Race car suspension Class

[Joshua Ryan]

No I’m not talking about the top link. But yes to the truck arm style. And I know about the spring loaded ones but this goes over the rear end and mounts to the back side. Mainly made by port city. Here is the link to the website. https://www.portcityracecars.com/Center-Pull-Rear-Suspension/ also a picture https://goo.gl/images/g5VFNy

 

[Ranger Mike]

Josh ,
Good catch on evolving technology. There are a few definite advantages for this J-Bar rear mount linkage on the asphalt rear suspension. I think 1962 Citroen 2cv pioneered this rear suspension with the rear mount concept to the point of actually mass producing it successfully. Note attached diagram for the Paved Track Stock Car Technology by Steve Smith. Every racer should have a copy of this.

1. It is easier to pull a chain then push it. This means pulling forces are easier to manage then trying to control the same forces pushing the same load. Looking at the typical 3 link suspension we have a single top link being puller as the car accelerates. The two trailing arms are pushing up on the cars suspension. The trailing arms must be beefier much the very nature of the forces they are dealing with. The mount location of the top link is critical as it must be mounted at the proper location proportional to the percent left side weight for maximum traction.
2. The J-Bar mounting handle force vectors twice as effective as you have two links transmitting the pulling force instead of just one top link. These two links offer a lot more adjustment and chassis tuning.
3. Braking may suffer but in round track racing, braking seldom wins races, acceleration and traction does. Road course racing usually means you have one maybe two places on a 12 turn track to PASS but many opportunity’s to out brake the other driver. I doubt braking will suffer with this J-Bar suspension.
4. It is indeed quite curious that the aluminum J-bar style mount has metal housing surrounding the J-bar. Speculation and rumors will abound I suspect. Could there be a compression spring device hidden? Could the wheel base actually be changed under acceleration because of this? The verbiage in the catalog says “This Style Allows You to Run The Trailing Arm Behind The Axle Tube For Less Roll Steer” and some cynical crew chief may take that to mean you don’t need STATIC roll steer with this device. But you can take out all the rear steer in the traditional 3 link set up. Hhmmmmmm?

 

[Ranger Mike]

Watts linkage
this is a good source of the pros of the WATTS linkage. I recommend this system for sedans turning left and right. the panhard bar is excellent for dedicated left turn only racing ..although this Watts linkage has many advantages over the P-bar and J-bar, if you can afford to buy it. I sure would use it just for the quicker set time and better control of rear roll center migration.
http://www.griggsracing.com/pdfFiles/Watts%20Links%20011414.pdf

 

[BEI77]

Hello Ranger Mike! You’re doing a great job on here. My question is about rear end set up. We run a 2800 lbs straight rail Pro Late Model on asphalt. A couple of years ago our rear end got slightly bent in a wreck. There’s a guy close that was the chassis specialist for Bobby Hamilton Sr when he won the truck series championship. He has shop now where he builds chassis’s and straightens rear ends.
We took ours to him and he suggested we let him put some toe in and camber in the rear end. He said under acceleration the toe would straighten out due to the force on the rear end. And upon deceleration the toe in would stabilize the car through the turn till you get back on the throttle. The toe was only a couple of degrees and it seemed to help. What are your thoughts on this.

 

[Ranger Mike]

welcome BEI
This is considered Advanced Race Car suspension class topic! good job!
read Post # 1326 page 67.also Herb Adams book “Chassis Engineering” has a great read on adding negative camber on page 81.

Wrote excellent advice. On left turn tracks negative camber on right rear tire and positive camber on left rear tire is desired. You will have rear end flex on the Ford 9 inch axel because the front face of the housing is weak, just by accelerating and braking. Adding a 1/16 inch toe in will help stabilize this condition. Toe out under acceleration will make the car oversteer or go loose. Make sure you do not have toe out when checking static rear toe in.

 

[BEI77]

welcome BEI
This is considered Advanced Race Car suspension class topic! good job!
read Post # 1326 page 67.also Herb Adams book “Chassis Engineering” has a great read on adding negative camber on page 81.

Wrote excellent advice. On left turn tracks negative camber on right rear tire and positive camber on left rear tire is desired. You will have rear end flex on the Ford 9 inch axel because the front face of the housing is weak, just by accelerating and braking. Adding a 1/16 inch toe in will help stabilize this condition. Toe out under acceleration will make the car oversteer or go loose. Make sure you do not have toe out when checking static rear toe in.

 

[BEI77]

I have a winters quick change, would it have the same flex you’re talking about? How would you suggest we square the rear end?

 

[Ranger Mike]

All aftermarket quick changes i saw were extra beefy just to prevent this flex. Most all can be equipped with camber snouts on each side. I am not sure about toe in being available on these camber snouts but i would call the factory to see ifin i were you. Personally, i would stay away from the toe in thing since a square rear end means one less variable to figure out.
once you get into dynamic variables like rear end flex, dynamic toe change, caster cross weight swing, roll steer and the like, you got a lot of variables to think thru once a handling problem occurs.
my opinion..

 

[BEI77]

All aftermarket quick changes i saw were extra beefy just to prevent this flex. Most all can be equipped with camber snouts on each side. I am not sure about toe in being available on these camber snouts but i would call the factory to see ifin i were you. Personally, i would stay away from the toe in thing since a square rear end means one less variable to figure out.
once you get into dynamic variables like rear end flex, dynamic toe change, caster cross weight swing, roll steer and the like, you got a lot of variables to think thru once a handling problem occurs.
my opinion..

Thank you for all the information and help you give.

 

[Latemodel94]

Wow! all I can say is this page forum is incredible. I have been using the tips and tricks here to apply to my new car, currently getting a brand new port city front clip. I run a 3000lb asphalt limited late model with a 108 inch wheelbase, stock or aftermarket fabricated camaro stub with stock lowers and stock type spindles. My question is a lot of teams have been reclipping cars and moving the front clip up. I haven't been able to find this addressed in the forum. would it be worth doing to my car? or since we are putting on a port city camaro stub would it be best just to leave it where it is on the chassis. I know this is being done but I haven't been able to find a what or why this is being done.
thanks
kyle

 

[Ranger Mike]

Welcome Latemodel

Read post 830 and 832 on page 42, post 1264 on page 64 and post 1314 on page 66

You need the steve smith book Street Stock Chassis Technology and a good computer software suspension program.

The short answer on why they move up the weld location from stock is to LOWER the whole car. The stock Camaro roll center is about ½ inch above the pavement. When you raise up the whole clip an inch you lower the roll center so now it is “ underground”. This is only step one. Steve Smith goes thru quite a detailed process of eventually getting the Roll Center to where it needs to be height wise and offset wise. Now Steve did not move the clip but made many ball joint changers and pivot arm re-locations to get there. For your case you will have to rely on t he computer program as you are making the moves. He did all the work to relocate the RC and make sure bump steer was correct and most important…the Camber Curve was as good as you can get. You are on hard spec tires and need this for maximum traction.

He ended up having a 3.375 inch RC height and did not disclose the offset but I would target 2.5 inch height for flat track to 12 degrees banking and 2.125” height for 13-18 degree tracks. Try for 3 inch offset to the right for proper down force on the right front tire.
Lower Roll centers mean less camber build. But means longer arm between RC and COG.

Do not forget the relationship between RC and Center of Gravity (COG).

Most door slammers run 18 inch CG Height and super late model cars with dry sump oil pans and aluminum heads get as low as 16” CG Height.
There is a method you can use to measure exact CG on your car with wheel scales and a floor jack. I use camshaft height as it seems to work pretty good.We have 18” COG and 3.375” RC so the fulcrum arm acting thru the RC is 14.6” long. Compared to the stock RC of ½ “ height, The 17.5” arm will cause a lot more action on the springs. No wonder these cars push going in!

Bottom line is to correct the camber curve as best you can with these hard spec tires and this will maintain better contact than the other guy.

 

[Wild Willie]

Ranger Mike and all other members of this very informative thread I would like to say thank you.

I just picked up a mini mod that I'm going to go threw from front to rear , top to bottom.
The car has gone threw 3 to 4 pit crews , so it will be interesting to see what has been done to the car.
It has a 3 link with j-bar located to the driver side.
Rack pin steering
Double A arm
I have cut out a few of the rules specs for review

I can get measurements in feet of the tracks corners and the one straight. Track is in the shape of a D
I also have located an excel spread sheet for your review
http://www.super7thheaven.co.uk/wp-content/uploads/2014/09/SuspensionCalculator.xls
This is my winter project . I'm in the process of stripping the car down.

 

[Ranger Mike]

welcome wwillie
you have a good start to a winning ride

 

[Wild Willie]

Hello RangerMike and members.
Have a question to a remark R.Mike made earlier about running the A-arms in parallel .
I'm a little lost as to setup for a top side run of the track
Running the top grove of the track.
With all this information of angles and offset centres
How does One go about setting up for the high side racing.
The tracks corners are over 15 degrees .
Corners 1 to 2 have a radius turn of close to 260 feet in lenght
Corners 3 to 4 are over 400 feet.
The off set curve length 460 feet
The straight away 440 feet
This a D type configured dirt track.
I get my gearing by looking at top runners lap times .
MPH = (3600 X length of track ) / lap times
Final Ratio = ( Avg. RPM X Tire circumference ) / ( Avg. MPH X 1056 )
I have a copy of Steve Smiths , street stock chassis technology, very good reading.

Another question ,how does off setting the rear coils effect turn in , if RR coil is mounted atop the axle , LR mounted 4 inches out front of the axle and down one inch.

 

[Wild Willie]

Here's how they have the rear coils setup

 

[Ranger Mike]

look at post # 1382 at top of this page. Take time to look at the force vectors acting on the two spring mounts you have now. I suspect these were fabricated to try to get advantage over the standard stock set up. Or could be case of monkey see monkey do. Is the center of spring to the center of rear end center the same on both springs? You may be dealing with two different motion rates because of the different locations. How much is it worth to you to make them both the same? Or is it worth it?

In my opinion, you can not set the car up to run the top until you have it handling going in, at mid turn and on exit. After you have this then you can adjust to run the high side.

 

[Wild Willie]

a few questions. to help me understand a 3 link
1: the further out front the instance RC , good or bad
2: for less roll instance RC closer to CG , more roll instance RC lower to ground
3: anti squat lower percentage better or higher percentage

Example
Wheel base 103 inches
Geometry
Roll center of 9.93 inches
Instance RC 9.5 inches
Instance RC leverage arm 227 inches
Anti squat 24.7 percent
Roll axes 0.57

In that example am I correct in thinking ,I have a long lever arm with very little body roll ,
I did the calculations with a longer upper link of 48 inches with a 3 inch offset front to back , back Height set at 25 inches front 22 inches
Top 3 link off set 5 inches forward of the rear axle and connects 25 inches off ground.
Lower link offset forward 1.5 inches and 4 inches above ground front of lower link to frame 4.5 inches off ground , 22 inches in lenght

 

[Wild Willie]

Mike I got informed by one of the builders that rear coil setup was, to put the weight back on the LR faster

 

[Ranger Mike]

WWillie

The longer the rear suspension Instant Center lever is, the less geometry error introduced due to the arc swing. Longer means less arc movement and more stable cornering.Anti-squat is the linkage taking up rotational movement thru the linkage and adding traction.

No anti squat means the springs will take any front to rear “ weight transfer” until compressed and we go to the drag racing funny car status of no anti squat and wheelie stand.

Read post # 314 on page 16 and post # 327 on page 17 before proceeding. Anti-squat is expressed in percentage. 50% anti squat means 50% of the rearward Pitch or “ weight transfer” is handled by the 3-link set up you have. 100% means 100% of the pitching or weight transfer is handled by the linkage and the body is actually lifted (wheelie time). Percentage is figured as the distance of Center of Gravity (COG) is compared to the Instant Centers location. Do not that the other point of origin of the IC is the tire contact patch. The COG perpendicular bisector to the pavement is crossed by the IC vector and gives the % anti squat. In the case of Duke Southard diagram from “Short Track Chassis Set-up” we see 20% on the 3 link figure #2.

I always used the camshaft center line as a good approximation of COG and worked for me. Production cars usually have COG between 20 and 22 inch above ground level. Round track cars will have it 18 to 20”. Street stock metric cars that have been worked over and lowered have 16.5” COG and super late model dry sump outlaw cars can be as low as 15” COG.
On our Formula car we ran low % anti squat mainly because the independent rear end was not the solid axle that you have, but has independent springs and swing arms on each side.

The rear pitch would load the individual springs unequally and the tire contact patch could shrink due to camber change. You do not have a lot of options on long links with these cars so 15% is huge.

Add to this the very low COG these cars have, and you are limited to % anti squat.

 

[Wild Willie]

Thank you ranger Mike
I'm finding with 3 link there is many more ajustments to the rear suspension
Moving with in lower forward frame connection points I can a just roll angle , by going side to side , by using washers to + - or zero
Instead of moving arms up or down
I now have the sim calculating an anti squat of 6. 3 % zero roll angle , think that will be a good starting point
Very interest how much a little degree of movement cause changes across the rear suspension in just one area
Looks like I will be building some lock down sliders with some square tubing . Like my j-bar has
I also found by moving my connection points on the rear of housing and frame up helps with ajustments for instance center Z axes
That was hard to get with my last post always giving me high anti squat %
Tuning in this anti squat for entering and exit

 

[Ranger Mike]

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[Wild Willie]

Something that's rolling around in my head
if you calculate and find the RC of both front and rear

How much split should there be between the 2 in height ?
How does this effect cornering ?

 

[Ranger Mike]

read post # 229 on page 12
mass centroid axis is shown on illustration 13

 

[Robby Johns]

Great read RangerMike!

 

[Surdge]

Hey guys I'm from south Africa the 3 link is pretty new and those who have it doesn't Wana help me I'm building a dirt car with 3 link and need help with the length of the bottom trailing arms and the angles thanx

 

[Ranger Mike]

welcome Robby and Surdge
3rd link stuff on post 81,253,261 and 707 but check out index for rear end stuff too

 

[Surdge]

Thanks u have been a lot of help just one more thing that I have that I can't seem to find is what angle do u use for the j bar

 

[Ranger Mike]

see index on page 1 ..i wrote a lot on Jbar

 

[Surdge]

Morning iv got a another question ur shock bracket witch angle do u mount it on

 

[Ranger Mike]

READ the posts ...do the research ,