Another thread http://www.jalopyjournal.com/forum/showthread.php?t=809350 today outlines a problem with a tub that wants to 'step out' over bumps at speed There was a lot of talk about panard bar length with an illustration showing a theoretical point of roll center, in line with the vehicle center line. I understand the need for the arc of the panard bar to come closer to level as the suspension is loaded, but I was also under the impression that any thing 'over center' of the vehicle center line was acceptable. See the photo attached. What can I expect with the set up shown?
Roll Center is a mythical point around which the vehicle rolls. Roll Center Height is the height at which a force would cause no roll. The distance between the center of gravity and the RCH is the lever arm that cause body roll. The CGH is here a lateral force is applied the RCH is the point that the ch***is rotates around. Raising the roll center gives less leverage or stiffens the roll resistance. NASCAR cars raise the panhard bar to increase rear roll stiffness and "loosen the car" and vice versa. Independent suspension complicates this with changing RCH and jacking effects.
In the other thread, it seems to me the problem is not of roll center but of spring rate and shock value. He stated the car "bounces" readily. My bet is the shocks are not either properly positioned or of such value as to control the suspension through the range of travel. My two cents. As to your question, the way you have your panhard bar set up will get the job done but not anywhere near optimum. The best panhard bar setup is the way the Pete and Jakes '46-48 setup is done or all the factory setups where a full width bar is anchored to the frame below the center point of the axle and below the center point, (horizontally), on the rear. A lot of rods have the bars set up as yo do which puts the center of instant, (roll center) very high on the car. If you drive a car with the panhard bar properly positioned you will never do a bar like you have again. The man to talk about this on here is El Polacko. The engineering on this is way above me but I do know what works and what is marginal at best. You don't see a lot of rods done properly because the bracketry to mount the bar is more complicated. Look under any race car or late model and you'll see what I'm talking about. Same deal on the front if your running a panhard to control shackle swing on a cross steer. My old red roadster had the bars set up properly and the damn thing drove like a sprint car! I try whenever possible, (and it's not always possible or practical to do so), to set up the cars that come out of my shop as close to optimum as I can. The lower bar is worth the effort!!! When you talk about "over center", I ***ume your discussing the length of the bar for lack of a better term. This stuff is second nature for race car guys but seems to get brushed over by rodders. You can adjust roll readily by installing an anti roll bar on the front but you will still need to get the panhard bars, front and rear, in the proper position or it is at best a bandaid in controlling body roll at speed. Remember to never install a rear anti roll bar without a properly valued front anti roll bar or the car will be downright dangerous. You can run a front bar without a rear though. Now that I've put this out here, your probably totally confused about how to place roll instant. Maybe some of the guys with roundy round experience can chime in here.
"When you talk about "over center", I ***ume your discussing the length of the bar for lack of a better term." Correct... I went this way because of the frame kick up in the rear. My main focus (15? years ago) was side to side movement of the rear axle.
I found this to be helpful in seeing what's going on. I was researching the jacobs ladder type locater devices as used on sprint cars & how they differ from the panhard bar. Good visuals. http://www.spitzracing.com/index_files/Page724.htm
Your panhard set up will work just fine. In answer to your question I posted what the optimum set up would look like. Street systems are a lot more forgiving than a race setup. I get into both here so I have crossed a lot of stuff over to street use although not everything that is good on the track is good on the street and vice versa. I couldn't take it anymore and posted on the other fellows thread. His problem is not related to roll center at all. Actually I only read his initial post when I responded to you. After looking at the pics of his car, the problem was obvious. If he fixes the rear as noted he'll think that tub is on rails.
The set up you have will keep the rear centered, until you hit a bump. As the axle and the frame move closer together and the bar becomes horizontal the frame will move to the right (as pictured). I try to install a panhard close to horizontal at ride height. And the longer the bar the better. For a good example look at a 69 or 70 Chevy Impala.
As Ognib noted above, a Jacobs ladder works well on a sprinter and the example is for a 14" panhard. On a street car it is not too hard to get a 42" or longer bar in place reducing suspension arc considerably. A Watts Link, which with a little imagination is a primitive Jacobs ladder, is actually a better solution on a rod. I'm not sure why you don't see more of them except they are a bit harder to build than a track bar. They all work well if set up properly.
Agree,a panhard bar works well if they are setup right ,along with other items like spring rate, i think the reason why you see them more over 'watts" links is they are maybe have a slighty neater and cleaner look
Koz has his **** together on my stock cars I tried to get it as low as possible which was about 6 to 8 in lower than the center of the pumpkinnnnn there are a lot of old stock car handling books that cover it with my background in stock cars and dirt I guess I am kinda appaled at the set up
A Jacobs ladder is only suitable in vehicles that turn one direction only. [eg: mounted on the RH side of a LH turning sprint car ] They serve 2 purposes: 1: you can get a relatively long Arc from a short compact unit [ in theory you can move the pivot point origin outside the vehicle, so mechanical bump steer can be minimalized. 2:Bodyroll in the ch***is is converted into downward force on the RH tyre ,when the car leans over it pulls on the upper link and pushes on the lower link and plants the tyre into the ground [ increasing "sidebite" on the corner ] If the "Jake" was mounted on the LH side pointing to the left, it would try to lift the inside wheel during bodyroll. A Jake is not suitable for a street driven car. A panhard bar can also be tuned to increase sidebite on corners. It needs to be mounted on the LH side of the ch***is and RH side of the axle so it is under compression during hard LH corners. To increase sidebite the panhard is either raised at the ch***is or lowered at the axle [ the side effect of this is mechanical bumpsteer in the rear ] A watts linkage doesn't have any bump steer and [ generally ] cannot be tuned for sidebite , the normal adjustment in a Watts is for roll centre height. By lowering the RC height you are in theory softening the roll stiffness which will cause the car to push. By raising the RC height the car will tend to get loose. Simple huh!
This is the first thread on the board that all responses were all correct and on the money.. Looks like a lot of old stock car racers here.. Bobby...
Disconnect the shocks/springs and drop a plumb bob to the floor in the center of the rear cross member. Mark the spot on the floor and then raise and lower the car around ride height and watch the plumb bob. I suspect it will move quite a bit because of the short panhard rod.
I'm not sure if I explained it better or worse here: http://www.jalopyjournal.com/forum/showthread.php?t=629334
Thanks for all the replies. Still trying to get my head wrapped around it... would it help me to know what the angle is (off horizontal) with the bed/additional cross members (added after pic) in place is? I think the angle is not as sharp with weight on the back. BTW... shock angle remained the same (15º) before and after that pic was taken.
Excellent explanation! I agree, this thread has all correct advice. Something new on for the HAMB. Also as noted above when I discussed the Jacobs ladder I should have elaborated a bit more and specifically excluded it from street use when I said to use either a Watts Link or Track Bar on the street. Just one piece of that race stuff that doesn't cross over, (unless your only planning on making left turns). My ***umption here is from you pic in post #1, when ou get the rest of the weight on your car the bar will set in the proper position, parallel to the axle. Some real good advice here.
Roll Center is a mathematical point around which the suspension pivots. Roll Axis is a line between front and rear points, around which the car rolls. Factories used a very low front, and a very high rear, for safe handling for the average driver. Sometimes the front RC is even below ground. Newer cars aren't as bad. Performance cars even it out even more. I think Corvette is about 3 inches front, 7 inches rear.
BUMP. So I've raised the back of my lump two inches to incorporate some big fat meats, because when you're runnin' a v8-60 traction is of the utmost importance. It rolls more in the corners now, maybe the higher CG. If I raise my panhard bar (rear) will it significantly improve roll resistance? Right now it is about an inch above the axle line and parallel. If so how much? Rear 'bite' is not a concern, I only brake the tires loose on my front lawn when my wife isn't home.
^^ Paging Kerry in NZ...and what happened to El Polacko? So from what I gather I can raise only the frame end of the P bar (easy) at least for a test, which will raise the roll center but leave the bar at an angle. Maybe I will try that. I may radius the fender another inch higher so I can lower it.
Sorry for the late reply, I was busy doing stupid **** in my garage [ welding galv steel while wearing flip-flops ] You are going down the right path with your thinking. Understand in most cars you have 2 parts to weight transfer at EACH end . You have total weight transfer at the centre of gravity . This is calculated as Wt x CGH x G’s ÷ TW [track width] And you also have weight transfer through overturning moment [body roll]. This is calculated as Wt x [CGH –RCH] x G’s ÷ TW [track width] This is the tuneable part. So unless the Roll Centre Height [ RCH ] is at ground level . the weight transfer through body roll is usually less than total weight transfer. By raising the rear you have actually increased the amount of weight transfer during cornering. The RCH of a panhard bar is where the bar p***es through the centreline of the car. One end is fixed to the frame and the other on the rear-end housing, so when you raised the rear 2” you also raised the RCH 1” .So the distance between the CGH and the RCH is getting wider, which also lowers roll stiffness. The panhard bar will need to be corrected back to horizontal again. Now here is the clincher: By raising the rear your vehicle will now understeer more. It will lean over more but the front end will also push on the corners. This is because when the rear wants to lean over more, a higher proportion of load is supported by the outside front wheel [ the greater the loads, the greater the slip angle ] There are 3 ways to remedy this: 1: stiffer rear springs which makes the ride harsh. 2: Rear anti roll bar to increase roll stiffness only. 3: Raise the rear roll centre [ as you have already discussed ] One more thing. By raising the rear end, you have changed the angle of the linkages in the rear suspension into roll oversteer [ when the suspension compresses the wheelbase lengthens] Because your vehicle has such a short wheelbase ,this will make it dart all over the place. You need the suspension links to be horizontal at normal ride height, so when they compress and move on an arc it shortens the wheelbase [ roll understeer ,which is more stable ]
Koz. I am rebuilding a 31 Chevy and intended to install a rear anti-roll bar. Ch***is is stock with 2 1/2 Sid's dropped front axle. stock spring packs, rear is 12 bolt, de-arched stock spring packs, 65 327-365. In the past there was some lean in cornering and lean to right with torque. What will I have to do to safely reduce roll and why is rear alone dangerous? Thanks Bob
If the car has severe understeer, rear roll bar won't be dangerous. Stiffing the rear takes out understeer or plow.
Rear alone is only dangerous when somebody is out of there depth. As "ago" stated it can be used to reduce understeer/ induce oversteer. Note the stiffest [and/or heaviest] end will always slide out first Always at the limit of adhesion As with Paul's car, by raising the rear also softened the rear. [which has the same affect as stiffening the front ] By softening one end allows the body to roll over more degrees of angle [for the same G's] Now ***uming the ch***is has a reasonable ability to control torsional twist, then if one end rolls over more degrees of angle, the other end will do the same. The other end will support more weight by being stiffer and it also has the suspension compressed more, so it will "trick" that end into being heavier at the tyre footprint. I hope this explains it in simple terms
I crawled under there last night for a few minutes, looks like my new panhard mount will put the roll center height at about the top of the center section (8" Ford, so about 4-5" above axle), hope that's not a ridiculous height.
I good clue on mounting a rear panhard bar is to look at production cars. They are mounted low. Think it thru. If you hit a bump with one wheel and that wheel goes up and the other misses and stays level, the rear will rotate around the un bumped wheel. If you draw an arc from the un disturbed wheel thru the end of the bar mounted on the rear, you will see a lot of sideways movement with a high bar. This will move the rear sideways and jerk the car.
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