Cascade Crew

blue89 · Posted: Wed Mar 05, 2008 9:34 am Post subject: a-arm suspension brace

i've been looking at this car for a while. Well, a couple of years. And I finally just noticed something. Their custom front suspension brace is incorrectly done! I drew on the driver side where the front a-arm suspension rotates and extended it with a line that intersects their front bar. I then drew another line that over laps the suspension brace that comes from the a-arm.

Those two lines NEED to intersect in order to work. Any other configuration will result in the a-arm pulling on the added brace and binding the bushings. Can anyone agree with me or enlighten me on why their geometry works?

Dewey316 · Posted: Wed Mar 05, 2008 11:33 am Post subject:

actualy, if the lines intercected it would be worse. My guess is that it is working, because there is not enough suspension movement for the bind to be very major. Or there are angles that we aren't seeing that allow the movement. Do you have any more pictures of it, or a better picture?

--John

blue89 · Posted: Wed Mar 05, 2008 12:03 pm Post subject:

I think it would bind. If the a-arm was big enough to have an extra bushing, all three of the bushings would have to be on the same axis in order to rotate. So any extra support coming from the a-arm needs to be on the same axis. Any other scenario results in suspension forming 3 points, and 3 points defines a unique fixed point in space. How far off the extra point can be is a matter of how much slop is in the pieces.

Granted the shot is taken below the chassis a little, it looks as if the brace is angled down towards the ground. I believe the a-arm axis is pointed up.

Dewey316 · Posted: Wed Mar 05, 2008 12:30 pm Post subject:

with it angled down a little , it might just work, without knowing more details it is hard to say, I think ti would bind, but it may not bind enough to matter, in th elimited range of movement.

blue89 · Posted: Wed Mar 05, 2008 12:37 pm Post subject:

I agree with you on that. Think it would be worth it? You wouldn't need urethane a-arm bushings, and it would probably create a more solid a-arm than going solid bushings, plus you wouldn't have the cracking problems that you do with daily driving a solid bushing car.

Under extreme braking I could see this being a benefit. However, taking the squish out of things just means when you push it over the limit, it will be less forgiving. I think it might be a good thing overall.

Dewey316 · Posted: Wed Mar 05, 2008 12:57 pm Post subject:

I really don't see a use for it. The direction it is bracing, isn't where the a-arm see's large forces. I would rather just box the arm.

--John

blue89 · Posted: Wed Mar 05, 2008 1:21 pm Post subject:

Under braking the a-arm is being pushed aft due to it trying to slow the chassis down. I wouldn't be suprised if a 1/2" of flex occured. Thats not much considering the bushing flex and how far away it is from the chassis. Thats only a 1/4" of squish in the bushings!

Assuming perfect adhesion to the road, you could have as much as 1500lbs pushing the tire back.

I have witnessed a wheel that appeared to be moving forward/aft when abs kicked in. Could have been due to something else though.

iansane · Posted: Wed Mar 05, 2008 5:51 pm Post subject:

Let me paraphrase this by saying I know little of the hardcore technical stuff you guys seem to know.

But, If the joint on the a-arm side was something like a heim joint it would let the arm move up and down fine and just prevent fore and aft movement?

rjmcgee · The Hammer Joined: 08 Jan 2004 Posts: 2328

Dewey316 · Posted: Wed Mar 05, 2008 11:37 pm Post subject:

Travis, That brace won't brace at all in the braking area's. The only way that brace will do anything, is if either tension or compression, the deflection of the bushings under braking, is is a torqueing force, it isn't pushing or pulling in the direction that that brace is doing anything.

blue89 · Posted: Thu Mar 06, 2008 8:13 am Post subject:

Thanks dew, that was the part I was missing. But won't there be some longitudinal forces due to the car being slowed by the wheels? The torque arm prevents the rear axle from rotating, but once those are subdue'd, all of the accelerating forces are transmitted down the LCA's to push the chassis.

Like wise the rapidly decelerating wheel will try to twist the strut towards the front of the car, but it is still trying to stop the momentum of the car by pushing backwards. I think that the force of that will still tend to push the a-arm aft towards the firewall. Maybe we should draw a picture?

blue89 · Posted: Thu Mar 06, 2008 9:03 am Post subject:

There isn't much you can do about the rotational torque applied to the strut. The a-arm won't twist though cause its only connected to the spindle at one point. If there is any squish in the bushings that connect the a-arm to the chassis it will cause the alignment to toe-out on decel.

Dewey316 · Posted: Thu Mar 06, 2008 9:48 am Post subject:

How is the ball joint going to put enough force through in that direction to defelect the a-arm? Think about how a ball joint is made, it can't transmit force like that. It the whole weakest link thing, if the force was going for/aft like that, I am pretty sure the ball-joint is going to be weaker in sheer than the a-arm.

blue89 · Posted: Thu Mar 06, 2008 10:33 am Post subject:

But if the all you do is rotate the spindle, the stut tower cap has less mechanical advantage than the king pin. Which would mean the king pin takes most of the torque. So yes, under smooth surface braking the king pin will have to be under sheer. And since the spindle to king distance is 6" and the spindle to strut cap is 2', the spindle will have 4 times the sheer as the cap.

Dewey316 · Posted: Thu Mar 06, 2008 10:37 am Post subject:

I am just saying, what are you trying to brace for? that brace won't do anything about bushing deflection, if you want to fix that, get some del-a-lum bushings for the a-arm. Other than that, you have a lot of other weak spots, before something is going to happen where that brace is bracing. If you were going to do a brace like that, you would also want to add in a mechanism to spread the load on the a-arm, with it just having a bold drilled there, it is doing nothing.

I'm not sure what you are looking for Travis, if you think it will help, build one, and see what it does, I just don't think it will do anything productive.

blue89 · Posted: Thu Mar 06, 2008 10:57 am Post subject:

Thats cool, thats why I asked for you opinion Wink

http://www.full-race.com/catalog/index.php?cPath=22_716_798&osCsid=30240881df5222cbc357074f178c9733

I saw this online and was thinking about it. Why it would/wouldn't be a good idea on the fbody. Then I remember seeing it on the Grace car. They even wrote an article about them.

http://www.full-race.com/articles/traction_bars.pdf

I totally agree that there is no way a bolt would help hold it together. I just wanted use an example of a fixed spot, no bushing. I guess since noone has really done it there is no need. The imports probably do it cause they have such weak suspension anyway.

91RSVert · Posted: Thu Mar 06, 2008 12:12 pm Post subject:

remember, most imports are also FWD. So theres alot more action going on up front then ours.

Ive been watching this thread and not really understanding what that bar would do Confused

Guess I'm not as understanding in suspensions as I thought I was.

Dewey316 · Posted: Thu Mar 06, 2008 12:16 pm Post subject:

The idea is, that it will transmit the force from braking, that would normaly be deflecting the bushings and the a-arm, and transmit them to the frame, this keeps your anti-dive and caster more controlled. I just don't see a need for it, or the the execution is very good for our cars.

I just think you can acomplish the same goals, by going about it in a better way, that won't give you such a weight penaly, and start messing with other geomety. When you add in there, when you add aother members like that, it is going to change your instant centers (svsa IC in this case). I would need to do some research on how this would change the IC, and effect the anti-dive instant.

--John

blue89 · Posted: Thu Mar 06, 2008 12:21 pm Post subject:

speaking of anti-dive, has anyone used eccentric bolts to change the a-arm geometry to play with that? My car doesn't dive that much so I'm not worried about it, but what can be changed on our stock setups?

Dewey316 · Posted: Thu Mar 06, 2008 12:26 pm Post subject:

I don't know anyone who has, I think to do it, you would need to weld up the existing a-arm bold holes, then drill a new one to change the a-arm inclination. I wouldn't want to add much more in there though, but a hair more AD wouldn't be bad. but the more A/D there is, the more stress on the bushings and frame where the a-arm attaches. A/D is sort of a strange beast.

--John