PDA

View Full Version : Pros and cons of a "J" bar



MuscleRodz
02-27-2006, 11:12 AM
I have heard most the arguements between watts and panhard bars, but what about "J" bars. They look better for packaging, but I understand the possibilty of more lateral movement being shorter depending on suspension travel. What are some of the more expert opinions on this?

Mike

Norm Peterson
02-28-2006, 03:44 AM
The roll center height tends to be rather high, being somewhere around the top of the pumpkin. Since the line of action of the force is between the J-bar's pivots (and is generally inclined), lateral loads will induce vertical loads that will differ in right truns vs left. Note that this inclination between pivots will aggravate the lateral motion of the axle in bump/rebound (this effect is likely to be greater than the "short link" effect). Structurally, it gets loaded in bending (in addition to the direct tension/compression) due to its shape, and is therefore slightly more flexible than a straight bar of identical cross section and length.

It works OK, I guess, in street rods that aren't really driven that hard - that's your packaging note at work more than anything, as concerns for appearance tend to over-ride functionality in many of those cars. On dirt track cars, where the environment is very specifically not symmetric (left turns only, other huge chassis asymmetries, different surface texture/conditions, etc.), its asymmetries can be used to some advantage.

Norm

wally8
02-28-2006, 05:15 AM
J bars are used in dirt track specifically to take advantage of the jacking forces induced by having a short bar. It's so extreme that there are now rules on how short of a bar you can run. It basically is used as a pole vault to give you a very radical weight transfer. Dirt racing is all about weight transfer.

As Norm said it's very asymetrical. That and the jacking is what the dirt guys are exploiting. Bad juju for the street or road course.

Myself, I'd stick with a long PHB as I think Watts are technically cool but overrated for most applications.


Later....

Wally

MuscleRodz
02-28-2006, 10:01 AM
The roll center height tends to be rather high, being somewhere around the top of the pumpkin. Since the line of action of the force is between the J-bar's pivots (and is generally inclined), lateral loads will induce vertical loads that will differ in right truns vs left. Note that this inclination between pivots will aggravate the lateral motion of the axle in bump/rebound (this effect is likely to be greater than the "short link" effect). Structurally, it gets loaded in bending (in addition to the direct tension/compression) due to its shape, and is therefore slightly more flexible than a straight bar of identical cross section and length.

Norm I was wondering how the bar loaded exactly, whether between points like a PHB or along the inclination. I did not think about the jacking effect it could cause with the inclination of the bar. I had already thought about the bending moment on an already bent tube and did not think highly of the compromise there. The only positive here would be the packaging and that alone is not enough to consider using it. Thanks.

Mike

wally8
02-28-2006, 10:13 AM
The bend is something to consider. If you've never seen one up close they are really thick. They use 3/4" heims, so when you see a picture of them you can get an idea of how thick they really are.

If you hit a wall pretty hard they will bend but car to car taps don't phase them.



Wally

RobM
03-01-2006, 06:05 PM
to put it simply having a panhard bar of any type where the end links are any thing but on the same plan as the axle will cause the vehicle to only turn well in one direction. In circle track they use the adjustable panhard bar to the advantage, raising the roll center helps plant the out side tire in a left turn. Unless you plan on turning in one direction more then the other and want to take advantage of that. Make the pan hard bar level with the axle and as long as possible. Thatís about as good as its going to get no need to over think this one. This will even be lighter and simpler then a watts which isnít very common in most forms of racing.

cagedruss
08-02-2006, 01:43 PM
Design a wishbone configuration for the rear and omit both!