Caster change by extending shackle (1 Viewer)

[pjohnson]

I could do this with geometry, but I forgot most of it long ago....

Maybe someone has already figured it out and has the answer. Or maybe someone wants to practice their geometry skills;p

My caster is about 3* and I want to increase it to about 6* by adding length to the shackle. The pinion will come up a bit, but that's ok. The question is, how much length...

Axle is spring over with a cut'n'turn. Stock front springs that have been flipped to move the axle forward. They sit pretty flat. The shackle is 5-1/4" center-to-center. It is shackle reveresed.

Does anyone know the answer?

Phil

 

[amaurer]

wouldn't you want to decrease the shackle length to get more caster?

EDIT: ooh, your avatar reminds me i missed your monthly meeting again!!! damnit!

 

[Pin_Head]

The sign of the caster change would depend on whether the shackles are in front or reversed. In front the caster angle becomes more negative.

Anyway, the change in angle is approximately 1.4 degree for 1 inch longer shackles.
The formula is:
arctan(1/40) = 1.4 degree
The springs are approximately 40 inches and the change is shackle length is 1 inch.

 

[pjohnson]

Oops.. I forgot to mention the shackle reversal. Edited the original post.

Phil

 

[Pin_Head]

The angle change also depends on the shackle angle. The calculation above assumes the shackle is 90 degrees to the spring. If the shackle angle is 45 degrees, you only change the caster by 1 degree.

Six degrees is a lot of caster. 3 degrees is about right. With more caster, you can lose traction in high speed turns and wear the edges of your tires.
Why do you wan't more?

 

[dgangle]

You are not going to get there changing shackles alone. Shackles and/or shims will be required. Both are going to change your pinion angle.

 

[pjohnson]

Also forgot to mention 37" tires... I think the move up to a 37" MTR from a 35" bogger along with flipping the springs has caused the change in caster and pinion. (although it doesn't make sense now that I think about it)

I get some DW when I hit bumps (yes, everything has been rebuilt and/or checked) and my pinion angle is actually up about 6* so this would help in both situations.

I don't think high-speed turns will be much of an issue with a 2F and 37's. It's actually not fun to go fast in this beast. It's also not a DD, just has to get to the trail and back.

<snip>Anyway, the change in angle is approximately 1.4 degree for 1 inch longer shackles.
The formula is:
arctan(1/40) = 1.4 degree
The springs are approximately 40 inches and the change is shackle length is 1 inch.

Does this hold true even when the axle is in the middle of the spring? Wouldn't that reduce the length needed for the same amount of angle? My shackles are pretty close to 90* to the spring. By your calculations then, I need about 2" of shackle length to get about 3* of added caster angle, correct?

Thanks,
Phil

 

[pjohnson]

Anyone else have experience doing this?

Phil

 

[Pin_Head]

Also forgot to mention 37" tires... I think the move up to a 37" MTR from a 35" bogger along with flipping the springs has caused the change in caster and pinion. (although it doesn't make sense now that I think about it)

I get some DW when I hit bumps (yes, everything has been rebuilt and/or checked) and my pinion angle is actually up about 6* so this would help in both situations.

I don't think high-speed turns will be much of an issue with a 2F and 37's. It's actually not fun to go fast in this beast. It's also not a DD, just has to get to the trail and back.



Does this hold true even when the axle is in the middle of the spring? Wouldn't that reduce the length needed for the same amount of angle? My shackles are pretty close to 90* to the spring. By your calculations then, I need about 2" of shackle length to get about 3* of added caster angle, correct?

Thanks,
Phil

Increasing the caster won't necessarily cure the DW, but it might change the speed at which it happens and the frequency. For DW, something has to be loose or have some slop in it.

The angle change will be the same all along the spring. 2 inch extenstions will give roughly 2.8 degrees max.

High speed turns would be anything above 35 MPH, like driving to the trail.

 

[dgangle]

Here is a recent thread of a very similar situation where this guy has +6 degrees caster, DW and is trying to cure it just the opposite way you are.

I am not saying either of you are right, I am saying you both have other problems causing the death wobble.

https://forum.ih8mud.com/40-55-series-tech/275392-caster-8*-whats-up.html

something to look at are:
Play in steering components
loose u-bolts
loose shackles/worn spring bushings
incorrect toe-in
loose trunion bearings
loose wheel bearings

 

[pjohnson]

Yeah, I remember reading that thread.....

I only get DW induced by bumps. Everything has been rebuilt/checked. Seriously, all of it.

Hmmm.... guess I'll keep looking for the solution. I am not convinced my idea is the right one anyway. Just running out of choices...

Phil