Write-Up: Installing a Front Strut Tower Brace (STB)

Buapo wrote:

Bert - I went to a specialty nuts and bolts place, and got equivalent strength bolts, as well as some nice washers. Just bring your bolt in there, you'll save a bundle of money and it will be the same (if not better quality).

And you have a '99 Riv, according to your sig - 98's and 99's are the lucky ones for this mod, it should bolt right up without any problems but you need the longer bolts and washers (of course).

Thanks, I will go by Fastenal on my way home from work tomorrow.

Bert

Sorry Bert, I don't recall the length. I think they were about 1/4" longer than the stock bolts. I took one into our local vendor and he found what I needed. I think they ended being a bit more than 1/4 longer, was all he had in stock. Need to make sure they don't contact anything on the underside of the mount.

1998 Riv wrote:

Sorry Bert, I don't recall the length. I think they were about 1/4" longer than the stock bolts. I took one into our local vendor and he found what I needed. I think they ended being a bit more than 1/4 longer, was all he had in stock. Need to make sure they don't contact anything on the underside of the mount.

Thanks Dave I was going to try and get the same type of bolt with the funky end on it but thats probably not necessary. I will just get some bolts from Fastenal approx. 3/8" to 1/2" longer. But I think those are "flange bolts" (the oversized head) Anyhow Fastenal will probably have what I need.
I bet that funky end on the factory bolt is to help keep the robot from cross threading the bolts when the robot is quickly moving the bolt into the hole with the bolt spinning...

My wife wants to know what I am doing to her car.

Bert

Just tell her you're fixing it. I usually tell mine I'm doing long needed maintenance...

On my 98 I heated the bar a little and flattened the bottom till it cleared the mastercylinder. A little gloss black and it looks factory and no washers needed.

^^^Let us know how you like it. I also dented my bar.

Update: took the S-turn at 85 mph this morning with confidence, tires didn't make a sound.

Jack, I was convinced the STB is primarily under tension after reading a strut bar theory page at: http://e30m3performance.com/myths/Strutbar_Theory

This link no longer exists, but here is a copy I found at a Honda enthusiast's board:

Page 1

It is my belief that a strut bar definitely does help. And during the explanation that follows I will try to provide a convincing argument for this.

Figure 1 shows the forces of interest in a strut bar analysis. For this calculation only horizontal forces need be considered. There are of course vertical forces, but since the sum of forces must independently equal zero in both the horizontal and vertical directions, we can concentrate on just the horizontal forces in this analysis.



We must begin by making some assumptions. First, consider an M3 cornering such that it experiences 100% weight transfer at the front wheels. This is not at all unusual on a modified M3. We have probably all seen pictures of an M3 in a turn with its inside front wheel in the air. That is a sure sign of 100% weight transfer.

Second, let us assume that our M3 is cornering at 1G. Again, on a modified M3 with R-series tires, this is very plausible. If an M3 weighs 2700 lbs and has close to a 50/50 weight distribution, then the outside front tire must generate a lateral force of 1350 lbs under the circumstances just outlined.

Thus F1 = 1350 lbs as depicted in the figure above. The figure is really a "free body diagram" which considers the forces that act ON the strut/wheel assembly (the blue link in Figure 1). These forces must sum to zero in the horizontal direction. Also, the sum of the torque's acting on the strut/wheel assembly must cancel out. Our goal is to determine the force F3 which is the force that the strut tower exerts on the strut assembly. There is an equal and opposite force exerted on the strut tower BY the strut assembly.

We can solve for F3 if we do a balance of torque's around the outer ball joint (where the control arm attaches to the strut). What we get is:

F1(L2) = F3(L1) or, F3 = F1(L2/L1)

Now, we already know F1 = 1350 lbs. And we can determine L1 and L2 from a quick measurement of an M3 (L1 = 24.3" and L2 =6.0"). Thus F3 = 333lbs.

So the conclusion is that when an M3 corners at 1G with 100% weight transfer at the front wheels, there is a 333 lb force pulling OUT on the outer strut tower. Since the inside wheel is un-loaded there is no corresponding force generated at the inside strut tower. Therefore a strut tower bar tends to be in tension, not compression as is often believed.

Page 2

Now we ask ourselves: How critical is a force of 333 lbs pulling on the outer strut tower?
This 333 lb load amounts to about 12% of the car's total weight. Even though the strut tower is designed mainly to manage vertical forces , 333 lbs in the horizontal direction is not going to permanently deform the chassis. But the problem is that this force is repeatedly applied over many cycles during the life of the car. The more you drive it hard the more cycles you generate. This can lead to fatigue failure of the material that forms the strut tower (or where the strut tower attaches to the inner fender well).

What a strut bar does is tie the two strut towers together so that they share the load applied at the outer tower. This gives you twice as much material to deal with the same cornering force and helps reduce fatigue stress in this area.

Another point to consider is that if your outer strut tower is deflected outwards 0.20" by this 333 lb force, then you just lost 0.5° of negative camber! If it deflects 0.42" you have lost a full degree of negative camber.

This demonstration has hopefully illustrated how a strut tower bar can be beneficial. But what about the possibility of a strut tower bar being under compression?

Contrary to the simplified analysis on the previous page, many people believe that a strut tower bar is predominantly under compression, not tension. This assertion is partially born out in some cars where the strut towers gradually move closer together over time. And I have heard of incidents where the strut tower bar was instrumented with strain gauges as the car was driven around. These tests show the strut tower bar is under compression as well as tension, depending on what the car is doing. One test showed that the highest loads recorded on the strut bar were in compression as the car was pulling out of a garage (sideways down an inclined driveway - we have all heard a stiff car twist under this condition).

So what is this all about? Is a strut tower bar under tension or compression? One likely theory is that it experiences both. It just depends on the driving conditions. Cornering on smooth asphalt induces tension. Driving in a straight line over bumps induces compression. A force diagram illustrating how compression forces result from driving in a straight line (over a bump) is shown in Figure 2:



The left side of the figure shows the resultant forces acting ON the strut tower assembly. Force 1 is the road holding the car up, and force 2 is the weight of the car. Forces 3 and 4 result to stop the strut from spinning (they counter the moment produced by forces 1 & 2). Force 4 of course has an opposite and equal reaction force which is Force 5. This is shown on the right (in green) and is the resulting compression force on the strut tower.

Bear in mind that when the car encounters a sharp bump or dip in the pavement that the chassis may momentarily experience 3 or 4 G's. This means that F1 and F2 in Figure 2 could equal about 2800 lbs! F3 and F4 (and therefore F5) are much smaller, but could still be quite significant. To calculate F5 more precisely requires some measurements. I will get to this eventually.

In conclusion, some cars spend most of their lives driving in a straight line. Such cars might experience the strut towers moving together over time. Track cars spend a lot of their time cornering at over 1G. Thus a track car might see it's strut towers spread apart over the years. Thus a strut tower bar can be under tension OR compression depending on the environment that the car is operated in.

(source: http://team99ers.2.forumer.com/index.php?showtopic=702)

^^ But thats speeding!

I've been wanting to buy an STB for a while now and I want to order one today and one dealership told me its $46 and the other one down the street from my house told me $63. I heard on here that its only $20 so why is it so much more in this shitty city. I want to make sure I have the right part #, is it 25653157? Also all i need is longer bolts because my Rivs a 98 right?

98RIV7777 wrote:

I've been wanting to buy an STB for a while now and I want to order one today and one dealership told me its $46 and the other one down the street from my house told me $63. I heard on here that its only $20 so why is it so much more in this shitty city. I want to make sure I have the right part #, is it 25653157? Also all i need is longer bolts because my Rivs a 98 right?

You will get a cheaper price if you order it online.

from gmpartsdirect? or where else? Also do I only need longer bolts for the install? I have a 1998 Riviera

I got mine for $32.70 to my door from http://www.trademotion.com/partlocator/index.cfm?siteid=214632

I got my fender clips from them too. They take a little while but were cheap.

dreww wrote:

I got my fender clips from them too. They take a little while but were cheap.

What did you need the fender clips for? Just Curious

Take off your chrome wheel trim and you'll see!

98RIV7777 wrote:

from gmpartsdirect? or where else? Also do I only need longer bolts for the install? I have a 1998 Riviera

gmpartsdirect.com is around $20, plus around $10 for UPS. I think my total was $33? You'll want to source 4 longer bolts of equal strength, minimum of 1/4" longer than stock. But that depends on how many flat washers you need to keep clearance over the brake fluid res, unless you intend to reshape your bar instead. I used a couple washers at each mounting bolt, I think 3 at the front bolt on the driver side. I'd guess the mounting brackets were slightly warped/crooked from welding and an extra washer was needed there.

Is 25653157 the right part number?

Oh and Id rather not take off my chrome trim unless I absolutely have to

98RIV7777 wrote:

Is 25653157 the right part number?

Yes, it's in the first post of this thread is big bold type.

Ya I know I saw it there but I wanted to makesure a new one was not listed after that somewhere in these 13 pages, cause I scanned them but was too lazy to look through all of them

monsterpartsonline.com says it's $22.75 + ship.

Unfortunately the STB does not fit on the 99Riv worth a crap.

I will attach some pics...

On the driver side I have to stack too many washers to clear the Master cylinder.
The hood bulge is slight but evident nonetheless.



With this many washers it just aint cool.


Let me know if I am doing something wrong.


I thought I had a pic of the clearance between the Master Cylinder and the STB but it did not take. Even with all these washers there is only approx. 1/8" clearance(or less).


Bert

Sorry to hear, Bert. It's hard to believe that the different years affect the fit that much, but if the bar is slightly bent, it could cause a bad fit for any year car. If you take the bar off and look down it lengthwise (like a pool cue), is it straight?

It looks lke there is approx. 1/16"-3/32" arch in the STB. I dont think that would solve my clearance problem though...
I will smoke it over and try and determine what the deal is.

Thanks for the tips.

Bert

WOW, that's a lot of washers! I wouldn't run it that way, unless I wasn't going to push it in the turns. Maybe your brake fluid res is positioned higher or the brackets are off a bit? I got by with 2 washers all around except for needing 3 on one driver side bolt. I have about a mm of clearance.

1998 Riv wrote:

WOW, that's a lot of washers! I wouldn't run it that way, unless I wasn't going to push it in the turns. Maybe your brake fluid res is positioned higher or the brackets are off a bit? I got by with 2 washers all around except for needing 3 on one driver side bolt. I have about a mm of clearance.

I took it back off.
I am not sure what the difference is.

Bert

Try bending the mounting tabs so that you don't have to use as many washers.