Quote:
Originally Posted by wedged
what made you decide to mount the pivot to the frame instead of putting the pivot on the axle housing ? I'd also like to hear thoughts of others regarding this.
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Thanks for all the encouragement guys!!!
Now, as far as the pivot being placed on the frame or the axle. Alot of discussion about it. Pros and cons for each.
I went with the frame mount for a couple reasons.
With a watts link the pivot point determines the roll center height. And Im not an expert here but I will describe it as I understand it.
Roll center at the rear is usually fairly high, compared to the front which is usually very low. So draw a line between the two points and you have your roll axis. Drive a rod through your car on that axis and thats how the body will roll, or more importantly thats how the loads will be felt on the body and suspension. Drive another rod through the car at the center of gravity. It will be above the other rod. But now you can see where the force of the CG is pushing on the other rod. The longer the distance (more leverage) the more the body will roll. See the front rolling more.
The front will want to roll more, picture all the weight is above the roll center (the front RC height is usually in the dirt), so there is more force to push it over. And at the rear, with a high roll center, say with a top mounted pan hard bar, the weight of the car is alot less above the roll center so it rolls less. One of the reasons you use a smaller roll bar in back.
But a high roll center isnt always great cause now the suspension cant control the body as well, its regulated to more of the vertical weight control and not as much for lateral load shift, the turns. I kinda imagine a really high roll center in the back will tend to push the rear end into a slide instead of allowing the springs to compress and do their job. Body roll is actually a good thing. Too much no, but yeah, you need some body roll on our cars. And Ive actually been able to take a large sedan through the cones that had a terrible amount of roll at a good clip. Sheriffs academy. Felt like I was peeling the tires off the rim, but it was predictable.
Ok, nuther point. Moving roll center height. With a frame mounted pivot the roll center height moves with the center of gravity as you drive. Remember that weight (weight above the center of gravity) above the roll center that wants to push the car over in a turn. Well it gives a more consistent feel to the driver. As you go into a turn that is rough, wheels moving up and down through the turn the body will roll at the same amount through that turn (you dont even want to see where the front roll center is moving YIKES). The rear roll center will be coupled to the center of gravity, stable and predictable.
Now on a axle mounted watts pivot the oppisite is true. The rear RC height is still the point where the pivot is. But now its linked to the moving (relative to the frame) axle. So as you go through the same turn the rear roll center is changing in distance relative to the center of gravity of the car. Say WHOA!!! The feeling it gives the driver first is yikes, too much speed bud, throttle lift. What it feels like is the rear going into too much roll and its too high of a speed for the turn, then it flops over to too much rear stiffness as the axle moves. Back and forth. Unsettling. The rear will feel like its out of control and could be. You are changing the load transfer on the outside tire due to changing in perceived center of gravity because the axle is moving with the road. The center of gravity isnt changing any more than the other set up but the distance between the center of gravity and the roll center height IS changing so the end result is the same. The lever between the two points is changing so the load is changing. Not a great feeling. You will have enough squirliness with the front roll center changes. Up and down and sideways. We can talk about the front next week, thats really a difficult animal to control.
So, you have two pluses IMO for a frame mounted pivot.
Consistent distance between the center of gravity and rear roll center HEIGHT.
And a lower roll center height available with a frame mounted pivot..
Ok, ya want some of the downers?? Umm, fabrication. Many cars just dont have the room for a stout frame to mount the pivot. I think both should be as stout. The whole system can see 2000lbs of load and from what I have read can go to 10,000lbs.
Nuther one. Most of the aftermarket systems use a axle mounted pivot. For the ease of packaging Im sure. Fayes over came that. They do the frame mount and at a great price. I highly recommend their product.
Ok... now that I burned yer ears off with alot of babble, not much of it real knowledge, just my opinion DIVE IN!!! LOL
Yeah really, you will do what works for you. Thats all any of us do right. Whats within our grasp. My only solid point. Build it SOLID. I have life insurance and AAA but would rather not use any of it yet
JR
Brian Hobaugh SCCA National Tour June 2014
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