Mr. Wizzard

The moderators have locked my original thread about the construction of my GS due to "inactivity".
As most of you know, I've been working to narrow the front tread-width of my Mongoose Grand Sport. I already have adjustable upper control arms to take care of the top, and I purchased new lower arms to build jigs to fabricate shorter lowers.
We've known for some time that the pre-1988 C4 Corvettes had a narrower tread width, but there's always been a mystery about how and why.
Just for the heck of it, I bought a couple 1986 lower arms from an Ebay source. Much to my delight, I find that there's about a 2" difference in length...which will put my tires under the fenders. We'll CNC some nice lower coil-over mounts for these shorter arms, as well as new sway-bar mounts....and they should be a bolt-on!
Here's a picture of the two arms side by side:

silvercamaro

I design suspensions as a profession, and I get uneasy when I read a post like this. Please don't install a shorter LCA without making other changes, but perhaps you already knew that. It will make some pretty wild issues with your roll center movement and bumpsteer.

silverslashstreak

Larry that is very interesting. I had no idea that the lower arms were that much different. That will make narrowing your car much easier. It makes me wonder if the front spindles on the pre 88 are a lot different, because to the eye the actual track width of the pre 88 looks very much like the 88 and up cars. Or maybe they made it up with the wheels.

I am sure some c4 experts will chime in.

My GS uses the 88 up suspension with the narrowed chassis, which creates a few other problems such as sway bar connections and using a proper rack that eliminates bump steer.

Jeff

Mr. Wizzard

Don't worry. I've been working with race car suspensions since the mid 60's, and we'll do what's necessary to properly address roll-centers and bump-steer. I'm just pleased that we don't need to fabricate new lower control arms.

jim lockwood

Yes, they are.

The early uprights are shorter (by about 2") than later uprights. A side effect of this difference is that the upper control arms need to mount at a lower elevation on the early suspension.

The big geometry difference between early and late C4 suspension is the king pin inclination (I think that's what it's called), which is greater on the later suspension. On later C4 suspension, the king pin axis hits the ground near the center of the tire contact patch (zero scrub radius). On early C4 suspension, the king pin axis hits the ground inboard of the tire centerline.

The upper control arms on early and late C4 suspensions are approximately the same distance from pivot to ball joint centerline. The significant difference between them being the mounted angle of the late upper ball joint. It has been altered to compensate for the change in king pin inclination.

In any event, the bottom line is that an early LCA that is 2" shorter will likely result in a front wheel position that is about 1" farther inboard.

Oh, and late C4 brake calipers will not bolt to early C4 uprights without some kind of adapter.

Jim

keystonefarm

Since I will be building a GS soon with Mongoose this makes me ask this question. I know it can not be as simple as this but. If I have Gary at Mongoose get a pre 88 C4 front end would that suffice to narrow the front track 1'' per side ? Is the upper control arm shorter on a pre 88 ? He is willing to duplicate the narrowed suspension mod that D&D did on my 049 car. I'm just wondering if this would do the same thing. Also on C4 suspensions. When you narrow the rear suspension you have to mount the coil overs on the inside of the frame tube versus the outside on a standard width C4. This brings along a problem of shock travel and suspension travel as mounting the shock inboard shortens the available room for shock travel. Jim Lockwood uses a modified C4 transverse spring on his GS . How much trouble /cost would there be to use a transverse spring either composite or steel on the rear like Chevy designed for the originals but narrowed by 2'' per side . Since the car weight of a GS is much less how would one design a new spring to give the ride and stiffness needed for a 2400-2600 lb GS. Using a transverse spring would eliminate the problems of moving the coil overs. Other than setting ride height what are the benefits of a coil over versus a transverse spring ? ------------- Ken McCorry

aworks

I'v have posted this in the past. Thats why I used 84-87 suspension on both my Grand Sports and my 60 with an SRIII chassis. With the correct knuckles on you will gain about 3/4 of an inch on each side ft and rear. I narrowed both my rears by cutting the half shafts then changing the dog bone mounts.

And by using the early rear brakes the e brake is very easy to set up. Find your self a Chevette e brake handle. It will mount right where the real GS cars have theres. And yes the Chevette handle was my design. I told Jeff Leach how to do it.

Also with the smaller brakes on the early suspension you can fit the 15 inch PS wheels on with no problem.

Ken
I used a stock fiberglass rear spring on my roadster. Get a later spring 94 and up. They have less spring rate. When you cut it it will change the rate. Then remove the rivets that hold the steel ends on. Cut the spring and cut a new whole with a hole saw. Reinstall the steel end and your good to go. Maybe an hours work.




Notice my wheels don't stick way out.




Look at the brake handle

silverslashstreak

Thanks for the information Jim, that really clears some things up.

Larry's picture also explains how the front sway bars are the same width but use different mounting systems to the lower a frame.

Brian do you recall what your seats in your roadster came out of or did you build them?


Jeff

jim lockwood

Hi Ken,

I believe this is true, due to the nature of the early C4 geometry, but I haven't personally tried to verify it.

No. The only significant difference is the angle of the ball joint mounting.


Boy, does it ever. Take a look at D&D #021 before its rear suspension got reworked. Notice the coil over is nearly bottomed out:



Later D&Ds, like your #049, were revised to permit more shock travel, but I think it might still be less than you'd get with a separate leaf spring and shock.


.

This is what I started with on our D&D prototype and it is very similar to #021, pictured above. Note the highly compressed coil over:



And this is what the car now has..... an '84 rear leaf spring, shortened about 1.5" per side, and stock '63 rear shocks which are located to give +/- 2" of travel:



Interesting problem. I'd toss it in the lap of the spring manufacturer by providing a set of design goals:

1. Loaded spring arc. You want the spring to be almost flat when loaded with vehicle weight to give the correct ride height.

2. Design load. The rear of our D&D weighs about 1150 lbs, as I recall. I'm a little hazy on this but I believe the spring load is a little greater due to the leverage of the wheel working through the suspension geometry.

3. Overall spring dimensions. Length, width and thickness at the differential mount point.


The "easy" way to adapt a C4 transverse spring to a GS is to do what I did..... get an '84 rear spring (look for code "BMF" stamped in the spring). The disadvantage of this is that the ride will be very choppy on a bumpy road; this is a stiff spring. The advantage, though, is that you'll have very little body roll and very predictable handling on track days.

Coil overs give you a broad range of spring rates and certainly make it easier to set ride height.

My observation from the conversion of our D&D back to a leaf spring is that the plastic spring plus standard shocks weighs less than coil overs. To me that's a plus.

If ever I wanted to change out the rear spring to provide a boulevard ride, though, I'm screwed.

Jim

silverslashstreak

Here are a couple shots of how they addressed the shock travel problem on my later design D & D with the lower attachment points of the shock,





Jeff

keystonefarm

Jeff thats what the lower rear shock mounts look like on my 049 car. The top mounts are located about mid tube height on the inside. This thread has opened up an amazing amount of suspension info. My thinking at this point ( and that has changed numerous times ) is to use a early C4 front end with a correct power rack and use a narrowed rear end 2'' per side with a narrowed 94 up rear spring to start. Timing with this info is perfect as the car has not been started yet !!! Jim, Jeff, Brian, Larry thanks !!!!!! ----- Ken

silverslashstreak

Ken I am with you, I really prefer the narrow chassis so you can run a wheel with the period correct deep dish offsets.

Gary told me the other day about setting up and building a narrowed chassis for customers that preferred the correct wheel offsets.

From what little I know about the C4 suspension pieces my suggestion would be to go ahead and have your chassis built to accept the 88 up suspension because of the advancements Jim described plus I think parts are more plentiful.

Here is a picture of the power assist Randy Sweet rack I have installed on mine with very little trouble and I feel confident I will have very little bump steer issues. It will go through its complete suspension travel with 0 toe change.



I agree with you , I would like to give the GS guys here on the forum a big thanks for all their help because there are no instruction books when it comes to building a Grand Sport.

Jeff

Mr. Wizzard

Great stuff guys.
Just received the opposite side lower control arm today. The left and right castings are identical, with the only difference being the orientation of a drilled and tapped hole that the shock plate mounts to.
So, the early arms are symetrical from side to side.

Larry

aworks

Jeff. The seats are early 70's Recaros. They fit the car very well. If you want an 84 rear spring I may have one. I would have to look. I never road raced either of my cars so I always went for the best street setup. Jim has a point about using the 84 spring. It is the stiffest C4 spring there is. Way to stiff for me but may be great for the track.

I wish I could afford to road race any Vette. The closest track to me is Lime Rock. No way I have the money to compete with those boy's. They spend more on there tow rigs than I make in two years.

This is the closest I will ever get to racing at Lime Rock. I did get to drive around the track once.

catchme76

Hey Larry, I finished building my mongoose grandsport back in september and I remember looking at many of your pics and I actually spoke with you once about your headers. I know you went with the knock off wheels and thats the reason for your fender clearence dilemna am I correct?

If so why not just go with the knock off look a likes? That would save a ton of work. Or just go with some ccw classics like I did ?

Mr. Wizzard

More video's please! Your car sounds great!
My plan is to ultimately go with a 15" wheel combination and some Goodyear road race rubber to achieve a look that's more original. The offset of most original-style wheels dictates that the axle/hub assemblies move inward, so that's where I'm headed with it. All I need now is time to fool with it.....

Larry

tyoneal

=================================
Ken:

While speaking to Jeff at Mid-America, he mentioned that he sells his cars two ways. One WITH the suspension narrowed, and the other WITHOUT the suspension narrowed.

The narrowing option is $1295.

I looked at one of his narrowed suspensions and the shocks were indeed moved inside the frame rails. It might be worth a call as there is no point "re-inventing the wheel" (so to speak ;-) ) to spend time figuring out how to narrow the suspension properly.

Just a thought,

Ty

Mongoose probably has a solution for this, maybe the same one, or maybe a different one. Might be interesting to see if there is a difference.

keystonefarm

Ty I have a D&D car chassis 049 and it currently is at Mongoose for some work. Gary also has one other complete D&D car there along with a semi built chassis/body so he has 3 examples of how D&D did the narrowing. They do vary with age of car !!! ----- Ken

keystonefarm

Anyone here have a photo of a MAI chassis rear suspension showing their version of coil over mounting ? Does MAI have a web site ? I can't seem to locate one. -------- Ken

tyoneal

======================================
Ken:

I'll see if I an get some pictures successfully posted for you.

It looks like these will work. Let me know if they are helpful at all. I received these from Jeff at Mid-America. I am really interested in finding out which way would be the optimum at this point in the Build as these decisions are very difficult to change once made.

Good luck, and please keep in touch.

Thanks,

Ty O'Neal