shock measurments

THE place for technical discussions concering the construction and preparation of SAABs for all forms of motorsport, Rally, Road Racing, Auto-X etc....
User avatar
Luke
Site Admin
Posts: 3387
Joined: Tue Sep 19, 2006 11:12 pm

Postby Luke » Sat Feb 24, 2007 10:11 am

Any thoughts on converting std racing bilsteins to an upper pin mount to bolt directly into the saab? I searched around and looked in the bilstein catolog and couldnt find any adapters. Pro shocks has an adapter but i don't know if they use the same thread size as billies.

Rallyho
Posts: 385
Joined: Thu Nov 16, 2006 12:31 am
Nickname: El Blanco
Number of Saabs currently owned: 6
Location: Camden, Maine
Contact:

Postby Rallyho » Sat Feb 24, 2007 10:55 am

I was thinking of going the other way and getting rid of the pin mounts...hate those things.
Rallyho

User avatar
Geoff
Team Turbo Troll Crew
Posts: 3888
Joined: Fri Oct 20, 2006 3:31 pm
Nickname: Geoff
Number of Saabs currently owned: 6
Location: Nude Humpshire

Postby Geoff » Sat Feb 24, 2007 11:36 am

as for the rear being important, I can say after yesterday (and other experiences) that it is quite important. I had to go on a business trip Friday and had a leaky right rear tire on Thursday and didn't have time to take it off and find the leak so I put some fix-a-flat in (its also quite worn so I know that if I had to get the valve stem replaced no tire shop would work on it). The fix-a-flat didn't get dispersed properly (even though I followed the instructions) and made the wheel unbalanced. I've also got a bad RR Bilstein on the car (which is probably better than a good KYB). I could feel some vibration but I couldn't drive the car faster than about 55 or 60 without feeling like I was going to end up off the highway.

Hans
Posts: 442
Joined: Thu Nov 09, 2006 6:50 pm
Number of Saabs currently owned: 2
Location: Rain City

Postby Hans » Sun Feb 25, 2007 1:55 pm

Luke wrote:I also plan on fitting solid rear upper links with spherical bearings in the hopes that they will allow more fluid suspension travel and less binding.

There is an old discussion on Saabnet about not using spherical bearings at both ends of the upper links because, for some reason, it'll bind. This came from Brad Schaffner, who's done tons of set-up on road-race and circle track 900s. If I find the thread, I'll post it...

Here it is:

http://www.saabnet.com/tsn/bb/performan ... ?bID=77055

User avatar
Luke
Site Admin
Posts: 3387
Joined: Tue Sep 19, 2006 11:12 pm

Postby Luke » Sun Feb 25, 2007 5:04 pm

I'm not sure that I buy into that theory of binding being worsening by the solid upper links. There is definatly some degree of bind built into the geometry but I think the spherical bearings can help prevent that from occuring in the first place but keeping the arc of the axle in its intended plane. Only one way to tell for sure... try it out.

Rallyho
Posts: 385
Joined: Thu Nov 16, 2006 12:31 am
Nickname: El Blanco
Number of Saabs currently owned: 6
Location: Camden, Maine
Contact:

Postby Rallyho » Tue Feb 27, 2007 3:01 pm

My pea brain is trying to figure this bearing mount thang out.

So, a stock 900 has a lower eyelet (rubber bearing) and upper pin mount. Fr or rr, as the susp arm moves up and down, it will travel in an arc, so while the lower eyelet will allow some rotation, the upper must rely on the give of the bushing sandwich. The harder this upper bushing material (like poly) or the tighter the pin bolt (duh...don't we all crank those down) the less movement aka, more bind.

If both upper and lower were eylets, and better yet, bearings with some degree of free movement in all angles/planes there would be less bind...right? And to go one better, if the stubby, rear locating links and the panhard rod had bearing links, everything should be happier. Right?

And, there are more shock options (without going to coil overs) if you have bearings top and bottom.

Thoughts?
Rallyho

User avatar
Luke
Site Admin
Posts: 3387
Joined: Tue Sep 19, 2006 11:12 pm

Postby Luke » Tue Feb 27, 2007 10:08 pm

We got off topic a little here. The spherical bearings we are talking about are in regards to the two rear upper links that connect the axle to the body. The shock mounts shouldn’t cause binding unless they were excessively hard.

The rear suspension on the SAAB is basically like a 4 link, except with the upper link facing to the rear. In roundy round talk they call this a Z-link. It’s basically a variation off a Watt’s linkage. Like this: http://www.brockeng.com/mechanism/Watt.htm

The point of the upper links is straighten the arc of the axle as it goes up so that the wheel is moving basically straight up and down and thus wheel travel does not have an effect of moving the axle in position front to rear thereby creating an unwanted rear steering effect (bump steer). This works all fine and dandy if all the arms in the linkage have the same dimensions. But Saab had some space contraints to work around so they couldn’t pull off a geometrically perfect watts link. The result is that a slight amount of rear steer is built into the system, not a huge issue and our cars certainly have less rear steer than most cars of the period that had simple single pivot trailing arm based rear suspension systems. No problem here.

There is no binding as long as both of the rear wheels are working in tandem, going up and down at the same time. The problems arise when the rear suspension tries to work independently. One wheel goes up which will cause the rear axle to twist slightly as the upper links try to straighten the arc out. Look at the animation and imagine if the center point (P) were hard fixed (as it in a saab) to the bracket that comprises points B and C. You can see that P would rotate slightly as it went up and down. But say the other wheel wants to go down at the same time. As it lowers the linkage does its thing and keeps the axle in a straightish plane, rotating the axle on its centerline to do so…but in the opposite direction! The axle can’t twist in two directions at once so the result is that the suspension locks up, binds. In a case like this most likely the side with more force being applied to it will win out and force the other side to move in a similar direction.

This phenonenom is eliminated in most race car setups by a device known as a birdcage. A birdcage is a bracket that contains the upper and lower links for the 4 bar suspension but with a large bearing where it mounts to the axle instead of being welded. This allows the suspension to stroke freely on both sides without bind but it also over complicates it because now the brake ears have to be mounted to the 4 link brackets and not the now freely rotating axle. For a rear drive car the axle still needs to be restrained from spinning around so another stabalizing link is added from the diff going foward.

Now heres the thing... The saab system is a modified watts link which allows some rear steer but also eliminates some of the twisting effect that can cause bind. Combined will all the rubber bushings back there, there is more than enough play to prevent any binding effect. I jacked up my car and removed the shocks and springs on one side. Then the burly 140 lbs of of me lifted one side of the axle from full droop to bump stop without any sign of bind up. This is with red poly bushings in the upper links.

Now for some more anecdotes:
I had a saab once that had really worn out upper bushing and allows a lot of play. It handled horribly! The turn in was super slow and in linked corners the loading/unloading of the rear suspension made the car very unpredictable. When I replaced with new ones the car became crisp and predictable at the limit.

So heres my theory and what I am going to try. I'm going to go ahead and make up upper links with spherical bearings at both ends and mount them on the car. I think that there is enough slop built into the other mounts in the suspension to prevent binding and the increased predictability of the rear will be worth it. Besides its a front wheel drive can and the point of the rear wheels is just to keep the muffler from dragging after all, right? They are easy enough to change so maybe we can swap them around at service and get some driving impressions on stage of the two setups.

User avatar
Crazyswede
Team Turbo Troll Crew
Posts: 4539
Joined: Fri Sep 22, 2006 3:53 pm
Nickname: Mongo
Number of Saabs currently owned: 97
Location: Vermont
Contact:

Postby Crazyswede » Tue Feb 27, 2007 10:21 pm

You are crazy!!

Image
I am the 73%


Return to “SAAB MOTORSPORT TECH”

Who is online

Users browsing this forum: No registered users and 3 guests