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?
https://static1.pt-content.com/images/pt/2008/07/2r7acqv-1.jpg

[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

https://static1.pt-content.com/images/noimg.gif

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 https://www.pro-touring.com/forum/images/buttons/viewpost.gif (https://www.pro-touring.com/forum/showthread.php?p=491466#post491466)
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.

--JMarsaWhen 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 . . . :eek:


Norm

[Bruning Auto Design]

It's expensive

Exactly! Woodward makes high quality products none the less!

Pardon the green tape...
https://static1.pt-content.com/images/pt/2009/02/_DSC5491-1.jpg

https://static1.pt-content.com/images/pt/2009/02/_DSC5507-1.jpg

[parsonsj]

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

https://static1.pt-content.com/images/pt/2009/02/Rack20and20mount20002-1.jpg

More here (http://www.iimuchfabrication.com/Front%20Suspension/photos/Rack%20and%20mount%20002.html).

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

[parsonsj]

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?I would have gladly paid Tony Woodward $800 for my rack too. He wanted more. :) My rack cost about $1400 altogether.

jp

[rjsjea]

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?
https://static1.pt-content.com/images/pt/2008/07/2r7acqv-1.jpg

Holy thread theft....

NateK, I think you will be good to go. Since you didn't mess with the arms, but only narrowed the frame. You got a custom rack right?

[parsonsj]

Holy thread theft....I don't see it. All of this information is germane to the OP. Nate is going to need a narrowed rack, and this is all useful to him as he goes down that path.

jp

[BRIAN]

The Vette stuff looks cool but have to ask why take something and create so many problems when Art Morrison or other co's have set ups that will fit your application? The AM set up is like $2200 which includes everything and is proven performer set up for your application. After you buy your rack and rebuild your stock parts you will be over or at that number.

All the number crunching is still going to require a trial and error process and who wants to do that with a fresh build where it can be avoided. It takes a lot more to design a suspension than it appears. Taking info off the net to design a suspension is a good and bad idea. A lot of opinions.

Looks like fun project and i am all for building it all but it is your suspension. Hope it works out.

[rjsjea]

I don't see it. All of this information is germane to the OP. Nate is going to need a narrowed rack, and this is all useful to him as he goes down that path.

jp

Just a sarcastic remark is all......:cheers: I am building a Demon, so I have some interest in his Dart.

By the way IImuch was one of the cars that got me hooked on building a road race/PT kinda car :thankyou:

[parsonsj]

Just a sarcastic remark is all...... I am building a Demon, so I have some interest in his Dart.

By the way IImuch was one of the cars that got me hooked on building a road race/PT kinda car Sorry... and cool! Thanks.

jp

[T_Raven]

^^What AM set up is $2200? You can get C5 arms with spindles and hubs from AME for $1800 or you can get the same thing on ebay for $800. The Camaro subs are $6k each. Yeah they figured out the geometry and made steering arms that lower the outer tie rod, but just because professionals build something doesn't mean a novice can't build something just as good. Plus it's more fun to do yourself, you can take pride in it, learn from the mistakes that you are bound to make, and hopefully save a good amount of money.

I bought a full C5 front suspension for $1000 shipped. I don't need the cradle, rack, leaf spring, or sway bar, so hopefully I can sell those to offset my cost. Metal to build a sub shouldn't cost more than a couple hundred bucks so even if a rack costs $1500 that's less than $3k for a full front suspension with C5 brakes. I might not end up with the "best" geometry by some people's standards but it will be much better than stock.

[BRIAN]

Complete Art Morrison suspension is $2695.00 I am all for building stuff myself but suspensions are dangerous to play with and take a lot of money and trial and error to get right. After you buy the custom rack $1500 plus the custom outer tie rods you are at $1700 already. Stock rebuild parts on Vette stuff has to be $250-300. Plus all the AM parts are new. Laser cut mtg brackets? Vette bearings are expensive.

Then add the tools required to measure your fabricated suspension. How are you going to measure camber and caster? Bumpsteer? Suspension program? These all require expensive set up tools.

Just trying to explain in the end it is easier to get a well established set up. There is nobody that has done the build themselves that has gotten it right the 1st time out. It takes trial and error to get parts to work together that weren't designed to do so.

If you are up to the challenge I applaud your effort and look foward to the build.

[Norm Peterson]

I can at least picture an alternative that probably wouldn't cost as much as a custom rack. I think something similar is already available for the early Mustangs.

Measuring alignment settings is lots easier than you might think. While the $$$$$ racks in the shop can measure a few things you might have trouble measuring in your driveway, it is possible to determine with sufficient accuracy the basic settings of caster, camber, and toe quite readily with less than $100 worth of equipment. Do a search on "DIY alignment" or "home alignment" either here or over at Corner-Carvers.

Even the OE's don't get it right straightaway at the initial drawing board & engineering stage. That gets you in the ballpark, and then you go testing. Maybe you can get away with less testing if performance is by far the most important characteristic.


Norm

[E.rodz]

well alot of questions and answers' charts , graffs. you already have the answers in your mock up . put the front end on stands put the front end at ride hight. mark the center of the tires with a scribe clamp your rack in and see what happens to your toe in and toe out. what you did by narrowing the front is fine you just need to use a CENTER STEER RACK. problem solved.good luck keep the dream alive

[JMarsa]

well alot of questions and answers' charts , graffs. you already have the answers in your mock up . put the front end on stands put the front end at ride hight. mark the center of the tires with a scribe clamp your rack in and see what happens to your toe in and toe out. what you did by narrowing the front is fine you just need to use a CENTER STEER RACK. problem solved.good luck keep the dream alive

I don't understand why you think that is an option for the original poster's car. I doubt it's rear engined.

--JMarsa

[Nate K]

I don't see it. All of this information is germane to the OP. Nate is going to need a narrowed rack, and this is all useful to him as he goes down that path.

jp

I did narrow the rack but it didnt cost me anything. I just used a rack I had and sectioned the center.

[parsonsj]

I did narrow the rack but it didnt cost me anything. I just used a rack I had and sectioned the center.

That's awesome, and not something you hear everyday. Got any pics of how you did that? Did you section it so that the pivot points came in too?

jp

[JMarsa]

I'm interested as well.

--JMarsa

[Nate K]

It isnt power

pics

http://www.forabodiesonly.com/mopar/showthread.php?t=23986&highlight=71+dart&page=4

[parsonsj]

Nice! So you cut and re-threaded the end of the rack itself?

jp

[Nate K]

exactly

[LowBuckX]

Nice! So you cut and re-threaded the end of the rack itself?

jp

That rack is 1018 steel With induction hardened teeth.(most are)