It wasn't that bad was it? Great minds do tend to ramble.

 

Ruslow

It's realy the opposete. Picture a huge piviot in the center of the truck so that the chassis would have exactly 0 roll stiffness per degree roll. At that point you could always put the link back in no matter how far you twist the suspension, the chassis will just go with the suspension. If the chassis is infenently strong, all deflection will be in the suspensioin. The real world is a little bit of both.

Hope this helps

Andy

 

Too bad I can't move this to the Road Racing forum of NLOC.net, because it would be a good topic to discuss further in detail. I understand the point that awhittle is making because I thought thru it in detail when it was brought up in the past. What is interesting is that a have a photo of my truck on a road and it is apparent where the roll center is on the rear when looking at the photo. See the picture below and imagine the pivot point at the top of the licence plate - compare it the the roll shown at the cab. Yet, the arc length is so small (ride height to fully compressed measured from bolt to bolt) that the bind is relatively small in comparison to the lateral movement of the axle (IMO). Since I have a my truck with me this week, I'll measure out the actual dimensions and CAD it and post the jpeg of the drawing. Whatever the outcome of this dicussion, the rear end on my truck is more predictable on hard corners with this P-bar.



Regards,

Spike

 

Spike I wonder if what you are feeling is an increase in rear roll stiffness caused by the "inharent bind" giving a new and different feel.

Andy

 

I tested this the first time it was brought up (by you, correct?) by removing the bar and driving it, putting it back on, removing it, etc. I also tried different bar/bolt attachments on the frame side (up and down) to get a "feel" for the change in stiffness during left and right corners, very much along the lines of what you are describing. That "feel" was apparent and noticable at the hole creating the greatest angle, especially if you load the bar by changing it's length.

But, with the bar attached at the setting/hole that I prefer and is as neutral that I can make it, the best way I can describe the "feel" of the truck is how it feels without it: there are moments on corners that seems as if a rear tire tugs the truck perpendicular (either way) to the direction I'm plan to go. Also, if I drive the same corner over and over, that "tug" is never predictable - when or where or how much. Once I reattach the bar, the corner becomes familiar and consistant. Because of that, I try the same corner again at a higher speed and the rear doesn't wander. BTW, most of this "testing" was done on the street and a few figure 8's close by, so the speeds weren't near what I'd do on a track.

What I will try at Thunderhill in March is putting my camera on the rear axle again and run one session with and one session without the bar and compare the axle travel on tape. If there is "stiffness" added, it should show up in a change to the deflection of the axle/suspension. I would do it at Laguna Seca in a couple weeks, but I'm not comfortable enough with course enough to change the suspension for this test.

Spike

Andy: Sorry about calling so late! I look foward to your phone call tomorrow. If you call me and get "dead air," I'm stuck in a metal box in Thousand Oaks. Just wait about 10-20 seconds and voice will come on.

 

Spike

Where you catching the white car or trying to hang with it? HEHE
There is something great about a guy that will road race a pickup truck. I use the L to set up autocross courses. If you can turn a fun run in an L then every competitor will enjoy the course. Boy have I amazed a few co-drivers in the L. We need to build a real cage for that thing and get that suspension to REALLY work.

Andy

 

Awhittle true on that point but you also have to take into account that the suspenson is mounted in rubber and rubber deflects.Ideally I would like to bar as long as poss to maximize the arc distance that is travelled so that the rear axle would move less side to side when going over bumps or in a body roll.but then the bar would not be a drill and bolt on install in your driveway.The suspension system that is going on my truck next week[maybe] has taken all this into account and is more of a no comprising type.But at the same time is NOT for the average person.Stan
PS in my last post I said per inch of deflector I met to say per degree of deflection.

Also the main purpose of the bar is to move the roll center.All of the bars that are on other vehicles as an add on will have the same problems too.Unless you redo the suspension geometry on them too.ie mustangs since they have a diagnal 4 link you will have some form of bind unless you take the top links off and use a torque arm.

 

We need to get Spike to mount that camera under th bed of his truck and take a few autocross runs and then figure out where this this is really moving. Spike, you got your ears on?

Andy

 

Andy

Spike did have some video from under the truck. I believe it was mounted on the rear bumper? You could see the axle moving from side to side thru turns.

Mike

 

You guys are thinking pretty clearly. The Panhard bar limits the lateral movement of the rear axle assembly, but it also induces a lateral movement into the assembly by virtue of the arc it decribes as the axle moves up and down. Granted, this movement is not a lot, and is linear to the vertical movement of the axle. The leaf springs also limit lateral movement of the assembly, but only after being deflected to one side in response to lateral loads on the axle. You can therefore imagine the increased stability you would gain by installing the Panhard bar. The resulting 'binding' or resistance of the springs to this movement is insignificant when compared to the reduced lateral 'play' in the rear axle assembly. If you're really curious as to how much moement is induced by the Panhard bar, disconnect it from the differential mount point and hold a straight edge vertically, even with the rod end bolt hole. Then move the rod up and down and note how for the hole moves laterally. Then get a buddy to push hard sideways on the truck and see how much it moves. You will be surprised

 

Thats what I have been trying to say.The main reason for the bar is to prevent lateral movement.Plus even if there is some binding the rubber mounted leafs spring deflect enough to compensate for the vast majority of it.You have to take the good with the bad.ILike I said before you can design a bar that will not bind but the install would be far more complicated for what benifits you would gain.Unless you are an all out every .1 sec counts on a 2 mile course.stan

 

I did mount the camera under the truck, and will do the same when I run Thunderhill in March and/or April (see post above). Regarding the camera, I may stick it on the front brakes at Laguna Seca. Hopefully, I won't burn it up. I will be testing a new pad at LC. They're an aggressive compound but still considered a street pad (made by Hawk). I doubt they'll be popular for the typical owner because they will dust the wheels much more than the stock pads.

Spike

 

I didn't feel like crawling underneath the truck tonight to take the measurements, but I think the following sketch is conservative or close. I'm guessing the bar length is about 20" and the axle travel is going to be about 2 - 3 inches. If anyone is interested in measuring this, I'll redraw it using your measurements.

Spike

 

But what happens when you get 4 deg and 2 inch bump at the same time (like cornering thru a dip) with two roll centers in the same suspension. Inquiring minds want to know. HEHE

Andy

 

I just took some quick measurements and on my rear suspension, it wouldn't matter because I'd probably be off the track shortly after that.

But, assuming a three degree roll with the roll center being at the panhard bar connection (my bolt locations), the axle will be extended on the unloaded side and bottomed out on the loaded side. The offset/bind in the spring connection is 0.148".

Spike