Caster and shock angles...am I overthinking this?

[Zandoz]

One of the open issues in my mind is the upper front shock mounts. I've not found any "off the shelf" units that will work. I will be using a front suspension configuration similar to this...

only with the lower shock mount pointing to the inside rear. The axle I'll be using is a 48" king pin width, with the spring perches 36-3/8" on center. This will put the lower shock mounts around 3-4" outboard of the frame.

With this set-up, as the caster angle is changed, the angle of the shocks...and their upper mounting point will also change. Without the upper mounting point changing, it seems to me binding would result. But I've yet to see an adjustable upper shock mount for these cars.

When configuring the front shock mounts, is it just a matter of settling for a happy medium for the shock/caster angle? If so, what angle is a good happy medium angle?

 

[iroct]

These aren't rock crawlers,
the suspension doesn't move all that much.

 

[choppinczech]

Depends on the suspension type.

With a parallel 4 bar set up, the caster won't change. The axle will travel vertically. Your shocks will compress in the same vertical path.

Split wishbone set ups like mine, or radius rods, being 3 point configurations, travel in an arc. So my shock mounts are at the same 7° back angle as my perch, axle and spring to follow that arc.

Kind of hard to see in the photo. But the mounts are tilted back.

 

[choppedtop]

I agree with iroct, the shocks don't really have much travel, depending on your spring stiffness. Mine don't travel up more than 3/4". Most of my ride comfort comes from the read end of the car.

 

[bobs66440]

Agreed. In almost 700 miles of traveling on super bumpy NY roads, mine has had less than 1" compression travel (checked with tell-tales on the shock rods). This amounts to a minuscule, virtually unmeasurable arc. Any potential binding will be absorbed by the rubber bushings in the shocks. I would say just make them as close to 90 degrees of the vertical suspension travel as possible and call it good.

 

[Zandoz]

I'm not talking about shock angle changes during suspension travel...thats a given. I'm talking about the angle changing during fine tuning of the caster angle. I've heard of caster angles from 3.5° to 9° for solid axle suspensions...a pretty big potential change of angle. If the shock mounts are welded on at say 5°, and after driving the finished car it ends up needing 8° to drive right, those shocks are going to always be binding. <shrug>

 

[ellis8500]

Here's pic of my front shock setup. 10 degree off vertical inboard. Use RPM mounts and shackle plates. You might be able to see the amount of travel on the shock rod as the seal pushed the dirt. Most of that is from my driveway approach. With this design, changing caster does not affect the shock mount since it's attached to the spring shackle.

 

[Zandoz]

Here's pic of my front shock setup. 10 degree off vertical inboard. Use RPM mounts and shackle plates. You might be able to see the amount of travel on the shock rod as the seal pushed the dirt. Most of that is from my driveway approach. With this design, changing caster does not affect the shock mount since it's attached to the spring shackle.

View attachment 11142

My suspension is a completely different configuration. It is spring over axle, with spring perch bolts through the axle, and the lower shock mounts on the bottom of the perch bolts. The shock mount angle will change fore and aft, as the axle is rotated to adjust the caster.

 

[2old2fast]

I think most would agree that w/these light weight , short wheelbase cars , 7*-9* caster is pretty much the "norm"... set your frontend up within those parameters & it's unlikely that you'll have a problem.... in the event that you do ,well , you'll have to deal with it... so yea , you're begining to suffer from
'paralysis by analysis !!"

dave

 

[ellis8500]

It looks like you would also twist the spring if you change the caster angle. Does the axle have a built in caster? If so, just go with that and you'll probably be fine. As 2old2fast mentioned, we often over think some of these issues. There is no exact caster angle for front suspension. Too little and car will wander; too much and it gets too hard to steer at stop or slow speed. I have mine at about 6 deg. Tacks true and slow speed steering is not too much. When you get it on the road you will find that the toe setting is more critical and has to be adjusted for your car. Too much In or Out and you'll find the Death Wobble kick in. Take wrenches and be ready to adjust. Start with about 1/8th toe in and drive it. If it wobbles, adjust out a bit, Check again. Keep adjusting until no more wobble. Even after about 500 miles I had to readjust due to the new, tight parts wearing in as one would expect.

 

[Zandoz]

I think most would agree that w/these light weight , short wheelbase cars , 7*-9* caster is pretty much the "norm"... set your frontend up within those parameters & it's unlikely that you'll have a problem.... in the event that you do ,well , you'll have to deal with it... so yea , you're begining to suffer from
'paralysis by analysis !!"

dave

LOL...analysis was my life for close to 30 years. My job was to avoid the unexpected, and find the hidden flaws. I was probably paralysed 5 seconds after I decided to tackle this project...LOL.

Fords recommended caster range during the 1930s was 4.6° to 9°. I've found frequent recommendations for a 5°-7° range...and a few even lower. So what angle should be "built in"?

 

[Zandoz]

oooppps

 

[2old2fast]

Keep in mind that 30's fords didn't have radial tires , offset wheels , & never went much over 40 mph.... this is one of those leap of faith times.... sounds like that may be a stretch for you , but ..... pick a number & go with it ......
dave

 

[Zandoz]

It looks like you would also twist the spring if you change the caster angle. Does the axle have a built in caster? If so, just go with that and you'll probably be fine. As 2old2fast mentioned, we often over think some of these issues. There is no exact caster angle for front suspension. Too little and car will wander; too much and it gets too hard to steer at stop or slow speed. I have mine at about 6 deg. Tacks true and slow speed steering is not too much. When you get it on the road you will find that the toe setting is more critical and has to be adjusted for your car. Too much In or Out and you'll find the Death Wobble kick in. Take wrenches and be ready to adjust. Start with about 1/8th toe in and drive it. If it wobbles, adjust out a bit, Check again. Keep adjusting until no more wobble. Even after about 500 miles I had to readjust due to the new, tight parts wearing in as one would expect.

No issue with spring twist for my set-up....the head of the spring perch swivels. There is no caster built into the axle itself. But there will be some built into the frame, by the angle between the mountings for the ends of the 4-bars. Before I started with researching the caster, I had assumed a 5 degree angle built in with the 4-bar mounting points...and the shock mount angle...thinking that would be a good happy medium. Now I don't know if 5° is a good built-in angle, or not.

Fortunately the toe setting does not effect anything difficult/expensive to change....LOL

 

[bobs66440]

If it means anything, mine is set at 7 degrees and it drives perfect. I think if you go with that as a starting point you should be real close. As long as your suspension is designed correctly, a degree plus or minus shouldn't be the difference...and I think if you have to change it significantly from there, you have other issues...

 

[2old2fast]

Ya'see there you go Bob likes 7* , mines at 9* , someone else will be at 5*....it's your call !!
dave

 

[Zandoz]

Keep in mind that 30's fords didn't have radial tires , offset wheels , & never went much over 40 mph.... this is one of those leap of faith times.... sounds like that may be a stretch for you , but ..... pick a number & go with it ......
dave

Yeah, I pretty much have no faith in leaps of faith...LOL. Besides being excessively analytical by nature, I'm doing this on a seriously tight budget, and a race with my diminishing health. Making a wrong assumption that takes a lot to fix could mean a major delay, and could be a project killer.

I'm trying to work out as many question issues while saving to have the frame built. The Minister of Finance (wife) is telling me we are close to sufficient funds, so now I have to figure out all those "I'll deal with that later" issues, that I've put off. This is one of those.

 

[ellis8500]

I bought my front axle from RPM and I believe it has 6 deg built when the batwing holes are in-line with a vertical line perp to the ground. Ron can verify. I set mine that way and it is at 6 deg.

 

[Zandoz]

My axle is a Super Bell, from Speedway. According to Speedway's tech support, there is no built in caster...but there is 1-1/4° positive camber built in.

 

[choppinczech]

Besides being excessively analytical by nature, I'm doing this on a seriously tight budget.

I'm the same way Zandoz.