The Ultimate Handling Guide Part 8: Understanding Your Caster, King Pin Inclination and Scrub

The Ultimate Handling Guide Part VIII: Understanding Your Caster, King Pin Inclination and Scrub

By Mike Kojima

Understanding what caster does and how to use it is a powerful tool in the box of a suspension tuner.  Since caster is not adjustable on nearly all FWD cars and usually not adjustable for many late model cars as well, we saved its discussion until now.  Discussions of caster pertain to the front suspension only as caster comes into play as steering angle is induced.

To read the rest of this series click here!

The Ultimate Handling Guide Part 8: Understanding Your Caster, King Pin Inclination and Scrub
Caster in a car is like fork rake on a bike.  They have about the same effect on both.

Caster is the angle from vertical of an imaginary line drawn when looking at a car from a side view through the ball joints of a multilink suspension car to the ground.  On a MacPherson strut car the caster angle is the angle of the line from vertical drawn from the top strut mounting point through the lower ball joint to the ground.  Think of caster as the fork rake angle of a bicycle or a motorcycle.

The Ultimate Handling Guide Part 8: Understanding Your Caster, King Pin Inclination and Scrub
This is positive caster in a car.  It is the angle about which the steering pivots in relation to the front and rear of the car.

Positive Caster is when the angle is sloping backward toward the rear of a car like fork rake on a bicycle or motorcycle.  Negative caster is when the angle slopes forward. Negative caster in the steering geometry just doesn’t work so we won’t discuss it any further. Vehicles do not use negative caster.

The Ultimate Handling Guide Part 8: Understanding Your Caster, King Pin Inclination and Scrub
This illustration shows positive caster, negative caster which no car ever has to our knowledge and the shopping cart caster analogy which we will discuss on the next page. You should be getting the idea by now!


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Scott Helmer
Scott Helmerlink
Sunday, June 12, 2011 10:40 PM
A most excellent article as always, Mike. Can't wait for the next article (seriously, you know I've been dying for some more in-depth information on suspension geometry)! I'm kinda hoping you'll throw us AE86/SA22 guys a bone and lay down some good tips on live axle link geometry, if at all possible. I understand that it only helps a select few (unless you count the muscle car crowd, of course), but it certainly couldn't hurt to have as much information as possible here on MotoIQ.

By the by, kind of off topic, but yeah I suspect that most of the crowd at LeMans may very well have been quite drunk (As you may have guessed, this is that Scott kid that recently friended you on Facebook; god damn man, have you got a lot of friends on there). Hell, I would be!
Monday, June 13, 2011 1:29 AM
Can't wait for the next article Mike, very nice writing!
This is very helpful for the average enthusiast. I am a student of automotive engineering and next semester we'll be looking into suspension design so I hope this info here will help me get aquinted with the subject quicker :)
Dusty Duster
Dusty Dusterlink
Monday, June 13, 2011 4:47 AM
Great article. My mechanically-challenged electrical-engineering brain actually understood it.

I'm not going to lie to you, though. The Kojima Hellaflush pic near the end made the article.
Der Bruce
Der Brucelink
Monday, June 13, 2011 6:44 AM
The more my brain digests these articles, the more I have to rethink even something as simple(or now not so simple) as a wheel purchase.

You wouldn't happen to have a top down view so that I could better visualize where the KPA and caster lines meet?

If I understand caster better now, does it mean that those (many hellaflush included) who end up with a larger overall tire diameter are decreasing positive caster wth the now smaller degree and therefore negatively effecting steering?
Monday, June 13, 2011 6:47 AM
I'm glad to see the Dave point was included in this article. I remember reading about it many years ago.
Monday, June 13, 2011 6:57 AM
This was slightly over my head but thank goodness for that, I can say without a doubt I learned something today. Probably not well enough to apply it but time will tell.
Der Bruce
Der Brucelink
Monday, June 13, 2011 8:03 AM
I reread it, the degree would be the same but for some reason I thought the relative angle would seem smaller.
Monday, June 13, 2011 11:51 AM
Still dont get it. More biker chick pics plese in the interest of true understanding......
Mike Kojima
Mike Kojimalink
Monday, June 13, 2011 1:57 PM
Do you know how hard it was to find a hot one?
Monday, June 13, 2011 2:29 PM
we went from oem +53 offset wheels to +15 and now the tires stick out way past the stock fenders. Is that bad?
Mike Kojima
Mike Kojimalink
Monday, June 13, 2011 3:05 PM
Depends the the proportions of things.
Monday, June 13, 2011 3:15 PM
In a FWD, going from a 15x6 wheel with +50 offset, to a 17x7 wheel the offset should decrease to 40~45mm?
Iron Giant
Iron Giantlink
Monday, June 13, 2011 3:25 PM
Does adding caster typically add anti-lift/dive geometry to a FWD/AWD car? Looking at pics it appears to move the IC forward, but I haven't actually heard anyone mention it or how much it makes a difference.
Monday, June 13, 2011 4:08 PM
5th page, 2nd. picture to the last, EPIC !
Monday, June 13, 2011 4:09 PM
I usually drive a Toyota minivan but recently test drove a 323i wagon. The steering feel was very high but the wheel was really heavy compared to the light steering and play in my Toyota. I was surprised that I really had to concentrate on turning and on holding the wheel.
Mike Kojima
Mike Kojimalink
Monday, June 13, 2011 4:56 PM
Supercharger, that offset is proportional but you might not have enough inside clearance. I would say you might end up using a 35-40mm offset.

Iron Giant, caster would only potentiality affect anti much on a strut car. I think the position of the lower control arm makes a bigger difference as far as screwing with the anti.
Monday, June 13, 2011 6:02 PM
Mike, I've had a question about caster/camber gain for so damn long I finally made an account just to ask it. I think I've been carrying this since the SCC days.

I've always been under the assumption that the only wheel gaining negative camber while turning is the outside one, while the inside wheel gains positive camber. If I'm totally wrong then this doesn't matter, but I'm basing it off what I've seen in photos, using the bike example, and fiddling with my own rides.

While that gain in negative camber seems to outweigh the positive inside wheel gain in grip driving considering the outside wheel is feeling most of the total forces, what about drifting/rallying when the driver is often at or near opposite lock? Doesn't adding caster increase the amount of positive camber in the outside wheel, and why isn't it better to try to have a more neutral setting?
Mike Kojima
Mike Kojimalink
Monday, June 13, 2011 7:17 PM
It's mostly because of the need for self steering outweighing the need for front grip because most of the weight is transferred to the rear at this point. Wow that was short.
Tuesday, June 14, 2011 4:08 AM
Short's better than my rambling. That makes total sense, thanks.
Tuesday, June 14, 2011 4:52 AM
what size front tire do you use on the front of dai's car?
Mike Kojima
Mike Kojimalink
Tuesday, June 14, 2011 10:15 AM
For drifting we use a 245/35-18 For Gymkhana we use a 275/35-18
Eric Hsu
Eric Hsulink
Wednesday, June 15, 2011 4:22 AM
Mike: too bad we don't have a video of the XS BNR32 to illustrate the massive jacking that occurs with excessive scrub and massive KPA. That would be a perfect example of what not to do.

That hella flush credit card pic is awesome BTW!
Friday, June 17, 2011 6:30 AM

Thanks once more for yet another great reading article.

Just one question here: you mention, quote"Sometimes when running much wider wheels and tires on certain cars like a 300ZX, you want to actually reduce the positive caster".
Is this always the case, even if you keep the original wheel offset, or just the fact of using wider tires, forces this geometry changes (if you keep the wheel radius/diameter the same, that is)?
Sunday, June 19, 2011 8:42 AM
The reason I asked tire size is because i have a 245 40 18 upfront now and its too tall. my car is not that low by any means and I am having alot of clearance issues with the foot wells and I still have alot of caster and I would like to take out another degree or so
Mike Kojima
Mike Kojimalink
Sunday, June 19, 2011 10:55 PM
If you don't have enough KPA, a lot of caster will tip the tire over so much it will have too much camber. Run a shorter tire.
Monday, June 20, 2011 6:40 PM
Thats the problem I dont want to drop to a 225 up front and I dont know if dropping to a 245 35 will be enough? a 235 35 would be ideal but is proving to be hard to find. I am also having a hard time trying to find a way to measure kpa and scrub. thanks for putting things like this up mike you have been a huge help and inspiration to me.
Tuesday, June 21, 2011 9:16 AM
Wow, I'm surprised I actually understood a good deal of this article! Well written and LOL at the hellaflush pic & comments.

So, how does one actually measure these things on a real car and figure out, "Yes, X wheel size with Y offset will be okay for my scrub radius"?
Mike Kojima
Mike Kojimalink
Tuesday, June 21, 2011 9:27 PM
I use a plumb bob and angle finder.
Mike Kojima
Mike Kojimalink
Tuesday, June 21, 2011 9:28 PM
Sometimes some trig too.
Friday, July 01, 2011 8:13 AM
Thanks Mike, I learnt a lot about KPI that I did not know before!

Now to find time to measure all this shit. Our Genesis Coupe racecars have next to zero self steer, and I'm not sure if something is off, or if it's just these Howe 1.5:1 steering thickeners that were installed in the cars. It sucks balls though!
Dave Coleman
Dave Colemanlink
Friday, December 30, 2011 6:27 PM
Mazda's new Skyactiv platform runs over 7 degrees of caster. More than any other front-drive car. Hooray for steering feel!
Thursday, February 07, 2013 8:01 PM
Just reread through this article - it recommends fwd/awd cars about 3-4 degrees in positive caster, which is about what the evo 8/9s have - a good number of the guys on evom have been experimenting with increasing positive caster, either by turning camber plates sideways or using something like Perrin's PSRS, and generally find all good results in performance except one main thing: every single one comments that it actually slows down/dulls steering response - not sharpens it. can anyone explain?

sorry my posts are so one-car focused.
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