allyolds68

I've been seeing this UMI brace in numerous local Craigslist postings and I'd never seen a picture of it installed so I Googled it and found this pic on a Ch&vy forum. The only load the shock towers take is a nearly vertical load from the shocks. What's the point of tying both shock mounts together?

Apparently those guys will buy anything that's painted red......

young olds

I think it's made to stiffen up the frame when you have stiffer springs and shocks especially when you go to coil overs. I also think it's marketed more towards protouring type builds and not straight line racers

rustyroger

Certainly they help McPherson Strut equipped cars for track and rally use, but the strut mounts have to deal with lateral forces as well as simple up and down movement.
I don't think they will be much help on a double wishbone arrangement as on most GM rwd cars.
The point of them is to stiffen the chassis. maybe it will help a bit on a double wishbone system, but I'm not sure the advantages in better handling outweigh the extra weight penalty.

Roger.

Koda

I see three things it might do.

1. It may help with the body flexing in and out, as in the distance between the two. This would be negated if the holes are slotted for tolerance in that direction.

2. It might help with torsion around the W (width) axis of rotation (as in parallel with the rear axle). So, like an upper sway bar.

3. It may help with body flexion around the L (length) axis, like the upper mounts bending up and in.

I think I would want video of what it does, and video of what my car did without it, before I would sacrifice the weight gain.

11971four4two

It lightens the car owner / buyer by a few buckskins
geometry stays the same

oldcutlass

This^^^^^, it does nothing but look pretty, there is already a frame cross support there. I can see doing something different to beef up the upper shock mounts and spring pocket if needed, but without the brace.

I would also assume it makes it that much harder to get to the upper shock bolts by taking up more of the limited space between the frame and body.

allyolds68

Koda,

You're an Engineer, right?.....

Explain to me how the addition of the bar accomplishes any of the above on an A-Body frame. It's located right next to a large C-channel shaped member which is designed to counter body flex. It's attached to a part of the frame that's only subjected to a nearly vertical load.

It's good for nothing except lightening your wallet and increasing the weight of your car

cherokeepeople

yup what he said!

joe_padavano

UMI sells a lot of brightly colored suspension parts with questionable benefits. As a structural engineer, I don't see the benefit of this part. Of course, I never saw the benefit of Mondello's straps across the main caps, either.

83hurstguy

Ha! For some reason, I thought I was the only one... and yeah, not seeing the purpose of this brace either.

Koda

I try to be one, Mike.

You'll notice I said may and might. I was trying to extrapolate the reasoning, if there were any, behind the design. I certainly am not standing up for, or defending the device.

I really only see one compromise in the GM design. That cross brace serves two functions, the first being holding the frame together and anchoring the spring and shock mounts, and the second being a mounting point for the upper control arms. The compromise is moving it forward to work for the second function, ideally, to hold it, it would be centered at the springs, not in front of it. Does it really matter? No, and the back cross piece in the tail of the frame holds flexion of the back rails together.

I would not use this device; if I were in a serious racing application, I'd make a tube chassis, otherwise, I'd be fine with the standard frame in this area, most I would consider is using a convertible frame under my hardtop 67; in fact, if I ever have to replace the frame, I will do that.

joe_padavano

The problem with any bolt-on reinforcements is the slop in the bolt holes. The load will always be carried by the stiffest path. The welded or riveted factory frame members will still carry the load until the frame deflects enough to take up any clearance in the bolt holes in the brace.

I'm also struggling to understand what a brace between the upper shock mounts is doing, anyway. Yeah, the shocks are at a slight angle, so there's a VERY slight inboard load when the shocks actuate. So what? The shocks have rubber mounts at each end. The rubber will deflect long before the stock chassis does. How does that brace ever carry any load? It certainly isn't carrying any up-and-down load (which is were an aftermarket brace COULD have some benefit). Sorry, but I can't see any possible benefit other than the aforementioned weight benefit from a lighter wallet.

allyolds68

Unless the connection is properly torqued and it's truly a friction connection. In theory the slop in the bolt holes is irrelevant then........I know.....In theory

joe_padavano

Sorry, but I've run too many modal surveys and vibe tests where friction connections like this were not carrying the load.

Koda

There's ways around it, like turning the fasteners so that they are in compression and tension versus shear, but I've cheated before on one-offs and made one fastener OD hole, plus a little, then drilled the second hole to the machine's specific dimension. Ideally, all brackets should have a datum hole, a sub datum slot, and all other holes should be slightly oversize. We'll shoot the datum bolt half down on the line, then blast the sub datum bolt all the way down, go back and hit the datum fully, then hit all other bolts involved when we put something on. I have made dual tools with two nutrunners that hit both datum and sub datum bolts at once, that works as well.

Guess I satisfied Mike as he didn't respond to me, just Joe, earlier.

joe_padavano

Of course you can make a bolted joint without slop. On critical space hardware we use shear fasteners like NAS1191 bolts with a diameter tolerance of +0.000/-0.001, installed in a hole that's no more than 0.001 over the bolt diameter. That's called a "light press". Yes, all of these are either match drilled or drilled with expensive, tight tolerance drill jigs and reamed to final size. Most of the highly-stressed joints we use are single shear or double shear - if you try to put the fasteners in tension, you then have to deal with compliance in the end fitting of the part, or a big weight hit. When you're paying $10,000 a pound or more to get to geosynchronous orbit, every ounce matters. I've even used tapered fasteners in reamed tapered holes on really stiffness-critical hardware.