Automotive Race car suspension Class

[z28xtreme]

good questions all... i have to rework the post on page 13 as it was done in haste a while back and not to scale.
are you using 4 link rear or 3 point rear suspension?
i can tell you one thing as of now..you are racing a classic paved track set up with the bars mounted on the right side.
look for in depth post latter on after the beer drive thru opens

the rear is torsion rear, birdcage and radius rods.

 

[Ranger Mike]

Track bars, some time called J-bars are more correctly called panhard bars.

(J-bars are mechanically equivalent to straight panhard bars of the same length from center to center. Their J shape simply provides driveshaft clearance.)

Designed to locate a solid axle side to side, or laterally. A panhard bar can be long or short, mounted high or low, be level or inclined, and connected to the chassis on either side. All of these variables affect handling.

By now I assume you know about what a Roll Center is and how its height and location affect handling. An imaginary line called the roll axis connects the front and rear roll centers. A turning car experiences a radially outward inertia force moving through both roll centers which tries to roll the chassis around the roll axis.

This force transfers this load the inside tires to the outside tires. Along with other variables, the relative heights of the front and rear roll centers affect the front-to-rear distribution of the car’s roll stiffness and thus the distribution of the transferred load. The end with the greatest roll stiffness will receive the largest percentage of the transferred load, and will tend to lose side bite first.

All else being equal, raising the rear roll center increases the rear roll stiffness and thus the percentage of the transferred load that goes to the outside rear tire. That loosens the car up. Lowering the rear roll center has the opposite effect.

All panhard bars swing in arcs, which means that the roll center moves up and down with suspension travel, including chassis roll. If the panhard bar is mounted to the right side of the chassis then the rear roll center will become lower as the chassis rolls to the right in a left-hand turn. That loosen the rear end. Right-side chassis mounts are the most common on pavement, while left-side mounts are more common on dirt. With a left-side chassis mount, the roll center will rise as the chassis rolls to the right, tightening the car up.

Panhard bars can also push and pull the rear end sideways as the suspension deflects, especially short panhard bars. Lateral rear axle movement generally causes rear roll steer. If the rear roll steer increases the right-side wheelbase with respect to the left, the car will have roll-over steer. Shorter panhard bars translate the rear more than longer ones, and inclined bars further increase the amount of axle movement if their inclination increases as the chassis rolls. A short panhard bar connected on the left and inclined downward toward its attachment at the rear axle will pull the rear axle toward the car’s left side quite a bit as the car rolls right. This is common on dirt race cars. The panhard bar controls the amount of lateral axle motion, and the geometry of the other suspension links determines how much the rear axle rear steers. Raising the panhard bar where it connects to the chassis will increase the amount that it pulls the axle to the left as the car rolls. With most dirt suspensions, this adjustment will increase roll oversteer, loosening the chassis up in the turns. Inclined panhard bars also produce vertical forces that act on the car’s chassis and the rear axle at the bar’s attachment points. This is the second fundamental reason that panhard bars affect a car’s handling.

When a panhard bar is level, the rear tires’ entire lateral grip is transferred into the chassis at the bar’s end horizontally. But if the bar is inclined upward to the left, the forces transmitted into the chassis will have two components – one horizontal and another vertical. The panhard bar contributes to chassis roll because it pushes up at the car’s left rear. There will also be an equal but opposite force pushing straight down on the axle where the bar connects to it. These opposing vertical forces increase as the inclination of the panhard bar increases, especially with larger angles.

If the bar is connected on the right chassis mount and inclined upward to the right, it will pull the chassis down on the right side, but it will also unload the axle with a vertical force up, where it connects to the axle and unload the left rear.

With inclined panhard bars – and all bars incline at least a little as they swing through their arcs – the point where the bar connects to the rear axle is important. If it’s at the center, its vertical force will be equally divided between the two rear tires. If it’s to the right, proportionately more of the vertical force will go to the right rear tire. The J-bars commonly used on dirt race cars are generally mounted to the right of the axle’s center line and inclined upward toward their left chassis mount. That sends more downward vertical load to the right rear as the car turns.

 

[z28xtreme]

phew, I've read this so many times now. I think I am further away from understanding than I was before.

The end with the greatest roll stiffness will receive the largest percentage of the transferred load, and will tend to lose side bite first.

Is this because it will have too much lateral force? and will shear the tire?

If the bar is connected on the right chassis mount and inclined upward to the right, it will pull the chassis down on the right side, but it will also unload the axle with a vertical force up, where it connects to the axle and unload the left rear.

I moved my panhard down one inch on the right side rear mount and up one inch on the right side chassis mount. so this will loosen the car? then I can add more left rear weight back into the car to get the drive off after the car goes back level? which will help with drive off? I am sorry I do better with pictures. I am not the smartest guy around :(

 

[Ranger Mike]

With dirt set up the roll center will arc up to loosen the car. The tire contact patch to RC angle is 22 degrees.
The paved set up means the roll center dives down which lowers the car body and load the right rear tire. Note the 33 degree angle.
You can fiddle with the angle and height all you want on a right chassis mount car on dirt but you will still push.

 

[marshalljr]

Track bars, some time called J-bars are more correctly called panhard bars.

(J-bars are mechanically equivalent to straight panhard bars of the same length from center to center. Their J shape simply provides driveshaft clearance.)

Designed to locate a solid axle side to side, or laterally. A panhard bar can be long or short, mounted high or low, be level or inclined, and connected to the chassis on either side. All of these variables affect handling.

By now I assume you know about what a Roll Center is and how its height and location affect handling. An imaginary line called the roll axis connects the front and rear roll centers. A turning car experiences a radially outward inertia force moving through both roll centers which tries to roll the chassis around the roll axis.

This force transfers this load the inside tires to the outside tires. Along with other variables, the relative heights of the front and rear roll centers affect the front-to-rear distribution of the car’s roll stiffness and thus the distribution of the transferred load. The end with the greatest roll stiffness will receive the largest percentage of the transferred load, and will tend to lose side bite first.

All else being equal, raising the rear roll center increases the rear roll stiffness and thus the percentage of the transferred load that goes to the outside rear tire. That loosens the car up. Lowering the rear roll center has the opposite effect.

All panhard bars swing in arcs, which means that the roll center moves up and down with suspension travel, including chassis roll. If the panhard bar is mounted to the right side of the chassis then the rear roll center will become lower as the chassis rolls to the right in a left-hand turn. That loosen the rear end. Right-side chassis mounts are the most common on pavement, while left-side mounts are more common on dirt. With a left-side chassis mount, the roll center will rise as the chassis rolls to the right, tightening the car up.

Panhard bars can also push and pull the rear end sideways as the suspension deflects, especially short panhard bars. Lateral rear axle movement generally causes rear roll steer. If the rear roll steer increases the right-side wheelbase with respect to the left, the car will have roll-over steer. Shorter panhard bars translate the rear more than longer ones, and inclined bars further increase the amount of axle movement if their inclination increases as the chassis rolls. A short panhard bar connected on the left and inclined downward toward its attachment at the rear axle will pull the rear axle toward the car’s left side quite a bit as the car rolls right. This is common on dirt race cars. The panhard bar controls the amount of lateral axle motion, and the geometry of the other suspension links determines how much the rear axle rear steers. Raising the panhard bar where it connects to the chassis will increase the amount that it pulls the axle to the left as the car rolls. With most dirt suspensions, this adjustment will increase roll oversteer, loosening the chassis up in the turns. Inclined panhard bars also produce vertical forces that act on the car’s chassis and the rear axle at the bar’s attachment points. This is the second fundamental reason that panhard bars affect a car’s handling.

When a panhard bar is level, the rear tires’ entire lateral grip is transferred into the chassis at the bar’s end horizontally. But if the bar is inclined upward to the left, the forces transmitted into the chassis will have two components – one horizontal and another vertical. The panhard bar contributes to chassis roll because it pushes up at the car’s left rear. There will also be an equal but opposite force pushing straight down on the axle where the bar connects to it. These opposing vertical forces increase as the inclination of the panhard bar increases, especially with larger angles.

If the bar is connected on the right chassis mount and inclined upward to the right, it will pull the chassis down on the right side, but it will also unload the axle with a vertical force up, where it connects to the axle and unload the left rear.

With inclined panhard bars – and all bars incline at least a little as they swing through their arcs – the point where the bar connects to the rear axle is important. If it’s at the center, its vertical force will be equally divided between the two rear tires. If it’s to the right, proportionately more of the vertical force will go to the right rear tire. The J-bars commonly used on dirt race cars are generally mounted to the right of the axle’s center line and inclined upward toward their left chassis mount. That sends more downward vertical load to the right rear as the car turns.

 

[marshalljr]

I reposted this by mistake, however on dirt, increasing jbar angle tends to tighten corner entry and center but can hurt forward bite on exit. A compromise is needed to optimize corner speed and momentum onto the straightaway. I only increase angle for very slick conditions.

 

[tom jennings]

hi -- I'm replying to an old post, as i make my way through this fantastic forum... i can't thank you all enough, concise, dense, courteous, wide-ranging. and especially Ranger Mike... on page 12 (2011), working to the present!

Squaring chassis set up

for cars with lower ball joints in front, i worked up (probably, re-discovered, in ignorance) a nice trick. i chopped the head off a Zerk fitting, inserted a cotter pin (loop end outside), slightly bent the legs to retain, clipped short, and screwed into the bottom of the ball joint. a hand side effect is that the cotter pin neatly holds a tape measure or string.

 

[Racer Dean]

Hello All

My name is Dean and have spent the last 3 weeks reading this thread from start to now and am extremely impressed with the content, ideas and the level of knowledge shown by all. Thoroughly enjoyed and thanks.

But I'd like to ask for some advice if possible,
I race a non wing Sprint car on dirt in Australia and wasn't completely happy with running the same set up as everyone else. (Torsion bar, Z link)
I've changed my suspension to a full coil car using a 4 link setup.
After reading this thread, I'm starting to realize the more I learn about racecars, the more questions get raised.
What I'm after is any ideas on a good suspension software that can work with roll centers using solid front axles.
I just can't see how I can raise or lower the roll center in the front?
To me it's always going to be in the center of the axle
Any help would be appreciated

Dean

 

[Ranger Mike]

Welcome Racer Dean, and thanks for the kind words..
I called Kevin at
PerformanceTrends.com
He said they are working on adding solid front axles to Suspension Analyzer. Right now the only thing is the Circle Track Analyzer. Front axle is assumed to be located by a panhard bar.Kevin Gertgen
Performance Trends
contact him at feedback@performancetrends.com

Actually I prefer the Circle Track analyzer even though it is 2D. it is quicker to put input in since you only measure t he distance from center line and height from ground. The more sophisticated Suspension Analyzer requires you to measure the distance from the front of the car for each data point.
plus it is cheaper!
click on link below, i think they have downloadable trial version
performancetrends.com/Circle_Track_Analyzer.htm

 

[Fiveothis]

Ranger Mike, a huge thank you to you and all the other contributors on this thread. I just spent a week reading ALL 55 pages. I understand almost all of the info but have a few questions.

First when talking about roll centers and jacking effect, is there a different thought process when setting roll center location between a car with independent suspension versus straight axle? Obviously from reading these 55 pages you believe in low roll centers that are offset 3 inches to the right, is that the same for a straight axle all things being equal between 2 cars except the obvious suspension design change.

Second question when you talking about setting the roll center 3 inches right, what about a car that has the suspension offset to the right 6 inches or more overall to increase left side weight? Is the center-line still the center of the frame or is it center of the contact patches??

 

[Ranger Mike]

Thank you for taking the time to wade thru 55 pages. Nice to know the effort was worth it.
We offset the front Roll center to add down force tot the right front tire when turning left. This is easy to do when you have control arm suspension and the chassis software. You can get the same benefit on beam axle or straight axle if you have coil over shock set up and panhard bar. If you have the traditional mono spring set up like the Ford Model T used, I am not sure you can offset.
Regarding the Roll Center offset and left side weight offset- They are not related and should be treated separately. Left side weight bias means the weight is shifted when you have the racer car setup on weight scales and the left front and left rear wheel weights are 55 to 60% more than the right side. It is a static placement. Wheel back spacing can be changed to help this. Typically you are mounting the engine offset to the left, fuel cell, battery and any ballast. Roll center offset requires the race car dynamics of chassis roll to add down force on the right front tire to turn the car, and proper static weight placement to cause the right rear and left rear tires to hook up and drive out of the turn.

I always scribe a center line on the chassis for reference. Using the tire contact patch is ok if you do not plan on swapping out different back space wheels. I got to many other things to worry about at the track so use chassis center line for all datum. Consistency is the key point here.

 

[Fiveothis]

OK Mike. Yes let's suppose a vehicle with straight beam axle, coil over shocks, and Panhard bar. Obviously the roll center height is centerered between the 2 rod ends but how do you go about offsetting the roll center left to right? Is it as simple as moving the location of the Panhard bar location? I was thinking in Carroll Smiths books he says with a beam axle the location of the Panhard bar left to right doesn't matter because the car "feels" the roll center left to right half way between the top mounts of the coilovers?

As far as the offset chassis and centerline the offset of the wheels would never change. But if I use the frame as centerline versus using the tire contact patch centerline the difference between the 2 is over 6 inches. So again should I not use tire contact patch centerline?

 

[Ranger Mike]

The roll center on a solid axle is the place the panhard bar crosses the axle center line, be it front or rear axle. you have unlimited means to design and build the best combo for your set up. If you plan on using contact patch center line, great. Just remember the offset wheels are the same every time.

 

[moosetallone]

Mike
On a dirt late model left side mount jbar, what's your thoughts on running soft right rear spring with spring rubbers and a lower jbar angle to help with loose off condition
Though being softer spring will allow the car to roll over easier and plant the tire with the spring rubber, keep the angle lower on the jbar to keep from pogo sticking the rear of the car up on the jbar.

53% left 53% rear

 

[Ranger Mike]

FiveO the RC is the intersection point of the panahard bar and axle. The example happens to be the p-bars center point as well. I agree with the software.

Welcome Moose..i am not a fan of spring rubbers as they wear out or fall out and are a variable to figure out when handling suddenly goes away. How much rear roll steer are you running now? What are the angles for the lower bars now? If you want more body roll soften the ARB (sway bar). To add side bite and bite off corners try lowering the panhard bar and add a little angle. You can take out the right side wheel spacer and add more tot the left rear wheel spacer too.

 

[Fiveothis]

OK Ranger Mike I got it now. Is there a certain amount of angle in degrees I could baseline a setup with? I have read and understand the broom handle theory and read many times your example of 3 to 4 inch offset to the right for roll center location. But for my application with the major amount of suspension/chassis offset it would seem to me and by looking at the picture of the performance trends software I need a roller center farther to the right than the typical stock car examples to get the needed angle.

 

[Ranger Mike]

I got not degree for your set up as there are too many variables. I will asy too much offset will lift the left front tire, overload the right front tire. use the pyrometer to adjust. If you have a wing you will add down force as well. all these add up so don't go big too early..gradual changes are better.

 

[Fiveothis]

OK Ranger Mike. Let me throw you another curve ball. Let's say we have 2 identical cars with the only difference being front roll center location. Everything else exactly the same. Car 1 has the front roll center 3 inches above ground, 3 inches right of centerline and that gives a force angle from right front contact patch to roll center of 20 degrees. Car 2 has roll center 6 inches above ground, 3 inches left of centerline, BUT the force angle from the right front contact patch to roll center is still the same 20 degrees. Will these 2 cars handle the same and will they have the same amount of download onto the right front tire?

 

[Ranger Mike]

Assume on this example , both cars have a track width of 66 inches and the engine is located at vehicle center line and cam shaft is 16 inch height. We use this as approximation of the Center of Gravity (CG).
Car 1 has a distance of the RC to Cg of a little over 13 inches ( 16” –3” = 13”, plus the 3 inch offset ..I did not do the triangulation but it is a little longer than 13”.
Car 2 has a shorter distance of 16” – 6” = 10 inch and change. This means the lever is shorter and has less mechanical advantage.
Now add to this, the fact that the RC offset to the right means 55% of the vehicle weight will rotate and plant on the right front tire. ( 66/2 = 33 center line and add 3 inch offset. 33 +3= 36...36/66 = 55%). Compare this to the RC offset to the left by 3 inch. Now you have only 45% of the weight rolling onto the right front using a shorter lever arm. .The left offset RC setup means the car will push going in and be loose off the turn. Forget about the jacking effect as you need to stick the right front tire first to cause the pole vault jacking effect.

 

[Fiveothis]

Got it Mike great explanation.

The only thing I can't wrap my head around is why the RF needs additional load to help turn the car. It would seem for maximum traction you would want to keep the roll center left and keep the LF tire loaded more since it does less work

 

[Ranger Mike]

like everything in life it is a compromise. Too much down force on the right front will make the left front tire lift, too little and the rt front skids instead of sticks. Ultimately you want all four tire carrying equal load. Thats why you use a tire pyrometer.

 

[marshalljr]

Then you either need a shock with more travel or you need to limit the travel via spring rate changes. Using a chain won't cure the problem, just keeps the shock in one piece.

As soon as any part of the suspension binds or locks or hits the end of a stop, the spring rate on the wheel effectively goes sky-high and that tire loses traction. If it is a solid axle like yours, it also changes the spring rate on the other wheel the same way, again causing a loss of traction.

By using a chain or a short shock on the left rear, you're making the right rear act like it has a soft spring when that car is flat and a hard spring when it hits the stop as it roll. This can change the handling rather abruptly with only a small change in roll angle and make the car a handful.

Get the chassis movement/roll under control like Mike is suggesting.

 

[Warriorracecars]

Assume on this example , both cars have a track width of 66 inches and the engine is located at vehicle center line and cam shaft is 16 inch height. We use this as approximation of the Center of Gravity (CG).
Car 1 has a distance of the RC to Cg of a little over 13 inches ( 16” –3” = 13”, plus the 3 inch offset ..I did not do the triangulation but it is a little longer than 13”.
Car 2 has a shorter distance of 16” – 6” = 10 inch and change. This means the lever is shorter and has less mechanical advantage.
Now add to this, the fact that the RC offset to the right means 55% of the vehicle weight will rotate and plant on the right front tire. ( 66/2 = 33 center line and add 3 inch offset. 33 +3= 36...36/66 = 55%). Compare this to the RC offset to the left by 3 inch. Now you have only 45% of the weight rolling onto the right front using a shorter lever arm. .The left offset RC setup means the car will push going in and be loose off the turn. Forget about the jacking effect as you need to stick the right front tire first to cause the pole vault jacking effect.

I wish this worked on my track. I can say that it does not. Actually creates a tight center. This is a very flat 3/8 almost round track. coldest tire on the car is the lf with 51% cross after loading 1 1/8 bar. I will be moving the mc to the left maybe 3" and see what happens. Mike when you move the mc to the right, do you notice any difference in the ability for the right to "get down"? The reason I ask is that with my car. I have equal springs left and right at the moment. Without any sway bar involvement the right corner is MUCH harder to compress in the shop and shows on the track.

 

[Ranger Mike]

are you still using bump stops?
and you are loading the ARB as well?

 

[Warriorracecars]

yes bump stops and 2-3 rounds in swaybar

 

[Ranger Mike]

warracecars, my advise and opinion does not apply if you are running on bump stops and adding in preload on the ARB. Both of these are counter to my entire thinking on making a race car suspension handle during cornering. good luck on your effort.

 

[Warriorracecars]

no problem Thanks Mike.

 

[Tyredone]

Ranger Mike: Thank you for advice - I bought and read Steve Smith's books. I now have a couple of questions about the car, where building for paved flat track with left and right corners:
1. Bump and Rebound travel - how much either way?
2. The third link pick up point on the diff? ... In your comment (Pg 38 / 749) you gave an example: "i.e. if car has 60 inch track width and 53% left side weight then 60 x .53 = 31.8 inch and 3rd link should be mounted 31.8 inch to the left of the RIGHT TIRE CENTERLINE." ... And in Steve's book he also suggests: "If a car has a 60-inch rear track width, and a 58% left weight percentage, the calculation is: 60 x .58 = 34.8. So, the centre of the weight mass at the rear is located 34.8 inces to the left of the center of the right rear tire." ... Please forgive my ignorance - I know my track width, but are you able to explain how to work out the left side weight so I can calculate as per examples above? (Sorry, I'm not the sharpest tool in the shed.)
Appreciate your help :)

 

[Ranger Mike]

Ranger Mike: Thank you for advice - I bought and read Steve Smith's books. I now have a couple of questions about the car, where building for paved flat track with left and right corners:
1. Bump and Rebound travel - how much either way?
2. The third link pick up point on the diff? ... In your comment (Pg 38 / 749) you gave an example: "i.e. if car has 60 inch track width and 53% left side weight then 60 x .53 = 31.8 inch and 3rd link should be mounted 31.8 inch to the left of the RIGHT TIRE CENTERLINE." ... And in Steve's book he also suggests: "If a car has a 60-inch rear track width, and a 58% left weight percentage, the calculation is: 60 x .58 = 34.8. So, the centre of the weight mass at the rear is located 34.8 inces to the left of the center of the right rear tire." ... Please forgive my ignorance - I know my track width, but are you able to explain how to work out the left side weight so I can calculate as per examples above? (Sorry, I'm not the sharpest tool in the shed.)
Appreciate your help :)

hello
tyredone
i am in chicago and away from my notes. suspension travel? what type car is it? a formula car will have a lot less travel, a street stock a lot more..
what is your question on left side weight. both calculations are correct. track width is distance from tire center line to tire center line. The percent left side weight is as you noted. the amount will be determined by local track rules. if you are hung up on the center line versus center of the tire (one single point) don be...use tire center line.
glad to have you stop by on this post..
rm

 

[Rajesh Shirsagar]

The suspension of an auto structures the basic interface between the distinctive components that work together to deliver its execution. Suspension is the thing that outfits the force of the force unit, the downforce made by the wings and the grasp of the tires, and permits every one of them to be joined adequately and deciphered into a quick on-track bundle.

Taking after the restriction on PC controlled "dynamic" suspension in the 1990s, the majority of the Formula One auto's suspension capacities must be done without electronic mediation.