Caster Talk: How much is right, How much is too much?

CTaulbert · Apr 14, 2013

Alright guys, I thought that it would be a good idea to put together some caster information for the HAMB. It seems like there are new “how much caster should I run” threads often, and they never completely reach a conclusion. The most consistent response I’ve seen so far is 7°, but does anyone really know if that this is right?

I was talking to a buddy who has struggled with tire shake/shimmy issue at high speeds on a specific car. He’s done several things to attempt to remedy the problem: shaved tires, balanced drums and hubs, checked wheel runout, and switched to Bilstein tube shocks (from stock Houdaille shocks). While the shock change did help a lot, it still had some shake. He finally switched to some radial Excelsiors, which have turned the problem off. That particular car is running about 8° of caster.

So how does 1° make such a difference? Well for starters, you need to make sure that you understand how caster is measured. Since we’re traditional here…..and 99% of the time dealing with a solid axle….let’s talk about the kingpin. Basically, the caster can be visualized as how much the kingpin is rotated in the fore and aft direction of the car. If it’s rotated forward, that’s negative caster. If it’s rotated rearward, that’s positive caster. The angle of caster is measured relative to the ground. The amount of caster needed is often a tradeoff between steering easiness, and wheel returnability. If you have too much positive caster, you can induce a tire shake/shimmy. Also as an important note here….underinflated front tires can also induce a tire shake/shimmy too.

The amount of caster directly correlates to the length of caster trail in a given suspension. If look at the side of the suspension and project a line from the kingpin down to the ground, and then measure to the center of the tire contact patch, you get your caster trail. It’s also dependent on the radius of the tire (all seen the first drawing).

You can see that as positive camber is increased, the length of caster trail also increases. As I mentioned above, excessive caster therefore excessive caster trail can create tire shake/shimmy. A great example here is those damn grocery carts….I always seem to get the lucky ones that shake all around the store. Fortunately, most of the hot rodders here on the HAMB are running the same general recipe of front tire….4.50/4.75-16, 5.00-16, or 5.00/5.25-16….all of those are just about 26” tall. So maybe, just maybe, we’re starting to reach a conclusion…….

Now most of the time, we don’t think about what a car does dynamically. We only pay attention to things in a static state. So maybe ½” of clearance between a valve cover and your firewall looks good statically, but once the engine is torqueing or bouncing around, you find that it may have need a little more. The same goes for your tires….. You may have seen some of the cool profile shots of a tire rolling before, and were surprised to find that it’s not round! It’s true….as the car is rolling down the road, the wheel is actually pulling the tire forward. The tire goes into a somewhat-egged shape, and the tire contact patch actually moves rearward. In my second diagram (and hey…I’m no illustrator here), you can get a visual idea of what happens.

Now of course my diagram is exaggerated to drive home the point, but the amount of deformation that does occur can create some side effects. For one, the length of caster trail increases.

Now remember how I mentioned that my buddy was able to cure his problem by switching to radials? So why did this help? Well, radial tires deform less at speed when compared to a bias ply tire. This means that you have less increase in caster trail when using a radial tire than when using a bias ply tire. My theory for his given suspension setup is that 8° of caster is ok with radials, but the tire deformation that occurs with bias plys increases the caster trail enough to induce his tire shake/shimmy at high speed.

I often see people setting their front crossmembers (and axles) up in a frame, and just throw in 7° of caster and call it good….but is the frame mocked up at ride height? If not, are you taking into consideration that it isn’t? Remember, caster is measured relative to the ground, with the vehicle at ride height. 7° of caster in your frame jig doesn’t necessarily correlate to 7° of caster once the car is on the ground. For example, if you built your frame on a table or in a jig with it sitting level, and setup your crossmember with 7° of caster but your ***embled car had 3° of rake (measured on the frame), you now only have 4° of caster in the crossmember.

So, I checked around to see what modern solid axle vehicles have for caster specifications. I took a look at a Ford F550. Per Ford, that vehicle is set at 3° +/- 2° of caster. So that means it could fall somewhere between 1° and 5°, and be acceptable for a F550. I really think that 7° is probably too much for our style of vehicles. We're probably creating some high steering efforts, and not actually needing the wheels centering forces that are being created.

So to summarize everything, make sure you setup your caster with the frame mocked up at ride height. Also, remember that you measure caster relative to the ground, and not the top of the frame rail. I personally run 5° of caster on my cars with a solid axle. I also run 32-35 psi in the front tires (depending on the front axle weight for that given car). Those cars have rolled down the road just fine, never getting into a tire shake/shimmy condition.

Hopefully this thread can lead to some good discussion, and serve as some solid advice for anyone in the future looking for guidance.

daddy_o's_diner · Apr 14, 2013

Great info Cory.

Thanks for posting.

.

'54Caddy · Apr 14, 2013

Great info! Thank you for sharing!

325w · Apr 14, 2013

Thank for the info. Mine ended up closer to 8-9 degrees. Everything works fine so I guess they may all be different to start. We set it 3 in the crossmember then 5 at the king pin and added some to get easy steering and wheel rebound. 26 inch tire.

hoop98 · Apr 14, 2013

Thoughts:

Caster Effects

Camber Effects

More Caster = good camber change (negative on outside, positive on inside) as more steering is added. Primarily low-speed-corners. At speed we don't turn the wheels to high angles.

More KPI = positive camber in both directions, not good on outside.

Weight Jacking:

Caster * Scrub Radius = diagonal weight jacking in the oversteer direction, as steering is added. Good for getting the car to rotate in low speed turns, helps with low speed plow.

KPI * Scrub Radius = weight lifting (jacking up the front end of the car), increases as steering is added.

Lateral Force Effects:

(Mechanical Trail + Pneumatic Trail) * Lateral Force generates steering wheel torque, driver feedback.

donut29 · Apr 14, 2013

Great info Cory, I talked to a few old-timers and ended up setting up my 34 ch***is at 5* at ride height.

Joe H · Apr 14, 2013

How does caster effect driving on rougher roads or dips?

I lowered my truck ( '37 Chevy with '41 steering box) in the front a bit, at the same time I added a extra degree of caster using wedges under the axle ( total of 4 degree). I also had to adjust the drag link to keep the steering wheel centered in the gear box. I now get a strange steering feel when going over bumps while turning, almost like the wheels are loose and the tires are turning / wobbling at random over the bumps. At highway speed it as smooth as my '03 Toyota. Dips in the road tend to steer it a little. I still have the straight axle and stock leaf springs, tube shocks, and a late model in line 6 (150 lbs lighter). The steering effect just seems looser now? Will to more caster tighten it back up?

Joe

Texas Webb · Apr 14, 2013

Really good info,I will save.

41GASSER · Apr 14, 2013

Great info, I have not welded my perches to the axle so I will measure twice and weld once. Thanks

daddio211 · Apr 14, 2013

Excellent write up.

Sent from my DROID device using the TJJ mobile app

stillrunners · Apr 14, 2013

good stuff....thanks....

3wLarry · Apr 14, 2013

how'd you get so smart...

TexasSpeed · Apr 14, 2013

Good stuff. I still haven't finalized my front end with my lack of free time so this is very valuable for me right now. Thanks for the great write-up.

35chevycoupe · Apr 14, 2013

I had a 05 harley sportster and at speeds over 75-80 it would get a little wooble or shake in the front end. I checked every thing and put a hi end tire on the front and it would still do it. I ended up putting a steering dampner on and it solved the problem 100% I use to p*** my ridding budys wih no hands doing 100mph. It handled very nice. Even if it isnt the best way to fix it, The steering dampner is a really good way to cover up the problem.

porknbeaner · Apr 15, 2013

Cory,
When yopu are talking about our style of vehicles I ***ume you mean old fords?

7 +/- is correct.

Here is something to think about, it is not just a caster adjustement that one needs to take into account, it is a combination of caster and camber. To be accurate you will need to go back to the big Ford and check both camer adjustment as well as caster and make a comparison to an old axle like we use.

You can run as low as 5 degrees and asd high as 10 degrees on one of our old hoopties and still be drivable, 10 degrees is pushing it to the outer limits but if it is going to run mostly fast in a straight line 10 degrees id OK.

dennis g · Apr 15, 2013

your drawings are excellent. Bias ply tires have to bend their tread so they are the ones which will show the egg shape you drew. I began at 3 degrees but my car was un-controllable so I shimmed it until it steered right. That setting was 8 degrees for my setup which was a 56 ford axle in a mid 70's jeep frame. (I use bias ply tires)

CTaulbert · Apr 15, 2013

Joe H said: ↑

How does caster effect driving on rougher roads or dips?

I lowered my truck ( '37 Chevy with '41 steering box) in the front a bit, at the same time I added a extra degree of caster using wedges under the axle ( total of 4 degree). I also had to adjust the drag link to keep the steering wheel centered in the gear box. I now get a strange steering feel when going over bumps while turning, almost like the wheels are loose and the tires are turning / wobbling at random over the bumps. At highway speed it as smooth as my '03 Toyota. Dips in the road tend to steer it a little. I still have the straight axle and stock leaf springs, tube shocks, and a late model in line 6 (150 lbs lighter). The steering effect just seems looser now? Will to more caster tighten it back up?

Joe
Click to expand...

Joe - it sounds like you've induced some bump steer issues when you changed up your front suspension. Maybe the geometry of the draglink is causing some issues now?

3wLarry said: ↑

how'd you get so smart...
Click to expand...

I never said I was Larry!

35chevycoupe said: ↑

I had a 05 harley sportster and at speeds over 75-80 it would get a little wooble or shake in the front end. I checked every thing and put a hi end tire on the front and it would still do it. I ended up putting a steering dampner on and it solved the problem 100% I use to p*** my ridding budys wih no hands doing 100mph. It handled very nice. Even if it isnt the best way to fix it, The steering dampner is a really good way to cover up the problem.
Click to expand...

Here's my view of a steering damper....they're a bandaid to the real problem. Looking at the suspension in a physics standpoint, you've just added resistance to the system. You could also add some weight (m***) instead to "tune" the resonance, or shake, to another speed or condition.

porkn****** said: ↑

Cory,
When yopu are talking about our style of vehicles I ***ume you mean old fords?

7 +/- is correct.

Here is something to think about, it is not just a caster adjustement that one needs to take into account, it is a combination of caster and camber. To be accurate you will need to go back to the big Ford and check both camer adjustment as well as caster and make a comparison to an old axle like we use.

You can run as low as 5 degrees and asd high as 10 degrees on one of our old hoopties and still be drivable, 10 degrees is pushing it to the outer limits but if it is going to run mostly fast in a straight line 10 degrees id OK.
Click to expand...

It looks like the F550 falls at 0.75°, but I don't know that the amount of camber is leading shake/shimmy issues since it often occurs rolling straight down a highway. Now I understand that the chamber/caster combination will create jacking for one of the wheels (the inside wheel jacks on a vehicle that has positive camber), but I don't think guys that experience the issue are steering enough at that condition to have camber contribute to it.

If you look at caster specifications for modern vehicles, 99% of the time it's 5° or less.

tommy · Apr 15, 2013

hoop98 · Apr 15, 2013

porkn****** said: ↑

Cory,
When yopu are talking about our style of vehicles I ***ume you mean old fords?

7 +/- is correct.

Here is something to think about, it is not just a caster adjustement that one needs to take into account, it is a combination of caster and camber. To be accurate you will need to go back to the big Ford and check both camer adjustment as well as caster and make a comparison to an old axle like we use.

You can run as low as 5 degrees and asd high as 10 degrees on one of our old hoopties and still be drivable, 10 degrees is pushing it to the outer limits but if it is going to run mostly fast in a straight line 10 degrees id OK.
Click to expand...

I would add KPI/SAI and Scrub radius to that equation.

CTaulbert · Apr 15, 2013

hoop98 said: ↑

I would add KPI/SAI and Scrub radius to that equation.
Click to expand...

In an attempt to not muddy the waters on this thread, I don't know that we need to discuss Kingpin Inclination....I don't think anyone here is changing what Ford designed into their axle, so discussing it may confuse anyone using this for caster information in the future.

hoop98 · Apr 15, 2013

I was referring to comparing Caster on the early ford and the 550. I agree no one will be changing it.....

In my opinion, the biggest thing about caster is really trail, and of course that includes mechanical and pneumatic.

This gives you the return to center/steering feel.

And the leverage on that is greatly effected by the Scrub Radius or wheel offset.

I have no idea what a 550 has for scrub radius, but I think that would be important when drawing conclusions/comparisons.

jm2c
Hoop

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Caster Talk: How much is right, How much is too much?

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