Narrowing a front suspension

[Nate K]

I installed a corvette C4 front suspension in my 71 dart and had to narrow it 4in so it would fit. Is this going to kill the geometry or will I be ok?

[sik68]

Did you narrow it by moving the frame rails closer together?

[Nate K]

Yes I made my own mounts and moved them in.

[JMarsa]

Related to the original post...

If you narrowed the width of a C5 front suspension by moving the control arm mounts in 1" per side on the horizontal plane and also narrowed the tie rod ends by the same amount, would this affect bumpsteer or anything else?

--JMarsa

[Derek69SS]

In order to keep the steering geometry, you'd need to take out of the middle (the rack).

[JMarsa]

In order to keep the steering geometry, you'd need to take out of the middle (the rack).

I believe you, I just don't understand why.

--JMarsa

[B Schein]

If you keep the svsa the same the rc will migrate most likely up

[JMarsa]

OK so help me out here. SVSA = Side View Swing Arm



I went and did some more research and I also printed out the diagram below and narrowed the track width 1" per side and compared the two. I also ran them through Perf Trends Analyzer and came to the same conclusion. Here's the math for the changes:

Std/-1"
Scrub Radius = .36/-.64
Roll Center Height = 2.92/2.83
Roll Stiffness ft-lbs/deg = 1957/1849
Anti Dive = 53.9/54.4
FVSAL = 320.6/319.6
SVSAL = 68.8/68.6

Anything to worry about other than the scrub?

--JMarsa

[E.rodz]

4" thats alot.you are definately going to have some serious bump steer.if you use a center stear rack it will be just fine.the corvette rack will not work.the pivot point on the inner tie rod socket is were you need to work from.you can't narrow the rack up because you won't have enough travel.flaming river,unisteer should be able to hook you up. you would be better off using a different offset wheel.and not narrowing it so much. keep the hot rod dream alive

[JMarsa]

I meant 2" overall, 1" per side.

Where's Norm when you need him

--JMarsa

[parsonsj]

Anything to worry about other than the scrub?

Yes. 1. How does your roll center move at 4* roll? (before/after) 2. How is bumpsteer affected?

jp

[B Schein]

sorry meant to say front view swing arm but you started researching the right stuff

[JMarsa]

Yes. 1. How does your roll center move at 4* roll? (before/after) 2. How is bumpsteer affected?

jp

I think I need some more training with the software. (I'm using the demo version right now). I changed roll to 4 but the roll center height stayed the same as the previous values.

Is the way to view bumpsteer (toe in gain) only in the graphs?

--JMarsa

[Norm Peterson]

Quote:
Originally Posted by Derek69SS
In order to keep the steering geometry, you'd need to take out of the middle (the rack).

I believe you, I just don't understand why.

--JMarsa

When you shorten the tierods you've changed the relationship of two steering pivots relative to the rest and tightened up the arc that the outer tierods and steering arms must follow. The result is bumpsteer that wasn't there before and which probably cannot be compensated for as well as the original either was or could be no matter what you try. The relationship among all the other pivot points was left alone, so things like camber gain remain as they were.

If you remove width from between the inner tierod pivots, then you've left the relationship between those points and all the others intact and nothing at all has changed geometrically. Finding a rack that does precisely this seems to be the "problem".


Norm

[JMarsa]

OK I think I get it. Thanks Norm.

So why is a narrowed rack a problem? There's a few custom rack manufactures out there. Sounds like a rack with the same specs as stock that's shorted by the same amount that the track width is.

--JMarsa

[parsonsj]

why is a narrowed rack a problem?

It's expensive, with long lead times. Not a show-stopper, but still a problem.

Keep us posted as you finalize this. I'd still like to know how your dynamic roll center is affected. 4* roll will move your roll center by at least an inch, and sometimes as much as 10 inches.

jp

[Norm Peterson]

4* roll will move your roll center by at least an inch, and sometimes as much as 10 inches.

jp

Sounds like somebody else has looked at the "McBearen" rear suspension . . .


Norm

[Bruning Auto Design]

It's expensive

Exactly! Woodward makes high quality products none the less!

Pardon the green tape...

[parsonsj]

Since we're comparing sizes... (without green tape):



More here.

jp

[JMarsa]

I really don't see how that's considered expensive part of the build.
You can't drive without it! Compared to some wheels, hood hinges and other extraneous bits I'd gladly spend $800 on a rack.

JP - I'll run the numbers tonight to get at how dynamic roll center is affected. BTW, who made your rack?


--JMarsa