|Sep9-09, 04:41 AM||#35|
Race car suspension Class
Overall height of the spindle upright effects Roll Center (RC). shorter spindle height ( upper Ball Joint to Spindle shaft center line) with everything else the same, will LOWER the RC height. This is because the shorter spindle produces a longer instant center. the shorter instant center to the wheel it is drawn from , the higher the Roll Center.
you can change the RC by shortening the lower BJ center distance relative to the Spindle shaft center line as well.
you can lower the RC by raising ( relative to the track surface) the inboard upper A-arm mounting point.
you can lower the RC by lowering ( relative to the track surface) the inboard lower A-arm mounting point.
get performance trends software to take the headache out of the calculations..
I would look at three link rear suspension to cut my teeth on,,,,the four link is very tricky to learn...
|Sep9-09, 05:27 AM||#36|
Panhard bar should be mounted level at the desired RC height to begin with... don't know about that rule of thumb.
minimum stagger you need is:
D + .5 (TW) x CL \ D - .5 (TW)
where D is the track diameter in feet
TW = rear track width of the car in feet ( divide inch measurement by 12)
CL = the left rear tire circumference
example- race track has 200 ft. diameter
car has 64 inch track (64 / 12 = 5.33 inch)
left rear is 86 inch circumference.
run the numbers and you get 88.33 inch right rear circumference or MINIMUM 2.33 inch stagger.
if you run a spool add 10 percent to minimum stagger..if the track banking is 0 to 10 degrees, add another ten percent..if banking is 15 degrees up..take off ten percent from min. stagger
Four link has advantage of good forward bite because the upper links are mounted up hill. anytime the forward link is up hill the the rear end is trying to go up under the chassis during acceleration and this increases loading on the tires, these 4 links setups create oversteer as the body rolls. the right rear is pushed back and the left rear is pulled forward which helps the car turn going through the corner without breaking traction.
all I can tell you is ball park settings on the links..uppers should be 15 to 17 degrees up hill angle. the right upper max angle is 18 degrees and min. angle is 14 degrees.
the left upper usually has 3 to 5 degrees less angle than the right so the two links will be parallel through the corner. the lower links are usually 2 inches shorter. this reduces roll oversteer. the most common setting on the lowers is 5 degrees down.
|Sep9-09, 09:20 AM||#37|
Kingpin inclination angle is 10 degrees. Hmmmm.........wut if I was to build a RF spindle with only .........lets say 6 degrees ?
I say I would like to run around 2" of stagger, because the super late models here run that kind of stagger. I figured that if that was good enough for them then I should be able to run the same. Theres guys out here looking for 5 and 6" of stagger for the sprint cars.......They must be nutz.
Dont get me wrong.....my cars not a pail, and I run very well, But Im allways interested in learning and going faster. I appreciate all your inputs.
Hopefully next year Ill have a set of scales, so I can try like you said Mike...............Put the car on the scales and see what it does as you turn left.
I have 1 race left this year.......Ill try a panhard change. I will start with the rear bar level and will lower the front bar..........Ill make a axle bracket with +1 and 2" drop and try them both.
|Sep9-09, 09:25 AM||#38|
I am going to measure our track this weekend...............when u say diameter of track Mike..........do you mean at the goove?.............out side of track ? Add inside and out side then divide by 2 to get an average ?
Im interested in seeing what this formula says I should have for stagger
|Sep9-09, 09:57 AM||#39|
also we ran nitrogen to make sure the stagger grew minimum amount...
you are right in that 5 inch stagger is HUGE...go with the super late model boys as starting point..
keep the rear roll centers about 1 to 3 inch taller than the front or it will push..and i know you know this but make one small change at a time and document everything.
before messing wit hthe roll centers try adding more up hill angle to the top links like 2 degrees at a time. then when you are really hooked up and start pushing..look at RC changes...
|Sep9-09, 12:37 PM||#40|
Yes.......I use nitrogen in the tires as well as tire reliefs.
Now...........heres an argument Ive had with more then a few people about setting tire reliefs.
If you ask Hoosier they tell u to set them 5PSI higher then your starting pressure..this doesnt make sense to me.........if my RR tire says it wants 15PSI.....should I not set my releif to 15PSI to start with, even when the tires cold ? I know you build pressure as the tire gets hot, so isnt that why we run a relief ??
Typicalaly what I set my bleeders to is
LF 14PSI RF 15PSI
LR 7-10PSI RR 15PSI
I have found that buy varying LR tire pressure that the car is much more responsive the using the adjuster nut on the spring ( Im told that 1PSI change in tire pressure = 1/2 a percent change in cross )
As far as hooked up and pushing..........thats where Im at now.........front end feels real light coming off
|Sep9-09, 02:29 PM||#41|
tire relief valves ( pop offs) are tire valves you can pre set to pop offor open at a specific PSI. ( kenny i know you know this,,,i detail this for the uninitiated)
these come with springs set for a PSI range. I have Longacre pop offs and they have a set of springs for 3-15 PSI and springs for 15 and up PSI. they tell us to set the pop off PSI 2 pounds above desired range. Just like a tire valve stem that has a spring to close the valve..these have a spring to keep it closed. The springs are linear and pretty accurate. so you just have to live with the preload..Rebco, i think.. has a higher default rate and wider range (which means the pop off value is not as sensative ) so i would check with the manufacturer. it is all relavent anyway as you will pyro the tire at three places to see the temps and adjust anyway..
|Sep11-09, 01:10 PM||#42|
Mike..............where does rear panhard bar adjustment most effect chassis handling ??
Through the middle and coming off ??
Front panhard bar adjustment gettin in ??
|Sep12-09, 05:51 AM||#43|
The distance between Center of Gravity (CG) ad the Roll Center (RC) is important. The panhard bar is easily adjusted ( hence the RC). the shorter the distance, the softer the springs because the lever length is shorter. with this in mind let review the three phases of handling problems..note if a car is loose going in to the turn, there are ten adjustments you can do, with brake bias , RC height , stagger, cross weight and spring rate being easiest to change.
Loose going in ( corner entry) oversteer
too much rear brake bias, RC too high, rt. ft. spring too soft, right rear spring too stiff, ft. sway bar too soft, too much stagger, more cross weight ( add wedge) rt. rear tire over inflated, shocks need tuned, wheel tracking- rt. rear wheel moved outward too far relative to rt., front
Middle of the turn car is loose
more cross weight
lower rear RC
rt. ft spring too soft
rt. rear spring too stiff
rt. rear tire pressure to high
ft sway bar too soft
loose coming out of the turn
stagger is main thing to look at, if loose coming out, reduce stagger
increase cross weight
soften rt. rear spring or stiffen left rear spring
rt. rear tire over inflated
if you have under steer..( pushing..) do the opposite of above
so instead of changing the springs you can tune by the panhard bar..but..ultimately you want to run lower RC than the other guy and this will mean changing to different springs
|Aug25-10, 12:21 AM||#44|
Back again............Mike, Im curious about the front roll center location left to right in a pavement sprint car. Ive noticed that Beast and a few others have the stand off on the right front for the panhard mount quite close to the frame, where as my car has the stand off quite close to the coil over. Lets say for arqument sakes that my front panhard bar is 6" longer then on a Beast chassis. Assuming that the roll center falls in the middle of the front panhard , then my front roll center would be 3" farther to the right then the Beast chassis ( also assuming that they both mount to the axle in the same location relative to the LF ) ??? Now Bob East is no dummy and Im assuming hes done this for a reason. My friends tour car has a front roll center 3" under the left front tire ( Kelly Tanner's old chassis ) I have to assume this was done on purpose as well.
As allways I look forward to your response......Kenny
|Aug25-10, 05:34 AM||#45|
I had to read your prior posts ..you have 10 high Roll Center..and run pavement, not dirt track
i know the Emod guys run RC offset 4 inch bias to right for jacking effect..but these Emod guys use the General Motors Metric frame..A-arms coil springs off big GM cars as cost saving measure..( cost savings and Racing is like saying you get lots of sex after being married for 5 years...yeah right...)
straight axle rockets like you have use leaf springs / panhard bar set up on the front which are least expensive, lightest , simplest form of positive locating method.
the axle does not move purely vertical, it arcs around the chassis pivot with radius equal to the length of the P rod, so either the axle or the chassis has to shift sideways a bit whenever the axle moves vertically up or down over bumps...you got perfectly smooth track , right??
one draw back is the spring tries to fight the panhard bar during movement so to keep the lateral movement to a minimum the longest Panhard bar as possible is used.
finally the roll center is located where the center line of the P bar intersects with the center line of the car so it moves some what when either axle travels vertically or the car rolls.
i would stick with offset to the right..dirt guys run offset to the left..which is their hot setup..exactly where is your front RC offset from vehicle center line?
|Aug28-10, 06:41 PM||#46|
Mike......my front panhard bar length is 31" OAL. From the center of the axle to the panhard mount on the left side of the axle is 11.5"
|Aug30-10, 04:21 PM||#47|
Mike.....one more thing. Should I make a " bolt-on " front panhard bar bracket for the axle so I can tip it up and down to keep the bar level as I move it up or down on the frame ??
|Aug31-10, 05:02 AM||#48|
i am in dallas for sales meeting but will post when i get back..
|Aug31-10, 09:34 AM||#49|
*Bookmarks and links to FSAE Forums, if you don't mind me doing so?*
|Aug31-10, 05:10 PM||#50|
|Similar Threads for: Race car suspension Class|
|Car Suspension Explanation||General Engineering||6|
|How does a Physics class differ from an Engineering class?||Academic Guidance||38|
|In class final for a graduate EM class?||Academic Guidance||3|
|Car Suspension||Engineering Systems & Design||3|
|Spring suspension on car||Introductory Physics Homework||4|