Steering, Suspension, & Brakes

tips, technology, tools and techniques related to non-driveline mechanical components

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Dawie
Dawie Coetzee
Cape Town, South Africa
(25 posts)

Registered:
04/17/2009 07:53AM

Main British Car:
1958 Morris Minor Subaru EJ25 (planned)

authors avatar
Morris Minor suspension theory: results
Posted by: Dawie
Date: May 20, 2009 06:48AM

I ran some numbers through the McDermott software to which Tom Lay pointed me (thanks Tom). It was initially disappointing - because there is measurable bump-steer - until I thought to try the stock suspension and so get some base bump-steer numbers. The lower-leading-arm set-up actually generates less bump-steer than the stock suspension lowered a few inches over the first few inches from ride height. Past 2" of drop, however, it went to 2-3 and more: too much however rarely one might see 2" of drop.

Then I tried upper and lower leading arms, but with the upper arms' pivot axes angled down towards the centre so as to generate a (constant) effective swing-arm, and reasoned that the steering link ought to be coplanar with a longitudinal axis through the arm pivots' intersection; and moreover that the angular spread of all the swinging elements should be the same (hard to explain). That worked! The bump-steer numbers are comparable to stock, and considerably less at extremes of travel. So is negative camber gain, which can be further improved with a steeper upper arm pivot.

I worked on a 100" effective swing-arm. I'll try 75" and see.

-Dawie


t.lay
Tom Lay
Grayslake, IL
(93 posts)

Registered:
05/13/2008 09:53PM

Main British Car:
72 mg b gt

Re: Morris Minor suspension theory: results
Posted by: t.lay
Date: May 25, 2009 11:12AM

Suspension design has a lot of compromise in it. Looks like you're working things out pretty quickly. I've heard a few people say trry to keep your swing arm length (ideally) between 100 - 150 inches. Shorter lengths can introduce some quirks like transitions from negative camber to nearly - or positive camber back to negative (decreasing radius turn). Try things out with 1 or 2" of bump, 2 or 3* of roll and some steering input.


Dawie
Dawie Coetzee
Cape Town, South Africa
(25 posts)

Registered:
04/17/2009 07:53AM

Main British Car:
1958 Morris Minor Subaru EJ25 (planned)

authors avatar
Re: Morris Minor suspension theory: results
Posted by: Dawie
Date: May 26, 2009 01:54AM

That's the thing about the converging-axis leading-arm thing: the effective swing-arm is pretty much constant. I thought about trying 75" because a conventional unequal-length A-arm set-up has the effective swing-arm diminishing with bump. I'll have to figure out how quickly the stock geometry reaches 75" from 100" at ride height. I'd like a bit more aggressive negative camber gain than stock, though.

But I did check roll and steering input, and so far so good. I didn't take caster into consideration: I'll have to make some adjustments. Stock is 0 caster, but with my set-up that goes through negative caster at some point, so I'll set static caster at 5 or so. I'm shifting the front wheels 5" forward, and I wouldn't be surprised if the Subaru EJ were lighter than the A-series, both of which argue for a bit more caster.

Thanks for taking an interest!


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