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Author Topic: Suspension design [age-old] question  (Read 2997 times)
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Jack Gifford
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« on: April 22, 2015, 01:02:37 AM »

Not limited to LSR, but it applies to many LSR machines- something that's bothered me for decades- actually about sixty years, since I first read about rodders "splitting the wishbones" on their early Ford hot rods:

How come nobody seems to question the inherent "binding" of a pair of radius rods on a rigid front axle (or a pair of "truck arms" on a rear axle housing)? If an illustration is necessary, just stick a front end model together- straight axle and a pair of arms (Erector set, Legos, toothpicks glued together, whatever). Simulate one wheel hitting a bump- raise one corner while holding the other three points down. See the glued joints fail, or the Erector set beams twist?

Yeah, it provides built-in anti-roll torque; but do we really want the axle and/or radius rods (or their fasteners) distorting to accomplish that? shocked
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« Reply #1 on: April 22, 2015, 06:59:13 AM »

Isn't there some improvement with rod ends or rubber bushings at the mounting ends?  They do allow "some" degree of twist with rotation.

I've seen lots of suspension with rod ends, no bind through the movement range.   Might take a "high angularity" rod end though.

 cheers
Fordboy
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« Reply #2 on: April 22, 2015, 08:32:15 AM »

How come nobody seems to question the inherent "binding" of a pair of radius rods on a rigid front axle (or a pair of "truck arms" on a rear axle housing)?

Split wishbones (or hairpins) look "traditional". End of story for some. For a modern racer, parallel leading arms are no less aero, and only a tiny bit more complex, and fix the problem.

Truck arms, if the forward mounting points are as close as possible to the driveshaft, keep binding quite small, within the rubber bushings' range of movement.

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Ron Gibson
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« Reply #3 on: April 22, 2015, 08:38:12 AM »

At the front, the wishbones are solidly mounted to the axle with no way to flex or move. To go over a bump it would have to twist the axle, (probably) or bend the wishbones.

Ron
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kiwi belly tank
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« Reply #4 on: April 22, 2015, 09:36:15 AM »

An original 'Henry' axle & a repo forged axle have the ability to absorb the twist but a tube or cast axle run the risc of breaking something. All street rods in New Zealand have to go through strict certification & a forged "I" beam axle is the only one accepted with split wishbones for that reason.
  Sid.
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Ron Gibson
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« Reply #5 on: April 22, 2015, 09:46:16 AM »

If you wonder if original Henry's axles can take the twisting, Speedy Bill's Museum has one mounted on the wall. It has been twisted 360 degrees at the ends with no cracks or breaks.

Ron
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Stainless1
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« Reply #6 on: April 22, 2015, 09:59:30 AM »

Ron Leslie has one of Henry's v8 connecting rods his dad twisted to see what it took to break one.... 7 twists made it a lot shorter but is stayed together... Henry must have had his own blend....
back to topic...  cheers
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« Reply #7 on: April 22, 2015, 10:27:27 AM »

Remember, Henry didn't split the rods.  He had them meet at a swivel point under the drive train.
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tortoise
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« Reply #8 on: April 22, 2015, 10:35:43 AM »

To go over a bump it would have to twist the axle, (probably) or bend the wishbones.

Ron
Or flex the frame.

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Ron Gibson
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« Reply #9 on: April 22, 2015, 11:41:24 AM »

In the early days, Ford owned steel mills so he could control the quality of the materials that went into his cars.
Sorry for the post drift. Back to regular schedule.

Ron
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« Reply #10 on: April 22, 2015, 11:51:34 AM »

Did you ever ride in a hot rod with a beam axle and radius rods attached to the frame? If so, you will realize the need for A arms and independent suspension which has been developed while the beam axle system development has stopped.

DW
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« Reply #11 on: April 22, 2015, 11:59:18 AM »

Both truck arms and Henery's axles have fairly low torsial ridigity.... ie they twist.  This is what allows split wishbones and the solid rear axle to work.  Every time I see a tube axle, front or rear, with split bones, hairpins, or boxed truck arms I cringe.  It's a mess waiting to happen.  On a fairly flat surface, salt flats for instance, it would likely be OK but any substantial cross articulation will result in very high stresses, such as pulling into a driveway.
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4-barrel Mike
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« Reply #12 on: April 22, 2015, 01:45:00 PM »

There were a lot of "three-springers" on the dry lakes back in the day.



Just sayin'.

Mike
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« Reply #13 on: April 22, 2015, 06:23:10 PM »

Based solely on this young guns experience, links with rod ends work well. I have been involved with several rock crawlers that have a huge range of motion only using a three or four link setup. In my mind an early hot rod style front end is a two link setup and uses the transverse leaf as a 'pan hard' or locating link. In a rock crawler the addition of a pan hard, or track bar accomplishes that, but it can also be done with proper triangulation. In those applications the length of the links drastically effects articulation as well as ride quality. In any well built rig using this setup not only do they work well on the trail, but most of them that have the correct geometry, ride nice and handle well on the highway also. The few times I have split wish bones, or used hairpins in a build I always use rod ends at the chassis end of the link to allow for the inevitable deflection.

I attached a picture of a common four link setup. And another of the amount of 'twist' you can get with a solid front axle.


* LONG ARM.jpg (47.06 KB, 640x480 - viewed 161 times.)

* 2-seat-roller-flex.jpg (94.85 KB, 700x467 - viewed 163 times.)
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Mr. Schimstock
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« Reply #14 on: April 22, 2015, 07:24:39 PM »

Be careful not to confuse articulation movement with the twist along the axis of the axle caused from split wish bones during articulation. In a 4 link type system with correct rod ends there should not be any twist along the axis of the axle. The connection between the split bone and the axle has less degrees of freedom than a ball joint end which results in a torque in the axle (twist along it's axis) or in the split wishbone. Given wishbones are tubes they don't twist very easily and an I-beam axle will.  Combine a split wishbone with a tube axle and one of them or the brackets needs to give.  If the articulation is small enough there's no issue, but with enough it gets ugly.  Somewhere on-line there's a video showing the difference. 
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