Automotive Race car suspension Class

[maverick 5582]

Here is what the left A arm looks like.

 

[maverick 5582]

I assume this is what you meant by "reducing the roll center offset".

 

[Ranger Mike]

height is great, offset needs to be about 3 inch to the right to put down force on right front tire so car can turn in without skidding tire. You are close to getting it spot on! where is the RC in 2 inch of roll?

 

[maverick 5582]

The roll center is at 3" to the right now. With 2" of roll it goes to 7" to the right. I tried using the "Optimize" button there but it always messes up the roll center. When I change the angle of the a arms trying to get the camber gain correct it messes up the roll center.

 

[Ranger Mike]

This is the most important thing you can do to get to a winning car. You may have to spend HOURS on the software. Try longer A-arm length, try milling the bottom of the spindles, different ball joint heights ( taller and shorter) maybe shim the bottom A-arms to move them out...it took me a week at nights messing with t he software to get RC with minimum movement and good camber build. it was worth it. keep copious notes so you don't start repeating your mistakes.

 

[maverick 5582]

How do you simulate a spindle change like an 8 3/4" tall spindle with a 2.5" drop?

 

[Ranger Mike]

you have to enter new data for upper and lower BJ and note any change to tie rod points. most Emods use the pinto spindle vs chevelle spindle. see post 960 page 48

 

[jsmith1984]

Hi all, first post here. I crew chief an east coast LMSC that we have been having issues with on and off for many years. I have been working with this crew for years with some good days, and a lot of bad days. I have been reading though this forum since last year, and found a ton of great information. I need some help learning more. Up to this point, we run .2 - .5 seconds behind leaders at the beginning of the races, and usually just as fast, if not faster at the end.

I don't have the suspension program right in front of me, but I think the front roll center is located 4 inches to the left, and 2.5 inches high. Now I could be wrong, and I will find out for sure tonight. We are running a BBSS setup, with coilovers, and a bumpstop setup. Truck arm rear, quick change, coil over springs. Total weight 3031 lbs, 1713 left, 1318 right, 50.1% cross.

We have been finding setup problems for quite a while, stuff other people had done in the past, that we have to fix. Certain things were passed down knowledge, but incorrect. Such as Stagger: Been told for years we don't need more than 3/4 - 1 inch of stagger. Well, I ran the calculation, and we needed a little over 2 inches. Found that out before the last race we ran lol. Car went from very tight all the way around, to very loose middle to exit.

We have a couple weeks off, so I need to start from scratch, and figure out what is going on. One of the big things I notice is the tire temp averages. The left side is consistently 40 to 60 degrees cooler than the right side tires. I feel if we can get the temps closer, it will run better. I had a quick setup that had equal temps all around the car, but unfortunately car was totaled before we could do anything with it. I have tried duplicating this setup on our new car, but haven't had any luck.

Thanks for any information you all can share!

 

[Ranger Mike]

Welcome ..good to know the posts are of benefit..

Without knowing the whole set up my first guess is the car is set up with big bar soft springs to use AERO down force to plant the right front tire. From what the tire temps say, it is doing it so the front Roll center is offset to the left to keep from adding more down force.
But when the left front tire is severely cooler than right front means it is not carrying enough load. Typical three legged milk stool. With 60 degrees difference the first thing to look at is excess cross weight (wedge) problem. But you are only at 50% and should be 55 to 57% ... what gives??
The Ideal should be 10 to 20 degrees cooler. Sounds like you are picking up the tire and carrying it thru the turns like the old sprint cars did when running a straight axle. There used to be 6 inch daylight under the left front tire! I suspect you are running on the bump stops and this is NOT the hot set up.

Stagger is usually a phase three problem (Turn Exit).
Not a lot of magic here..I would run what everyone else is running and get tire temps sorted out first.
and start using the cars suspension like it is supposed to be used and don't turn it into a go kart with those bump stops. my opinion but its free and you know what free opinions are worth..

 

[jsmith1984]

Yes, my opinion of the bumpstops is still in the air. A lot of the guys we run with started the season on bumps, took them off for the second race, and put them right back on for the third race. The drivers I have talked to have a more positive feel through the corners. I don't like the thought of the "gokart" scenario, but I digress. We have to run an external bumpstop linkage, because of the rules concerning bumpstops and coilovers. We are running bump springs, a 1400 lb on the left, and an 1800 lb on the right. The left spring compresses about 3/4 inch on corner entry, and the right compresses about 1/2 inch. They are not coil binding.

I went down to the shop last night, and checked a few more items. The rear axle was pushed over to the left almost 1/2 in., measuring by the contact patches. The axle was setback 3/16 on the left as well. We lined up the contact patches, and put the axle back straight. I'll be refining front roll center tomorrow. I looked at a previous roll center calculation (from the last time we measured) and it started at 3 inches to the left, and moved to 20.3 inches to left! I have no idea how we thought that was acceptable, but we did. I am going to try to make front RC between 2-3 inches to right ( If that sounds ok).

I will grab my notebook and list some specs in the next post.

Thanks for the help so far!

 

[Ranger Mike]

fr roll center three inches to the right is good start..

 

[jsmith1984]

Ok, I'm going to nail that down tomorrow. As promised, here are some other specs.

LF RF
LR RR

Qualifying Run Tire Temps/Fastest run with this car, still .3 seconds slower than leaders
159 147 142 190 196 195
158 156 159 195 199 205

Corner weights while resting on bumpstops, no coilover springs attached
870 618
834 681

Practice from the last race, temps with conventional coilovers, no bumpstops installed
194 189 189 226 228 218
184 188 189 234 232 228

Post-race temps, 100 laps, roughly 55 degrees F ambient temperature, on bumpstops, and changed RR spring from 275 lb to 250 lb
159 162 165 188 188 183
144 150 152 189 186 185

Current spring rates/ with bumpstop setup
175 150
225 250

Conventional Setup spring rates (No Bumpstops) Draco front springs
250 275
225 250

 

[Ranger Mike]

Since i do not know motion rate of the suspension, assuming it is after market control arms, you are a little light on the spring rates.
A base line (at least on paved 1/3 rd mile medium bank track around here) 3000# car
LF - 350 RF - 350
LR - 225 RR - 225
with 220# ARB (Sway Bar)
front end total spring rate is 350 + 350 + 220= 920#

You are running 250# and 275# ft springs so to
to get the same total spring rate the ARB should = 395#
if this is not the case you are probably pushing going in and loose off

I can not tell a thing about the set up until you get it off the bump stops and get fr RC correct.
The whole point of BBSS is to drop the car body super low to the pavement to block out any air underneath the car and plant the entire car body down to increase traction. This is done with lighter than " normal" spring rates and a bigger ARB to handle the load in the corner. It takes a whole lot of work to get this right AND you have to back off the front Roll center offset so you are not overloading the right front tire with too much AERO down force.

 

[Martin1]

Hello Mike,
I was doing some online research and came across this thread. Very interesting reading. I am just in the middle of rebuilding the front end of my mini stock and debating on a staggered wheelbase. I run an open diff, and find that wheel stagger does not really help on our track, which is a paved medium bank 1/4 mile oval. It is mostly banked turn with short straights. But I am repairing some previous damage and discovered that my car is longer on the right side than the left. My car was very fast last year and I am wondering if the wheelbase stagger actually helped that. But is it the same effect as wheel stagger? In which case I would be better of squaring everything up again because setup is easier. I also run a fair bit of castor split as well. 4 and 8 degrees last time I set it up.

Thanks

 

[Ranger Mike]

welcome Martin..I ran mini stock a couple of years...
When the left front is placed further back than the right front, it is called putting lead into the chassis. Putting lead into the setup takes weight off the left rear and puts it on the left front;
this will loosen the car going in and somewhat coming off. Generally, the car can be made to handle correctly without resorting to adding lead to the car. Leading the right front tire ahead of the left front will help the car turn into the corner better, especially on the gas. The drawback seems to be that it will hold the car on the right front longer and hurt traction off the corner. A mini stock is low power and it s all about maintaining momentum thru the turn. Cranking in lead on the right front ended up being one change to the car that driver can really feel. Its usability really depends on the type of track you run on and what your driver wants in the car.
When racing a totally tune able suspension, square it all up. When you have to run a super restricted class, adjust the heck out of the set up and keep good notes..

 

[jsmith1984]

Ok, I'm down at the shop now, Just finished all the roll center measurements. I am using Circle Track Analyzer from Performance Trends to find roll center. I made two sets of measurements, because I am not sure which way the program is designed to handle the centerline. The first measurements, I used the centerline of the front tire contact patches as the centerline measurement, and left the track width measurement centered, with each contact patch being 32 inches from the centerline.
The second set of measurements were made with the centerline being the actual centerline of the chassis, which is 17 inches to the center of each front frame rail, and the track width measurement offset 33 inches to center of right contact patch, and 31 inches to center of left contact patch.

I will be upfront and say I have no idea which one to use. I assume the use the second method, and the program calculates the actual centerline. Anyway, on to the measurements... These are all baseline figures, haven't moved anything yet. My previous RC numbers were way off, I guess someone was playing with the program and punched some odd number in.

Method 1: Static roll center Left 1.8", 3.2" high
Dynamic RC with 2.3" dive, 1 degree roll: Right 9.5", 1.5" High

Method 2: Static roll center left 4.6", 3.3" high
Dynamic RC with 2.3" Dive, 1 degree roll: Right .1", 1.6" high


I am going to make some changes to fix the roll center now. I checked the ARB stiffness using the programs calculations, it is an 1 5/8 diameter, with 1100 lb per inch rating. Motion ratio of front shocks are .655 right, and .796 left.

Were your recommended spring rates for coilovers, or big springs? I am building a complete set of shocks, with your recommended rates, to try in practice next week.

Thanks so much for your help!

 

[Martin1]

Thanks for the info Mike. Our class is pretty wide open and it sounds like I can achieve the same result with corner weighting? I think I will square the car back up and go from there.

 

[Ranger Mike]

Martin, i would square it and run it..makes things a lot simpler. good idea!
Jsmith, i always referenced the center line of the car. When you go off the contact patches, you run into potential screw ups if you change offset wheels. The ball park spring rates are for coil overs, stocks springs are a lot stiffer. Shoot for 1.5 to 2.5 RC inch height ( lower is better) and 3 inch offset tot the right. Try to keep the RC migration to under and inch. I know it is a lot of work but you will find it well worth it. You may have to shave 1/4 inch of the bottom of the spindle, raise the upper BJ or re-locate the A-Arm mounts but it can be done. This is the one thing you can do to correct the handling and properly down load the right front to turn the car.

 

[jsmith1984]

Thanks! I spent all day Friday measuring and punching in numbers, I think I got it nailed down. I don't have my numbers close by, but I got the RC height to 1.4, and right 3 inches. I still have some work to do, so far as RC migration, but time constraints have shot that down for this week. We have a race this weekend, and going to try to practice Friday.

I still need to build another set of shocks for the heavier springs, and we will try that in practice also.

Thank you for the help! I will update as work progresses!

 

[Ranger Mike]

Racing the Front Wheel Drive (FWD) stock car

I have kept away from commenting on this class of racing because I have not competed in it and have not had time to talk to those who do. I bumped into my old mini stock driver who recently ran in this class and put together some notes.I am by no means an expert on this subject so its value is dubious.

On the plus side, this is an entry level class that can teach any racer the important lessons to learn on competing at a weekly series. Things like chassis set up, Tech inspection at the track, qualifying, learning the race rules on track give and take, post race tech are a few of the big ones. Other life lessons, tow truck and trailer night mares, running up the credit card on tires, not properly managing time between race car flogging and family, and learning that all wives, GF think the car is their competitor for your feelings/finances/ love interests...well, it gets real “REAL”” some times.

You can get into a class pretty cheaply and have a ball racing. These cars are typically smaller and lighter than the Rear Wheel Drive mini stocks and other “traditional” race classes. You will never get these cars to go power “ loose” on exit like most RWD rookies experience.
It is a funny thing, this forum is on Physics but the FWD is a paradox in Physics. When you bomb into turn One, and put on power, you loose grip at the front drive wheels because the “ weight transfer “ goes to the non drive wheel rear then comes back to the front and hooks you up into a Push ( under steer ) situation. You can not put power down, and when you do manage to, it pushes.

FWD was originally designed because of EPA fuel mileage requirements. Lighter means more MPG. A side factor was reduced manufacturing costs because they're easier and cheaper to build. From a Sales and Marketing standpoint, in normal (street) driving conditions the average driver will be better able to control a FWD car. A RWD car is more likely to spin out in low-traction conditions like rain and snow. And you had more leg room space available in the passenger compartment.
From a Racers point of view, FWD cars typically have slightly less drive-train loss, i.e. no drive shaft needed from the transmission to a rear differential. It is also slightly lighter as a result of this same fact.

Down side is And you're using the same wheels to drive the car as you are to turn it. This means that it's difficult to feed more power and increase speed during a turn without resulting in a huge PUSH or under steer. And they under steer at HIGH SPEED. You lose traction and its all over. Grab a shopping kart next time at Wal-Mart. Pushing from the rear is ok but grab the front of the kart and see how difficult it is to make the kart trail you when you want to turn. The kart wheels lose traction and its skid time. As long as things go slow and easy, these cars are a great grocery getter. Once out of their element, watch out.

Another problem is torque steer. If you do manage to add more HP and get the tires to hook up, since the front wheels are putting down the power AND turning the car, when you hit the gas, the steering wheel wants to rip out of your hands.

My main beef with most FWD is the Macpherson suspension. Granted it has less unsprung weight, and is narrower than the Double A-Arm suspension, has more room for half shaft C/V joint drive. But it is taller than the Double A-Arm and must mount to car body. Has very small camber change during body roll and zero camber change while cornering. Requires mucho static camber for racing. The MacPherson struts also have problems working with wider wheels that have increased scrub radius, where you would need extra effort to steer. It has terrible camber compensation in roll; the suspension does not gain negative camber when compressed like a double A arm does. There is a huge problem with the small camber change with vertical movement of the suspension, which means the tires have less contact with the road during cornering.

For me, my main beef with most FWD is the Macpherson suspension is that no way can you change the front Roll Center location with the stock rules in place. So you can not change how the right front tire gets more down force by body roll. It is my opinion that if you are going to race FWD class - find a small, light car with a double-A-arm front suspension like some Honda Civics or convert a Ford mustang to double A-Arm if rules permit. see video below on why I like the double A-arm.


You can stop reading from here unless you are bound and determined to race a Macpherson strut car or can not afford a double A-arm car.

Tips for racing a Macpherson Strut car

1. Make sure you have a Limited Slip Differential drive (LSD). You will need all the help you can get putting power down during mid turn to Exit. Get both wheels working for you and reduce inside front wheel spin.

2. Biased much more heavily towards entry over steer. You want as much front weight bias as you can get away with. This will hurt braking, but you aren't going to be braking all that much and you need every ounce of weight on those front wheels. Realize that Turn entry is the only chance you get to rotate the car. Rear diagonal load transfer is going to the right front drive tire. Bias the static weight on the inside front, and get more grip out of that inside front tire. Your rear tires are just along for the ride. Get as much left to right load transfer as possible WITHOUT lifting the left from tire. You do not want to open the LSD and lose traction.

3. Set the rear bar and/or spring until the inside rear lifts off the ground at entry.

4.. Tune the front spring to where power-down started to suffer. Get as much left to right load transfer as possible WITHOUT lifting the left from tire. You do not want to open the LSD and lose traction.

4.. Reduce the positive camber gain and to set enough static negative camber to give you maximum tire contact path on the right front tire.

5. Learn how to properly BRAKE to set the car. Practice to get the over steer you want with hard, short, quick braking. We want corner-entry over steer, and given that we want a lot of rear lateral weight transfer (to help plant the inside front). We are going to lift the inside rear off the ground on corner entry. We may have to add more front spring / ARB rate.
Good references
http://www.hotrod.com/how-to/chassis-suspension/ctrp-0401-ministock-suspension/

Building the Mustang Ministock is available from Steve Smith Autosports Publications.

 

[maverick 5582]

I have spent the last 8 weeks modifying the old chassis for vintage modified. I need some help on spring rates for the coil overs. The weight is 2650 lbs. The motion ratios for the 4 corners are: LF= .679, RF = .481, LR = .708, RR = .609, the ARB I have is a 1.250" hollow bar. Roll Center Height: 2.95" Roll Center Right: 3.18"

 

[Ranger Mike]

Looking good Maverick
the car I assume is for paved track..looking at previous posts I think you got a winner.

LF 350 RF 350
LR 200 RR 200
220# ARB

your front motion rate refers to ARB mounting. look at wheel rate.
I would ball park 350# on both fronts and fine tune to 300# LF and 325# rt front springs if you get really hooked up
from old screen shots of the frt suspension results in previous posts you made I figure
you have 17” lower bj to frame mount length and 14” lwr spring mount to lower frame mount and spring is mounted at 20 degrees
so 14 / 18 = .823 and squared it is .677
20 degree cosine is .94 this squared =.883
.667 x .883 = .597
so a 350# spring would give you 350# x .597= 209# wheel rate

look at post # 589 on page 301. The total amount of momentum to be countered by the the tire contact patch and spring / ARB package during cornering

Tw= Gs x car weight x CGh / track width
we assume the camshaft center line is the CG at 15” and the spec tires are crap and hard as wood pecker lips so we only get 1.0 G grip. Track width 66”

1.00Gs x 2650 x 15 / 66 = 602 total load transferred during cornering.
602 load transfer has to be countered by two springs and one ARB so we divide it by 3
602 / 3 = 200
typically you would l run a 220# ARB (sway bar)

ball park rears are 200# LR and RR with 3 point link
but typically you will end up with
175# LR and 150# RR

 

[maverick 5582]

Looking good Maverick
the car I assume is for paved track..looking at previous posts I think you got a winner.

LF 350 RF 350
LR 200 RR 200
220# ARB

your front motion rate refers to ARB mounting. look at wheel rate.
I would ball park 350# on both fronts and fine tune to 300# LF and 325# rt front springs if you get really hooked up
from old screen shots of the frt suspension results in previous posts you made I figure
you have 17” lower bj to frame mount length and 14” lwr spring mount to lower frame mount and spring is mounted at 20 degrees
so 14 / 18 = .823 and squared it is .677
20 degree cosine is .94 this squared =.883
.667 x .883 = .597
so a 350# spring would give you 350# x .597= 209# wheel rate

look at post # 589 on page 301. The total amount of momentum to be countered by the the tire contact patch and spring / ARB package during cornering

Tw= Gs x car weight x CGh / track width
we assume the camshaft center line is the CG at 15” and the spec tires are crap and hard as wood pecker lips so we only get 1.0 G grip. Track width 66”

1.00Gs x 2650 x 15 / 66 = 602 total load transferred during cornering.
602 load transfer has to be countered by two springs and one ARB so we divide it by 3
602 / 3 = 200
typically you would l run a 220# ARB (sway bar)

ball park rears are 200# LR and RR with 3 point link
but typically you will end up with
175# LR and 150# RR

Yes, the 36 Chevy Vintage Modified is for asphalt. Where should I set the track bar for the correct roll center? The roll center height is 8.97" high and roll center right .310". The track bar is about 36" long with the frame mount on the right.

 

[Ranger Mike]

there are as many different ways to mount the track bar as there are colors to paint the car. I like the location you have and the height. try to make it level to the track to begin with. sounds liek this is going to be a very good race car.

 

[z28xtreme]

I was going to PM you mike. I saw some things sporadically throughout about northeast modified. I am trying to find roll centers on my car but the front and rear is located with a panhard bar. front is chassis mounted to the right and is mounted to left of center in front on axle. the rear is chassis mounted to the right and on the front of the pinion input shaft. my car pushes in and is loose off. so I am trying to see if its the roll center problem. thanks for all you do for the forum on this.

Patrick

 

[Ranger Mike]

welcome patrick,
i am not that familiar with your class of racing. Are you racing dirt tracks? Panhard bar suspension is easy to figure the roll center. Mid point between the mounts. Thats the good news..the bad news is you are pretty much stuck with what you have. see page 13 post # 246. A lot of handling depends on the bar angle.

 

[z28xtreme]

welcome patrick,
i am not that familiar with your class of racing. Are you racing dirt tracks? Panhard bar suspension is easy to figure the roll center. Mid point between the mounts. Thats the good news..the bad news is you are pretty much stuck with what you have. see page 13 post # 246. A lot of handling depends on the bar angle.

yes I race on dirt tracks. I don't see much about angles in that post. only swapping the j bar sides. I only have 2 inches to play with on the left side on the axle. but I have the whole frame mount to go up or down on the chassis side. most people already run the bar almost down as far as can be on the chassis side.

 

[Ranger Mike]

is there any reason you have the panhard bar set up with the chassis mount on the right side?
did you deisgn it this way?

 

[z28xtreme]

is there any reason you have the panhard bar set up with the chassis mount on the right side?
did you deisgn it this way?

nope that is how all these guys run them here. the axles are already preslugged so you can just mount your bracket. like a post from years back, we are different a lot so its hard for me to comprehend all what's going on. here is my scaled car
LF 5 1/2 - 431 lbs - spring 250
RF 5 3/4 - 370 lbs - spring 175
LR 6 1/2 - 717 lbs - torsion bar ~ 156
RR 6 3/4 - 640 lbs - torsion bar ~ 140
total 2158
cross 50.3
left 53.1
rear 62.8

 

[Ranger Mike]

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