Caster Curve

ntsqd

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A friend & I were talking about his up coming live axle front linkage trying to define what he what he wanted in each different aspect of the geometry. Figured we could start down the compromises road once we had a set of ideals. One change he won't make is going to IFS.

My thinking on the caster curve is that ideally it is 'S' shaped. The system should gain caster in both droop and bump. I'm inclined to say a mild gain in bump and a significant gain in droop. My thinking being that if you land on the fronts only, having a lot of caster will make the vehicle want to go straight (rather than wanting to turn & flop) and the angle of the vehicle at the contact point is going to effectively subtract some of the castor angle. Likewise at full bump, a vehicle that wants to go straight in that condition should be an easier thing to drive.

Am I missing something, or just totally out to lunch ? Any idea how much it should gain at the extremes of travel ? He has a conceptual design that gains 1* at each extreme and another design that gains 3* at each extreme. I'm inclined to suggest he go with the 3* design if he can't come up with a design that gains more in droop.

TS

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Chris_Wilson

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Since you are talking front live axle, is this a 4wd? If so, is torque steer affected by caster?
Something to consider.
 

ntsqd

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Interesting, I replied to this already. Must have turned into net gremlin food.
Torque steer is usually the result of axles with different torsional spring rates (some live axles are prime candidates for), but the axle housing seems to cancel this out by tying both fronts together. At least that's been my experience. Perhaps someone who has driven any distance in 4hi w/o a rear driveshaft knows different.

I thot sure some of the heavy weights on suspension design would respond to this. Oh well.......

TS

"It only seems kinky the first time"
-- Bumpersticker seen in Lost Wages
 

Jack

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I have had to drive home on the front a few times with my CJ5 and yes there is torque steer. That being said is not a big problem if your pulling from all corners, the big problem will be the drive shaft angle and u joint angle, to get the best steering geometry, the u joints will not live, and to get the best u joint angle the steering will be crap, so you must compromise.
 

FABRICATOR

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ntsqd,
It sounds like you are on the right track but there’s a lot of information missing. Is the solid axle a drive axle? Does the axle use FWD (front wheel drive) type spindles with FWD type wheels, or is it the old standard type with outboard hub and bearings and standard wheels? By linkage, do you mean no leaf springs? What type of steering system are you using? Are you quietly building the system Bob was working on? All of these can make a big difference.

<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>
 

ntsqd

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This is more of an old school layout than a new type. It is a part time high pinion D44 with late D60 outers. Not too concerned about UJ angles at speed and since the D60 'C' brackets aren't installed yet we have the ability to make both angles realistic (@ ride height anyway).
There's some debate on the steering. Depending on the final locating linkage design (no leaf springs, either Fox or SAW coil-overs according toonwer preference, which isn't me) it may be a traditional drag link - trac bar design or the drag link may follow the trailing arms forward to a GM type knuckle mounted steering arm.
Hadn't really followed the thread that Bob was working on. I'll dig it up and review it.

My design idea was to first lay down what we wanted for each aspect of the total package. Ideal Caster curve, bump steer, UJ angles, wheel travel, etc and then see what compromises would have to be made as each aspect's interference with others comes to light.

TS

"It only seems kinky the first time"
-- Bumpersticker seen in Lost Wages
 

Jack

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With that in mind, I think your first post is on track.
 

ntsqd

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I think the ideal curve would have 1* gain at full bump, but 3* gain at full droop. Not sure what it would take to get this. I'll guess something that starts to look like an non parallel Watts link. Does 3* gain at either extreme sound like too much or too little to anyone ?

TS

"It only seems kinky the first time"
-- Bumpersticker seen in Lost Wages
 

FABRICATOR

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There are things built in to most OEM spindles to take care of castor that should be considered. Generally, the only reason a spindle tries to go straight in the first place is because the spindle raises that end of the axle when turning, thus raising the car. It’s the weight of the car that tries to straighten the spindle. In full droop, throttle on, the castor will increase (positive) along with the increased angle of the chassis. There is extra castor at this point, but the front end is light and the extra degrees are needed and used. In full compression (or bump as you referred to it) extra castor is not needed, even though it will naturally loose some due the same reason mentioned above. This is because in full compression the extra weight on the axle makes up for the small loss in castor. Both of these serve to keep the tendency to go straight fairly constant throughout wheel travel. (This explanation is slightly simplified and is a bit different for FWD or “live” spindle designs)

Too much castor can be worse than too little, street or dirt. Too much makes it difficult and/or sluggish to turn. You should not need to build in any more. Castor is far more critical on the street than the dirt, largely due to differences in traction. Larger tires also increase castor. If the design or modifications negatively affect the steering, you may end up disappointed with the outcome. Castor is important, but should not take significant priority. Some castor should be maintained at full compression, but no extra is needed. Factory castor angles should be close to what you will need.

<font color=orange>The best ideas are the ones that look obvious to the casual observer.</font color=orange>
 

ntsqd

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Thanks.

TS

"It only seems kinky the first time"
-- Bumpersticker seen in Lost Wages
 
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