Automotive Race car suspension Class

[alpharomeofifetwo]

LF static compression 1” + 3.25” dynamic =

I think I’ve discovered a major issue. The LF has a 2.25” drop roll bar upper shock mount so it’s pushing the whole shock down and The shock is bottoming out. Here is what it has currently and I’m going to change it to a roll bar swivel style (yes, I know the phot is of a different roll bar diameter lol)

 

[Ranger Mike]

I looked up the set up for a street stock car on dirt for a 3200# car, 400 # more than your car
I had setup for dirt car 2800#
Lt Ft 1000 Rt ft 1050 Lft rear 250 rt rear 225
so i still think you are too soft on frt springs but we have to get the camber, frame rub and shock mount right before we can get good tire temps.
my opinion
rm

 

[alpharomeofifetwo]

so i still think you are too soft on frt springs but we have to get

Been doing my due diligence on this refrigerator with wheels the past two days, love some good math problems.

Here’s what I’m guesstimating:
Load transfers
Front loads (measured with a load stick: https://pitstopusa.com/products/weh...MIzOqMy_-AhwMVQVJ_AB1hzAXBEAQYAiABEgLZkPD_BwE) at dynamic travel are about 2000lbs on the RF, LF is about 1800lbs.
RR load is getting to 1075lbs
LR load is going to 700lbs

We run tubular after market and front uppers and lowers so I’m 110% not even coming close to frame clearance issues or bottoming out.
As far as spring package goes, my understanding is that all I’m doing is changing how fast I load and unload a corner.
Stiffer spring = Quicker loading, quicker unloading in relation to the tires
Shocks control how fast the springs load/unload in dynamic travel

What I’ve deduced is that my issue is one thing only(with the exception of the few known for certain issues aforementioned): load transfer
Issue #1 Load Transfer via ballast placement
With me having roughly 160lbs of ballast mounted high and centered over the rear end, I’m transferring too much load laterally to the RF and RR, effectively unloading the LR.
Possible solution- Move ballast lower and to the left. Reduce CGH and load transfer

Issue #2 Load Transfer via Shocks
This is a little tougher to track down for to an exact fix but we’re getting closer to fine tuning.
The LR could have have too stiff of a compression not letting the LR get the weight transfer under acceleration.

The RR could have too soft of a compression letting excess load transfer to there under acceleration.

Less likely but still possible variables in this:
The LF could have too stiff of a rebound, transferring load to the RR

The RF could have too soft of a rebound, not letting the LR load under acceleration

 

[Ranger Mike]

what is motion rate of the spring? Now this makes sense.
i assumed you were running stock A-arms. are you running slider attached at different angles and mount point from stock? look , if you want my help...tell me your setup!

 

[alpharomeofifetwo]

what is motion rate of the spring? huge!!
i assumed you were running stock A-arms. are you running slider attached at different angles and mount point from stock?

I’ll have to get you that information later tonight or next weekend. I’ll measure middle of inner pivot point -> center of spring then inner pivot to outer ball joint. MR= D1 / D2

I figured that’s what you were getting at because I realized where those spring rates you recommended were coming from. Keeping the car up off the frame.

 

[Ranger Mike]

If you have GM metric A-Arm and have 700# spring in stock location
not enuff, too nose heavy , loose off

 

[Savem c10]

Looking for decent base line set up for a straight rail asphalt late model racing on a 1/3 mile flat track 8/12 degrees of banking , 2800 lbs 57% left side 101.5 wheel base on 8 inch slicks with coilovers per rules . This car has jacking force front end geometry I would like try it this way . Also arb is 1.5 spline bar I don’t want to change that . So I’m looking for spring rates all corners wheel weights / rear trailing arm angles. Thanks .

 

[Ranger Mike]

Savem,
you have been on this forum too long , have learned much , and are a racer with advanced knowledge.
You know me and my fanatic requirement about roll centers!

1. where is the front roll center located, height and offset.
2. Where is the 3rd link located relative to the track width

8 inch slicks tell me you are in spec tire and car class, true or not?
1.5 spline bar ??? wtf????????
this don't mean sh** to a tree??? clarify! asking me for base line spring rates when i do not have any clue about ARB spring rate is .. well Savem, come on man?

did you read post 1721 above?

Savem, Yes, I am harsh but you have too much potential!

 

[alpharomeofifetwo]

If you have GM metric A-Arm and have 700# spring in stock location
not enuff, too nose heavy , loose off

I’m slowly coming to terms with this idea.

I did get to check the MR on every corner but the RF
LF 2.5” shock travel to 1.5” spring travel(5:3)
LR and RR and pretty close to a 1:1 ratio
RF Guesstimate based on load stick #’s would be a 2” spring travel to a 4” shock travel 1:2

No spring sliders. Springs sit on a cup mounted atop the center of the axle tubes, vertically 90°
The rear shock have a screw stud mount aft of the rear end tubes that comes down from above at pretty close to 90° and mount on the back side of the of the lower trailing arm bracket.

Keep in mind I had two issues in the suspension:
LF bottomed out shock
LR shock mount bolt broke

Shocks:
The set up I’m running right now is a tie down RF it goes down and stays down… unless you wait 3-5 business days for it to come back up.
The LR is very hard to compress and wants to stay up. Pinning the RF down
LF is an easy down, but has a good amount of Rebound
RR has a is a medium comp/Reb
I have the shock data if you have Roehrig software

We have a test n tune next Friday (12th) and race on Saturday. I’m going to bring all my springs and make changes to ballast, wheel offset etc to see what we can do

 

[Ranger Mike]

alpha
think about it. if you have the front roll center right, putting down force on the rt ft tire, going IN to the turn, why keep it mid turn and coming out? tie down shocks are a Band-Aid because the roll center is not correct.
you tie all the weight down to the right front tire until h222 freezes over thru mid turn and corner exit. Mean while the lft rear shock is saying, no no no, do not come back Mr. Weight. Tie down shock keeps weight on rt ft forever . You already are mid turn and need the weight to hook up on EXIT. Does this not sound like nose heavy? Now on corner exit you have no traction? wonder why?

 

[Savem c10]

So there two types of roll center geometry that I know of. Most common is kinetic roll center does work. But over the last year I’ve been working with jacking force geometry on and off and found it works better than a kinetic based roll center geometry. Does anyone on here know of racers using jacking force geometry and have knowledge on or know of any literature to read on it . Rm… Mender ??

 

[Ranger Mike]

By kinetic i take it you are using the automation mode on software like Performance Trends that show Roll center migration on roll and dive, right? Is the jacking force function on some other software?

 

[Savem c10]

By kinetic i take it you are using the automation mode on software like Performance Trends that show Roll center migration on roll and dive, right? Is the jacking force function on some other software?

I don’t have any computer software as of yet iv always mapped roll centers out life size on paper. I have no way of knowing migration . Based on recommended starting points for roll centers is what I’ve done. I do not know if there is a function for that type of geometry u would know better as u have the software.

 

[alpharomeofifetwo]

Tie down shock keeps weight on rt ft forever .

Finally got to run Saturday night.

Through processes of elimination, I firmly believe a shock change is due.

Shop work
Double checked all chassis mounts/measurements to make sure everything was square. Ran rear roll steer numbers and got about a 1/4” max trail in dynamic on RR
0 lead/trail on LR. Rescaled car. Stiffened up springs

Track/chassis adjustments
We stiffened up the front 200lbs each spring, stiffened LR to a 20” 125#, softened up RR to a 200, moved LR in 1”, moved RR out 1” via offsets, moved lead lower 8-10”, tightened chain on LR RR and LF, cranked cross / bite in it. Pulled a 50lbs lead chunk off the right side low. No matter the change, we still got the same results.

Race day notes
Bumped those tire air pressures up to 17-22 psi, Couldn’t get an accurate temp readout due to the infield being very wet from rain during the week. But we burnt the rr up and every other tire looked exactly the same as it went on the track, No feathering on any tire except RR. I think we had it set to 22psi and noticed it did wear significantly more across the center treads so might’ve been a tad much on air. Started the race with 1/4” of reverse stagger just to try a last ditch effort

Car had “the usual” handling; great getting in, lacking drive mid off… even with all the “positive” changes. I’m chalking this whole issue up to the shock package and shipping them off today to be redone like I mentioned in a prior post .

 

[Ranger Mike]

Right rear is worn out because it is not hooked up. Still loose off. Why? front springs too soft plus you have no arb. Good news is the car is going In ok. No traction corner off.

 

[Ranger Mike]

https://www.msn.com/en-us/sports/ot...1&cvid=523a751deaa848d0bc475e3efe569885&ei=17

one fine racer, mill be missed

 

[alpharomeofifetwo]

I’m chalking this whole issue up to the shock package and shipping them off today to be redone like I mentioned in a prior post .

Reference: https://www.physicsforums.com/threads/race-car-suspension-class.326355/page-51

Sent the shocks off to be redone and the shock expert said exactly what we were thinking. It was holding too much load for too long on the RF and not getting any back to the LR.

They made a huge difference on the track this past Saturday! Without changing much other than shocks, it handled really decent compared to before,I’m just fighting a loose entry/ RR tire hot problem still. It was loose to the point I had to run the very top high side where the banking was the steepest to keep from losing grip on the entry to mid part of the corner/ braking going straight before turning. I believe ballast placement is the next area that needs to be addressed. I have roughly 200lbs directly above the rear end center section just below the decking.

Tire temps were equal across the tread so I’ll just abbreviate the avg per wheel:
LF 110 RF 127
LR 125 RR 175

 

[Ranger Mike]

LF 110 RF 127
LR 125 RR 175


RF to lft rear temp should be with in 6 to 10°, is is 2°, not enough cross weight - LOOSE


RR to Rt Front ave temp , the rt rear should be 10° cooler, it is 48° hotter ..LOOSE


rt rear not getting enough traction, it is spinning and heating up. It needs more weight so ft springs are too soft.

 

[alpharomeofifetwo]

So how would I differentiate between the RR getting too much vs too little dynamic load? I was under the impression I was transferring too much load laterally and sliding the tire. The feathers on rr are toward the centerline of the chassis (laterally, not longitudinally)

 

[Ranger Mike]

Your tire is shredding due to lateral momentum. The tire can not provide grip due to not enough downforce on it when you get on the gas at corner exit. Read post 1519 on page 44.


Why not enough down force. The car body has finished rolling over to provide down force and traction for the right front tire on corner exit. You still do not have enough weight on the right rear tire.

Your springs reason - too weak on rt front spring rate , too stiff on rt rear spring rate. .

 

[alpharomeofifetwo]

Your tire is shredding due to lateral momentum. The tire can not provide grip due to not enough downforce on it when you get on the gas at corner exit. Read post 1519 on page 44.


Why not enough down force. The car body has finished rolling over to provide down force and traction for the right front tire on corner exit. You still do not have enough weight on the right rear tire.

Your springs reason - too weak on rt front spring rate , too stiff on rt rear spring rate. .

 

[alpharomeofifetwo]

Yeah I had a ton of RF travel (outboard mounted shocks, see motion ratio from earlier post) RF 6” LF 3”

Going to stiffen RF from 650 to 800, LF 700 to 900, RR from 200 to 175

 

[Ranger Mike]

a great start. Please only do one side at a time. Stay with right side changes and try it, take tire temps. add cross weight at the track. run it. get 6 to 10 degree Average from lF rear to right front. car is balanced.
then worry about lft frt tire. see if you improve drive off better with this setup , then we can work on super fine tuning. I suspect you will be carrying the lft ft tire like the old sprint cars and lft ft will have less temp. This is good. next we can make lft ft tire do more work. but we have to make calculated, slow changes one at a time. I like the 25# lesson rt rear idea. not major change but gradual. good job!

 

[alpharomeofifetwo]

you should see what I’m looking at on my screen… I purchased the Performance trend software and the front roll center is ugly. 2.5” high, 12” right (this is what it was from the previous race)

 

[Ranger Mike]

Front roll center offset too far to right will lift up the left side of the car. Has too much leverage. This will load the right rear tire heavily under acceleration. This will heat up the right rear tire and make you loose traction to the point that you totally loose all traction on that tire. From earlier conversations I thought you had the roll center pretty on track? Do not make major changes until you fox the frt roll center offset to 3 to 3.5 inch to right. You can change the rear spring to go a little softer but we do not know is causing rt rear tire temp to be so big? Could be roll center or springs. In any event you have to fix the ft roll center first. Then spring swap is easy.

 

[alpharomeofifetwo]

I did have it set pretty well but we’ve rescaled it and lowered the car 1/4” since. Also took my measurements without someone sitting in the car so I’m sure it’s off a tad. I’m going to take another look at in today and rerun the numbers.

 

[Ranger Mike]

Why RC location can be too offset
Let us look at a Late model asphalt Car weighing in at 2800 lbs. with 66" track width. If we calculate the unsprung weight total we have about 500 lbs. leaving 2300 sprung weight.


If we place the front Roll Center (RC) mid point we have it located 33" from the center-line of the left front tire. If we place the 2300 lbs sprung weight on the chassis we will see that half of this weight will rotate thru the RC to plant down force on the right front tire during cornering. The other half will push sideways with lateral momentum. In simple terms we have 1150 lbs. (half of 2300) acting thru a 33" (2.75 ft.) lever (2.75 x 1150 lbs. = 4,125 foot lbs. of torque.

A pound-foot (lb⋅ft), abbreviated from pound-force foot (lbf · ft), is a unit of torque representing one pound of force acting at a perpendicular distance of one foot from a pivot point. Conversely one foot pound-force (ft · lbf) is the moment about an axis that applies one pound-force at a radius of one foot.

​

If we move the RC to the right by 3", we now have 1150 lbs. times 36" (3 ft.) = 3,450-foot lbs. torque because we now have 54% of the unsprung weight rotating thru the RC to plant down force on the right front tire.

Now we find the front RC is 12" offset to the right! Our RC is located 45" (3.75ft) from the center-line of the left front tire. We have a whopping 68% (45" divided by 66" track width) of the unsprung weight rotating thru the RC to plant down force on the right front tire.

3.75 x 1150 = 4,312 foot lbs. torque. This is toque will lift the left front of the car and add down force to the right rear tire. The rt rear tire will quickly overheat and loose grip or traction. Car will be real fast for a few laps then drop off the cliff!

But wait there's more! This assumes we have 50-50% left to right side weight. What if we have 58% left side weight? These figures will be much greater.

The point of this post is to show you the importance of proper RC location as step one of any successful chassis setup.

Unsprung weight table from "Short track chassis set-up" by Duke Southard
other pic from Steve Smith- Paved Track Stock Car Racing Technology -

 

[alpharomeofifetwo]

I’m completely bamboozled at this point then. I reset RC to be dead center, up 3”.

Raced tonight and still had excessive roll and lateral load transfer. Very loose mid/off. (Turned 4th quickest and finished p4)
LF OAT+5° RF 130°
LR 130° RR 180°
Feathering laterally across tires (RR Extreme)

Even changed up my rear set up to the following after qualifying
LR 100lbs 20” RR 225 w red Longacre rubber
Static load LR 600lbs RR 225+rubber
LR 83” tire on 3” offset
RR 86” tire on 4” offset (with 1/4 spacer so 3.75”)

 

[Ranger Mike]

written 21 August 2024 above
LF 110 RF 127
LR 125 RR 175


RF to lft rear temp should be with in 6 to 10°, is is 2°, not enough cross weight - LOOSE


RR to Rt Front ave temp , the rt rear should be 10° cooler, it is 48° hotter ..LOOSE


rt rear not getting enough traction, it is spinning and heating up. It needs more weight so ft springs are too soft.


ft Springs are too soft and you need RC offset to right 3" to put down force on Rt ft tire. and you stiffened the rt rear spring!! ??????

 

[alpharomeofifetwo]

written 21 August 2024 above
LF 110 RF 127
LR 125 RR 175


RF to lft rear temp should be with in 6 to 10°, is is 2°, not enough cross weight - LOOSE


RR to Rt Front ave temp , the rt rear should be 10° cooler, it is 48° hotter ..LOOSE


rt rear not getting enough traction, it is spinning and heating up. It needs more weight so ft springs are too soft.


ft Springs are too soft and you need RC offset to right 3" to put down force on Rt ft tire. and you stiffened the rt rear spring!! ??????

We went up on the front springs
900 LF / 800 RF

Had a test session Friday night and tried a 175lbs spring on the rr and had the same results tire temp wise. I was getting 5-6” of dynamic spring compression on the RR. LR was static loaded to 600lbs and getting 700-800lbs total of dynamic load.

I just wonder if my lead is mounted too high and rolling over. Using the “crank + 5in” for CG height, I’ve got all my lead mounted above this by 6-7” (200lbs)