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Modifications to LH-UC K-Frame


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#1 Heath

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Posted 07 June 2017 - 05:33 PM

Okay so we've all done the modification where we install UC upper arms, and mount them as low as possible on the inside to increase camber gain.

But how can you take things further? If I like cutting stuff up and welding it back together, are there other geometry improvements that I can readily attain? Some of our members who are knowledgeable in dynamics often mention the shortfalls of the front end design (namely that the McDonald Bro's and Rod Shop alternatives don't address these shortfalls). Is there any way to mount the lower arms differently, for example, for an improvement?

Any dead weight I should know about? Weak points? I just picked up another crossmember to start off with a blank canvas, I'm going to mount it a little further forward than usual to increase the wheelbase a tiny bit, and am keen for other suggestions!

#2 axistr

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Posted 07 June 2017 - 06:56 PM

More positive caster wouldn't go astray. I would make the lower inner control are mounts adjustable. Even with UC upper arms 2 1/2 deg is about the maximum you will get without buggering up the camber. Somewhere around 4-5 deg of caster would be nice with power steering.



#3 76lxhatch

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Posted 07 June 2017 - 07:21 PM

I machined out a poly bushing at the front of each lower arm and machined down a Commodore IRS camber correction crush tube to offset the mounting. Gains a bit of caster without causing camber problems, and does so by moving the wheel in the opposite direction that caster gain from the top arm does. It makes a big difference, ended up with a bit over +6 degrees caster, and still less than -1 camber. Before I was struggling to get over +5 caster with marginal camber issues.
http://www.gmh-toran...e-8#entry849759
edit: bear in mind that I positioned the steering rack to suit a minimum of +5 degrees caster, if you get too carried away you'll need to make alterations to minimise bump steer.

The advantages of a clean sheet front end would be designing the geometry into the arms and mounting points from the start, better caster, improved camber gain, better Ackermann, even improved scrub radius if you are changing out the stub axles and using big wheels. The bolt-in units on the market ignore any geometry changes and seem to focus on (maybe) weight savings and extra room for big headers, drag racing stuff really. If I were adventurous enough to build my own cross member it would be something to suit readily available Corvette or similar parts, using their geometry as much as possible to avoid re-inventing the wheel.

Toranamat talked about significantly altering the length and mounting points of his control arms, may have been on the Cardomain site? Of course you have to start measuring and checking absolutely everything from scratch if you do that.

Edited by 76lxhatch, 07 June 2017 - 07:23 PM.


#4 Heath

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Posted 09 June 2017 - 10:15 AM

I should have been more clear, sorry. I'm not making a crossmember from scratch, just starting with an old LX RTS shitter that I don't feel bad modifying, rather than my pretty painted bushed one that someone else can enjoy.

 

I'm already mounting this crossmember 10mm further forward in the car, so offsetting the lower arms to put the wheels further forward (while fairly easily done) is probably not in my best interest. I think I could have just as easily done that instead, oops! Keep in mind I'm not expecting to have power steering so don't know if 6deg of castor is a good thing.

 

With the upper wishbone mounting points being at LX RTS height (quite low), should I still seek more camber gain (on bump) in the form of upper balljoints with longer pins? (lifting the outer pivot point of the wishbone)



#5 76lxhatch

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Posted 10 June 2017 - 07:03 AM

Yeah I was just referring to the issues with the aftermarket setups since you mentioned them, you are somewhat more limited with modifying a factory item but might give you some ideas. Adjusting caster by tweaking the lower arm is also good because helps keep the upper arm shims at a sane amount and keeps camber in check, I suppose a modified upper arm could move the ball joint back even further to achieve a similar thing if you're concerned about wheel position?

I drove mine for a while with wide tyres and 5 degrees of caster without power steer, wasn't too bad but obviously its better with the ability to turn the wheel faster. If you want it to handle OK I would think 4 degrees bare minimum.

The extended ball joints are something I would like to try at some stage, probably requires some measurements to confirm they actually help more than hinder. Moving the lower control arm inner pivot up a little could be useful to bring the front roll centre up a bit, depends what you're doing with the back end and would require a lot of checking and measuring to make sure you don't mess up other things. Beyond that you really can't make any significant improvements to camber gain and roll centre without getting serious and modifying the length of the control arms, again has a lot of flow-on effects that need considering.

Also bear in mind that a UC steering rack is a bit wider and will likely give better bump steer results with this sort of setup.

#6 76lxhatch

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Posted 10 June 2017 - 07:06 AM

Probably the first thing you need to do is set up a rig to take some measurements, camber, caster, bump steer etc under controlled suspension movement.




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