don't bind. Lots of older cars had rubber bonded ball joints that did bind. But our rubber suspension bushings bind. Imagine tightening the suspension at full droop. Then expecting the bushing to twist as you go into full compression in a corner. That bushing will act as a spring and restrict movement of the suspension(suspension bind). You need to tighten the suspension mounting bolts at ride height, so the bushing has equal resistance in both directions and have enough elasticity to work within the normal range of movement. This is criticle on NC and ND cars. Their 5 link rear suspensions are terrible for bushing bind.

 

The same theory of using the same method works for toe measurements also. Lots of people use strings at the shop. Measuring from the wheel lip to the string box. The subtracting to get a toe measurment in inches or millimeters based on a 15" wheel. then at the track they use toe plates for a quick check. Toe plates are generally 24" long. Roughly 50% longer than the wheel diameter. Now the angle of toe has not changed, but the method of measurement has change from 15" to 25". So you math with be different. This is hard to explain, so you need a visual aid. Grab a paper and pen.

 

Draw a horizontal line about 3" long.

 Now draw a verticle line at the left tip of the first line about 6 inches long. Should look like a sideways "T"

Now at the other end of the horizontal line draw a verticle line at a 30* angle. Should look something like this "l-/" but connected.

Imagine the horizontal as your wheel spindle. measure up 1" and down 1" to reflect the wheel diameter. Make marks.

Now measure 1.5" each way, this reflects the toe plate (roughly 50% longer). Make marks.

Now measure from the 1" marks to the angled line, subtract and get a toe reading.

Now measure from the 1.5" marks and get a toe reading.

The 1.5 marks are 50% larger!!!!!

 

The toe angle never changed, but we measure the difference and get an inch/mm number. That can vary wildly based on measurement technique.

You must use the same method of measurement all the time. String boxes are accurate, toe plates are accurate, big fancy laser machines are accurate. But the measurements they supply do not always match each other. FYI, the big fancy machines and the specs published by manufacturers are based on wheel angle converted to inches/mm, based on a 24" wheel/tire diameter.

 

Hope that helps

dave

[Johnny D]

Welcome Jonathan.

I like to say how impressed I am. A pic and using your name, very nice. We don't get that a lot on the 1st post.

J~

[lillyweld]

Nice job, Dave!  Thanks.

[J.L.Racing]

Thank you Dave for the prompt & thorough response.

 

I was just thinking whether I over-estimated my front toe out setting when I set my car up last weekend (first time doing it myself) because I was aiming for 1/8" toe out on my 15 inch wheels, whereas the race shop that recommended the setting to me has "big fancy machine" that may be utilizing a different diameter measurement, 24" as you mentioned. 

 

I have a related follow-up note on suspension binding. Since control arms only move up and down, is it correct to assume that the polyurethane bushings if greased properly at installation would minimize chances of suspension binding? Also, would one advantage of spherical bearings in spindles for example is the complete elimination of binding?

[Bench Racer]

Dave, excellent write up.

Jonathan, Dave will return with an answer ^.

Not knowing your background and so on, have you reviewed the "Downloads" Spec Miata Setup Guide and compared to what your doing?

[davew]

The advantage to spherical bearings (aka heim joints) is no deflection and no binding. Poly bushings eliminate most of both issues, but not completely. The only poly bushing we can use in SM are the offset front upper control arm bushings. So we only eliminate a small percentage of the overall binding in a SM.

 

dave

[J.L.Racing]

Thanks for the update.

 

As for my background, I track a Miata and race a Nissan 240. I don't do spec Miata racing up in Canada as it's nearly non-existent here, but I find there is a wealth of information on this forum.

[davew]

There appears to be  a pretty popular SM class on both coasts. We ship lots of parts to Canada. Somebody is using them.

 

dave

[J.L.Racing]

Thanks Dave. To correct myself, the series is currently not present in Ontario, but there are some presences in Atlantic and Western Canada. It may have to do with the type of race tracks we got here. 

 

I think it would be a really fun series to run without breaking the bank.

[SaulSpeedwell]

I agree with Dave here, emphasis in bold is mine: 

 

"Or you can come up with a method that is repeatable and works for you. Just remember to always use the same system. Then only compare your system to your system on identical cars.

 

My method:

Set steering wheel and rack to center position, adjust if they don't match

Turn wheel 1/2 turn in one direction, measure camber. 

Turn wheel opposite direction 1 turn (1/2 turn the other way). Measure camber and record difference. Wether you turn out, then in or vice versa only tells you if you have positive or negative caster. Since no new car has been built with negative caster since the early 1970's (bias ply tires) our number is always positive.

Now do the other side and compare."

 

The only difference is I use 1/4 turn because I am also looking at the "weight jacking" in the % cross numbers at those points, and 1/4 turn is closer to where we are on track.  I want to understand how the % cross is changing dynamically for diagnostic reasons (is something bent?) or to sort out a performance problem.

 

For a racecar where we don't care about steering wheel return  effort and climbing crowns of roads, etc., I just think of caster as "More camber gain as I turn the wheel".  If you think you have a camber problem in a slow 2nd gear corner, you can get more effective camber in that sharp corner by increasing the caster, without changing (much) the effective camber in a Kink or under braking.

 

Which brings up another point - more camber is generally disadvantageous for lowered production cars when it comes to braking ... and when it comes to, say, RR wheelspin in the Keyhole at Mid Ohio! 

 

There are speedy guys in SM that run max caster, and speedy guys that run minimum caster, and the NA and NB have different caster ranges. It is more important to follow the bold advice above and monitor your car than trying to figure out what Hille/Drago are running for caster. 

 

Caster split can make a dramatic difference at tracks that are more oval than road course.  If Nelson Ledges ever hosts the Runoffs, go max caster left, min caster right!  

 

Measuring your wheelbase is another good idea - since it changes as a function of caster.  Whenever you find a car that requires very different caster bolt settings to achieve the same number left to right, it is a pretty good indication of something being bent.  If the wheelbase measurement is in the same direction as the caster difference (low caster side is shorter wheelbase side), you can be 95%+ sure something is bent.