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Race car suspension Class

by Ranger Mike
Tags: class, race, suspension
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rwstevens59
#109
May31-11, 10:36 AM
P: 16
Sorry, Mike,
Terms and being very specific about what you mean always seem to get us in trouble in this complicated business.

I am referring to the amount of roll resistance provided by the spring pair at one end of the car. In a steady state corner when the inertia force acting at the CG rolls the car right in a left hand turn the spring pairs front and rear produce a resisting couple (torque) to counter that roll. My use for that is, that in load transfer calculations, the resisting moment produced by the spring pairs front and rear is considered the elastic portion of the sprung weight transfer. The other portions of weight transfer are the sprung geometric weight transfer transmitted through the suspension links, this governed by roll center heights. And the unsprung weight transfer, which is basically uncontrolled, except for tire spring rate, caused by the rolling of the unsprung components such as a solid rear axle housing.

The quick and dirty formula for this is: K(angular)= 1/2 Kroll * Spring track width^2/57.3 which yields an angular roll resistance for the spring pair in lb.in./ deg. The kicker is what is the value for Kroll which should be in lb./in. for a spring pair on a solid axle with split and an asymmetric layout. For simplicity lets assume the springs just sit on top of the axle with no linkage to add a further motion ratio to the mix.

All I really want to know is how stiff is my elastic (spring) resistance relatively speaking front to rear. The stiff end gets the weight transfer and therefore the more unequal tire loading left to right and therefore less grip. If you stiffen the front springs relative to the rear at some point it's gonna push.

Hope that makes some sense.

Thanks,
Ralph
Ranger Mike
#110
Jun1-11, 03:56 AM
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as you said at the start - The problem comes in when I ask or try to research what the roll resistance is for a car running solid axles front and rear with spring split, and with the chassis offset to the left for oval racing. There are many simple explanations of solid axle cars as long as a. The spring rates are equal left to right and b. The car is symmetrical about the average track center line.....
As I posted earlier in this post, we can calculate the total weight being transfered during cornering if we know the G force in the corner. The left side weight of the car will not change this total. Regarding the equal spring rates- these are ok if you are running a road course but not optimum if your racing left hand turn only. Cross weight or diagonal weight transfer from left rear to right front must be in the mix as well. This is why the right front spring has higher spring rate than the left front spring..rear springs are also not equal rate.
question - what kind of front spring set up do you have?
rwstevens59
#111
Jun1-11, 08:16 AM
P: 16
The cars I am referring to here are Northeastern DIRT Modifieds if you are familiar with this 'breed' of car. The solid front axle has coil over front springs mounted just behind the front axle on the radius rod brackets, think of a T-bucket roadster hotrod only without the dropped axle. The cars are 2500 lbs and the average setup calls for a 250 lb/in LF spring and a 150 lb/in RF spring. The rear is a solid tube Winters quick change (not live) with a spool. The rear is suspended on a pair of torsion bars (sorry don't have my setup notebook with me and I forget the exact calculated spring rates at the axle). The rear torsion arms rest on rollers below the axle centerline, so unlike a sprint car they are a spring only and do not play a role in longitudinal location of the axle.

There is a one line mention of calculating spring split roll resistance in Milliken in the chapter on Ride and Roll Rates can't remember the page. And quite a bit in Warren Rowley's book on race car engineering dealing with solid rear axles. Both of these sources state that a pair of springs (or wheel rates on a front independent) provide a roll resisting torque equal to: K(left)*K(right)/(K(left) + K (right)) with K(left) and K(right) being the respective spring rates in lbs./in. This produces an offset spring center located somewhere closer to the stiffer spring. If you plug this value in for K(roll) you get an awfully low roll resisting moment in lb.in./deg. Now springs on any beam axle produce a much lower roll resisting moment than their respective ride rate just because the springs are inboard of the wheels with a spring track that is quite a bit less than the track width. All of the above (spring center calcs) I can prove from first principles just using a simple beam supported by two different rate springs as my model as well as just placing a bar over two small springs with different rates, shim the soft side up to level on a surface plate and find the point where the beam rotates evenly with dial indicators. All well and good, but is that how the car behaves and when you start adding in the asymetries of the car CG centerline being offset 4 inches to the left, I'm starting to get lost in the woods without a compass.
Ralph
Ranger Mike
#112
Jun2-11, 04:31 AM
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I admire all the work you did on the calculations..more than I ever did.

About all I can offer is from all my notes on Dirt modified using metric GM chassis days, I can only reply with the following:
We ran 100 lbs stiffer spring on the right front vs left front...was 3 link rear suspension.
55% left side wt. 56% rear wt. 51% cross weight. if my notes are right we used 200 lb springs on both rears..
why are you using a higher rate left front spring? This is a "reverse Split" and can be used to keep the car flat at tracks with large amounts of banking or for when you want the car to lean over on the right rear...right?

Are you running a big wing on top?
rwstevens59
#113
Jun2-11, 08:12 AM
P: 16
That is the chassis builders recommended setup and brings us to why I am scratching my head. The bodies of these cars don't have a big wing on top but do create a fair amount rear down force which leads to an aero push almost always. The rules really don't allow for a proper aero balance. The tracks we run are flat, less than 10 deg. at 5/8 mi. and semi-banked bull ring at 1/4 mile. I am questioning the spring choices and trying to decide which way to go and by about how much. I don't just want to take a WAG at it because the car is close. The overriding complaint by the driver is drive off the corner. Now, I have never believed you can calculate your way out of these situations because it is just too dynamic. But, calculations will give you clues as to whether you are fine tuning or making major changes. The other problems as always are money and time. We are now into the season, hot laps are just that and only tell you nothing fell off the car and the engine is running OK. The first opportunity to see what you have is in the heat race, experiment too much, and you don't qualify well. Doesn't offer a lot of space for test and tune. But if we stick to the 'Golden Rule' and change one thing at a time and only one thing at a time we will improve if for no other reason than finding out what doesn't work.

I have asked this question concerning roll resistance of spring pairs on solid axles several times in different engineering circles and am finding out that it really isn't simple and it is actually easier to calculate on an independent suspension. The seemingly simple beam axle becomes quite complex when viewed dynamically.

Ralph
Ranger Mike
#114
Jun2-11, 12:47 PM
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i would equal up the front springs as change one.
when is the driver needing bite off the turn..on wet track/ tacky track? dry hard packed track?
can you adjust rear wheel track. i forget ..are you running a panhard bar?
rwstevens59
#115
Jun2-11, 01:35 PM
P: 16
The car exhibits push in all phases of the corner when wet and sloppy early in hot laps depending on the group you go out in. As all tracks in the northeast the track does not stay wet and sloppy for long. The complaint of lack of drive off on late corner exit occurs when the track is dry. The 1/4 mile track transitions to an interesting condition when it dries out. I would say the clay sand content must be quite high as the track does not get hard packed and shiny but rather drys out and continues to abrade into powder which blows off and it almost never takes rubber.

The baseline setup starts with a 2 inch spacer in both the left rear and right rear, so yes, there is some ability to adjust rear track. The rear currently runs a 14 inch wide rim with a 5 inch back set. Three inch back set rims are also available.

The rear suspension is a torque arm with two lower links and a short panhard bar mounted to the frame on the right side (rear view) and the center section at the rear end on the left side of the pinion (rear view) in front of the axle centerline. Yes, this is the total opposite of what you would see in a current super dirt late model, but that is how all three current chassis builders construct these cars. To best picture the lay out think of the old GM 'F' body cars like the Camaro, the layout is similar but not the same.
Ranger Mike
#116
Jun3-11, 03:45 AM
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now we are getting some where
have you measured the rear track under compression/rebound?
I am trying to determine if there is rear roll steer in play?
is the wheel base on one side growing or shrinking when in bump?
what is angle of third link? what is angles of both trail arms?
i am not too worried about panhard bar mount as much as angle
Attached Thumbnails
roll steer.jpg  
rwstevens59
#117
Jun3-11, 06:54 AM
P: 16
Attached is one of my sons pictures to give you a better idea of what the heck we are talking about here.

The rear suspension on these cars would technically be considered a two link with panhard. The torque arm consists of two plates bolted to the quick change using the side bell through bolts either side of the drive-shaft. The plates go forward about 28 inches and attach to a large heim which attached to a slider plate mounted on two shafts attached to the chassis. The heim is mounted to the plate with a rubber biscuit to cushion rear end wrap up on throttle.
The two lower links are mounted on clamp on brackets at the outer end of the axle tubes below axle centerline. The forward end of the lower links have three mounting holes on the chassis to adjust link angle and therefore anti-squat, which also effects the amount of roll steer. This is kind of hard to picture if you have never seen the design, but it follows all the principles of a torque arm two link with panhard. When I say torque arm do not automatically envision a typical late model torque arm with fifth coil. These cars are laid out like an over grown sprint car with the driver centrally located with the driveline and torque arm between his legs and under the seat.

This rear linkage design has inherent roll oversteer in all lower link mounting positions. This is crutched (in my opinion) by using a spring rod for the right rear link to induce power on axle steer to the left to counter the roll induced oversteer.

The panhard angle is cockpit adjustable for angle, which to me just adds or subtracts a jacking force more than moving the roll center height. If I want to move the roll center I move the bar up or down equally at both ends while maintaining the angle.

Hope this all makes some sense.

Thanks, Ralph
Attached Thumbnails
256034_204447956263259_100000941687229_499550_7727948_o.jpg  
HowlerMonkey
#118
Jun3-11, 11:16 AM
P: 276
We're using the aim evo4 with smarty cam on our twin turbo ford GT.

That said, reducing the data from runs or laps is tough because of even tire width and where the contact patch (if not flat and perfect) will affect "spring rate" as seen by the car.

It makes reducing any useful date require a lot more figuring.

That said, look for our current 252.9mph standing mile record to be pushed past 270 in a few weeks.
Ranger Mike
#119
Jun4-11, 06:14 AM
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pushing going in - rt front spring too stiff
too much cross weight
too much rt front tire pressure
not enough stagger
too much panhard bar angle
my opinion
mender
#120
Jun4-11, 09:48 AM
P: 563
Sounds like an interesting set-up. You're right, figuring the roll centre when the springs are essentially the suspension pivots can get messy. I don't do dirt, so take my questions with a pound of salt!

What little I know seems to indicate that most of the action is at the back, and your description of the suspension seems to confirm that. The roll oversteer to get the car to point, then power-on understeer to transition into a set for the rest of the corner means that the rear changes position noticably when power is applied, right? Could it be moving too much and unsettling the tires, giving the poor drive off the corners? I'd be tempted to set everything closer to neutral in a test session, then move towards the recommended set-up and see if the car really needs that and if so, how much is helpful before going over the line. Does the car feel unsettled with on-off throttle adjustments mid-corner?

Having the reverse front spring split would seem to be an attempt to help traction off the corner by partially compensating for the torque reaction in the diff when throttle is applied; with a narrow rear track, that effect is amplified, so playing with the track might indicate how much that is affecting things. Again, I'd consider reducing the amount of things happening at the back end of the car, then adding them back in as you get a handle on what each adjustment does. Again, finding the range of front spring split for your car might require a test session. From what I understand not a lot of people scale their cars after baselining for the season; do you?

Have you talked to the chassis builder about how his design is supposed to work? Has the car been wrecked? Have you tried alternate set-ups?
Ranger Mike
#121
Jun5-11, 04:30 AM
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I been thinking about this a lot...i still come back to roll understeer since it sounds like it is pushing all the time..entering mid and exit the turn. it takes a lot of work but i think you should measure the wheel base as the suspension goes through compression and retraction to see if the rear end is cocking toward inside of the track in the turn.
I been involved in getting many many used races from racers who wanted out of the game..besides the safety consideration,,,checking quality of the welds, wiring, fuel cell and plumbing...we always stripped down the , took out the springs and mapped the suspension travel in bump and rebound..checked the bump steer front and rear and checked ackermann when we were rebuilding the steering...you have to do this in event you have a real bad crash and have to replace parts..thats why i love the software program for performance trends..shows the linkage action and roll center changes dynamically...
ifin you don't do this your just putting a bandaid on the situation
rwstevens59
#122
Jun6-11, 01:18 PM
P: 16
Quote Quote by mender View Post
Sounds like an interesting set-up. You're right, figuring the roll centre when the springs are essentially the suspension pivots can get messy. I don't do dirt, so take my questions with a pound of salt!

What little I know seems to indicate that most of the action is at the back, and your description of the suspension seems to confirm that. The roll oversteer to get the car to point, then power-on understeer to transition into a set for the rest of the corner means that the rear changes position noticably when power is applied, right? Could it be moving too much and unsettling the tires, giving the poor drive off the corners? I'd be tempted to set everything closer to neutral in a test session, then move towards the recommended set-up and see if the car really needs that and if so, how much is helpful before going over the line. Does the car feel unsettled with on-off throttle adjustments mid-corner?

Having the reverse front spring split would seem to be an attempt to help traction off the corner by partially compensating for the torque reaction in the diff when throttle is applied; with a narrow rear track, that effect is amplified, so playing with the track might indicate how much that is affecting things. Again, I'd consider reducing the amount of things happening at the back end of the car, then adding them back in as you get a handle on what each adjustment does. Again, finding the range of front spring split for your car might require a test session. From what I understand not a lot of people scale their cars after baselining for the season; do you?

Have you talked to the chassis builder about how his design is supposed to work? Has the car been wrecked? Have you tried alternate set-ups?
mender,

All of your suppositions are correct as I understand the basic design layout of these cars.

With 63% to 65% rear weight the cars basically drive off the back with the throttle. I'm not necessarily convinced this is the best way but it is what is currently being produced by three different car builders. The right rear will move at a maximum of 1/2" forward on throttle depending of course on the right lower link spring rate and preload (more preload reduces movement. With that much movement all dependent on available traction and 'finesse' with the throttle, as you might guess, dropped throttle response in the middle of the corner is not good.

I agree with your assessment of the reasoning behind the, to me, rather large front spring split. I am moving in your direction, in that I would like to see a more balanced car overall, starting with mid-corner, then entry and finally exit. I would like to get all four tires working as much as possible and right now that is not the case.

All I need now is more time and money, time being the most important.

Thanks,
Ralph
rwstevens59
#123
Jun6-11, 01:22 PM
P: 16
Quote Quote by Ranger Mike View Post
I been thinking about this a lot...i still come back to roll understeer since it sounds like it is pushing all the time..entering mid and exit the turn. it takes a lot of work but i think you should measure the wheel base as the suspension goes through compression and retraction to see if the rear end is cocking toward inside of the track in the turn.
I been involved in getting many many used races from racers who wanted out of the game..besides the safety consideration,,,checking quality of the welds, wiring, fuel cell and plumbing...we always stripped down the , took out the springs and mapped the suspension travel in bump and rebound..checked the bump steer front and rear and checked ackermann when we were rebuilding the steering...you have to do this in event you have a real bad crash and have to replace parts..thats why i love the software program for performance trends..shows the linkage action and roll center changes dynamically...
ifin you don't do this your just putting a bandaid on the situation
Mike,

When I can get the car away from its owner and spend some 'alone' time with it all of what you have mentioned about mapping will get done. I'm fighting the usual battle of my slow engineering troubleshooting approach versus the 'hot' fix of the week from the 'rail birds'. I'm sure everyone on here knows the syndrome.

Ralph
mender
#124
Jun6-11, 02:29 PM
P: 563
Quote Quote by rwstevens59 View Post
The overriding complaint by the driver is drive off the corner.
When you say the drive off the corner (dry?), is the problem caused by power application or roll? Not planting the tires or becoming a handful? I'm assuming the car is loose coming off, and also that you've played with the anti-squat.

Dynamically, the car will handle the best when all four tires contribute but there can be times when getting that requires a set-up that has a very narrow window of drivability. What is your driver telling you he wants to be different about the car?

How do you like Warren's book? He was my jazz improv instructor at university; versatile guy! I hope he includes a personal section in his next book describing some of the head games he used to play as crew chief!
rwstevens59
#125
Jun6-11, 03:25 PM
P: 16
Quote Quote by mender View Post
When you say the drive off the corner (dry?), is the problem caused by power application or roll? Not planting the tires or becoming a handful? I'm assuming the car is loose coming off, and also that you've played with the anti-squat.

Dynamically, the car will handle the best when all four tires contribute but there can be times when getting that requires a set-up that has a very narrow window of drivability. What is your driver telling you he wants to be different about the car?

How do you like Warren's book? He was my jazz improv instructor at university; versatile guy! I hope he includes a personal section in his next book describing some of the head games he used to play as crew chief!
mender,

Warren is a really neat guy. We have exchanged a few emails (I have been encouraging him to keep writing, although I realize he went through quite some period of illness and is just now catching up). From a practical and educational standpoint his first book, to me, is head and shoulders above the Millikens, but being the first of it's kind the Milliken book is still considered the bible of engineering texts and of good historical value, I guess. Being a toolmaker I love the two sided nature of his work-OK here's the engineering theory and now lets go out in the shop and build a test rig to see if we really understand this. I don't know where the man found the time.

Well, there's the other part of the problem. I'm not absolutely confident in my driver feedback yet as I am also playing driver coach. From what he tells me and what I observe the car seems to turn in well, but he has a tendency to over use the throttle early and over rotate the car, so he is sort of playing 'pitch and catch' at the apex which makes him late on the exit and then he is experiencing wheel spin off at late exit. So...some is chassis and some or more is driver induced, at least in this old mechanics mind (blame the driver when all else fails :-)).

I have tried more and less anti-squat. The car looks better to me coming out of the hole with the higher anti-squat, but same driver complaint. I want to try taking gear out of the car to reduce the wheel spin and force him to drive smoother, albeit maybe slower, but have not been successful to date.

It's just not that simple! :-)

Ralph
rwstevens59
#126
Jun6-11, 03:37 PM
P: 16
mender,

Forgot to mention the 'dry' part. When a dirt track has high moisture content at the beginning of the night during hot laps and qualifying it is much more forgiving on the chassis setup, more driver than car. As the track drys and gets hard packed it can go one of two or more directions. One will be the dirt will start to 'take' rubber just like the groove on an asphalt track, sort of. The other is a condition where the track stays dry, does not take rubber and the dirt continues to abrade into powdery dust. The later condition is what we face most in the northeast in short 25 to 30 lap races. In longer events you can almost guarantee the dirt will take rubber. In the dry abrading condition the setup becomes more like asphalt, but think of running asphalt that is damp or oiled down. Available grip is very low.

Ralph


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