Automotive Race car suspension Class

[drobbie]

Do you use tie down shocks? They will take some time to return to static height. It is tough to reproduce this in the garage.

 

[mikey531]

Yes they do take time to return, we are also going to make some bars to replace the shock spring combo when traveling.

 

[Ranger Mike]

i would not. its a lot of work. chances are you damaged the shock and this is not a common everyday event.

 

[Thayne14]

Ranger Mike the last 10 nights of reading this has been fantastic. Thank you.

I run a late model on a 1/3 mile banked asphalt track. 8 inch Hoosier. My car is a '14 port city chassis.

I'm a front runner but points are real tight and we have 15 cars that can win any weekend. The best field of cars I have ever been apart of. My car works real good in and to the centre and is getting off the corner okay but this is where I need to make gains. Bite off. I run a straight up solid 3 link suspension. And our rule is written as "no 'active type' rear suspensions" so basically no bird cages. The car that has my number right now is using the centre pull mount on his lower trailing arms and has apparently shortened his trailing arms to 18 inches or so. I'm pretty old school with the lower bolting up on the shock mount. I will put my specs below. Hopefully you will have an idea on what I can do to help side and forward bite.

Trailing arm angles
Lr- 3.6degrees
Rr- negative 0.5 degrees
Centre- 4 degrees

Pan hard is 3 7/16 lower on right side than left (this is only part of setup I have changed from last year and I have loved it)

Springs
Lr- 175
Rr-150

Crate motor car. And we run a LR bump.
LS 56.5
Rear 47.6
Cross 51.8
Total 2775

Thanks for any help or ideas.

 

[Ranger Mike]

Welcome Thane...always nice to hear from racers.

One thing jumped out right off the bat. Rear weight is only 47.6% (1320#) and ideal weight would be 50.5% ( 1401#) 80 pounds is a big difference.
The left side weight is a little low too. 56.5% vs. ideal 58% the cross weight is light as well at 51.8% vs. ideal 55 to 57%
All this will make your car loose off.
Springs look about ideal on your rates.

where is the 3rd link mounted? See post # 253 page 13 and post # 707 on page 36.

Trailing arm angles
Lr- 3.6 degrees and Rr- negative 0.5 degrees means you have the car inducing rear steer due to differing angles. Measure the wheelbase for change in length left to right. You should find it is changing, maybe to roll over steer. Try setting them parallel with the ground initially.I hate rear steer and prefer to get bite thru the 3rd link angle, next go to spring loaded trail arm then mes with differing trail arm angles.
Center link angle you have is 4 degrees..not enuff.. ( try 7 degrees and not over 10 degrees) this will tighten the rear end coming off the turn.
go slow ..get the weight percentages right. This may throw off your turn in handling. Set trail arms level...dial in 3rd link..keep good notes on all changes...

rm

 

[Thayne14]

Thank you for the response!

Our rules allow 50% rear and 56.5 left so I'm kind of stuck there.

Where you talking about snubbers and the like when you said keep the trailing arm angles the same and use spring loaded arms? Maybe you could go into detail on that a bit? I will put rear weight to 49% it's difficult to get much more. And put the angles to 0 and the upper to 7 and try it in testing tomorrow night.

 

[Ranger Mike]

I hate legislated limits..but we have to live by them at the track...
Still need to know about 3rd link mount versus track width..it should be mounted 57% to left.
The adding of rear % weight will help. The 3rd link angle should really help.

Unless i misread your specs on trail arm angles, it looks like you got left side trail arm running up hill and slight down hill on right side trail arm at static position. this means
it will produce rear steer only under acceleration is by staggering the height of the two trailing arms in the three-link system. If we mount the left-side trailing arm lower than the right-side trailing arm, then as the rear end rotates under acceleration due to roll then the LR wheel will move rearward more than the RR wheel, causing rear steer to the left to a small degree. This promotes forward bite without causing the car to be tight on entry or in the middle of the turns.

This sounds like the set up you now have. Do not mess with it until the last step in seeking forward bite. I like a level trail arm set up but if you confirm you are producing rear under steer as described above..do not monkey with it. Leave the trail arm angles alone. work on 3rd link angle and cross weight and stagger. If you can not get bite off the turn ,add a spring loaded 3rd link ( pull bar). This will add rear steer and not impact handling going into a turn.

 

[Thayne14]

You have it right. That's exactly what I am doing. And okay I won't move that. The 3rd link I am not sure. I'll have to measure tonight.

I just want to be very careful because I am extremely good from landing to roll and I am making a lot of my passes by cutting under guys through that transition that I don't want to hurt that.

On the topic of the spring loaded upper link...this has been what we have been talking about recently. I'm not a fan of the snubber style lowers but I do believe that upper setup would work but what one? Cole,an has a double action one that in theory looks interesting. What is your opinion on them and what would you think a good baseline would be to try them? http://www.colemanracing.com/Third-Link-Dual-Action-Aluminum-P4730.aspx#

 

[Ranger Mike]

you should find out if you are inducing rear under steer or over steer...easy enuff to do with a tape measure and jacking up rear of the car. but you need to find this out!The 3rd link mounting should be at the track width % you have regarding % left side weight. If you have 65" rear track width and 55% left side weight then
65 x 55% = 35.75 so the 3rd link should be mounted 35.75 inch from the right of the right rear tire center line. A 65" track width means 32.5" mid point so the offset would be 3 1/4 inch to the left. If it is mounted now at the mid point this change to the left will tighten the car on turn exit.

When you have a solid-rod upper third link, the instant you accelerate off the corner all the Torque is immediately dumped to the rear tires. “Depending on the track conditions and the chassis setup, this may be too much torque dumped too Quick to the tires and they begin to slip, causing a loose condition and loss of forward bite.”

When we add a spring to the 3rd link we have a torque absorber. The energy we dump when nailing the gas pedal is stored in the spring ( or rubber bushings —depending on the style). This allows the rear tires to gradually hook up, pick up the momentum from the drive train and produce BETTER forward bite. Down side of these spring loaded 3rd links is the spring can break over time and you are scratching your head trying to trace it down unless you maintain a good maintenance program. The bushing type 3rd link is usually on lighter cars running dirt. It is a lot more forgiving on misalignment and can be offset both vertically and horizontally to really adjust for bite.

When used with trailing arms that are not level they assist in the inducement of roll steer. As the link compresses the spring, it increases the change in wheel base and adds to the rear steer.
check out
http://racingcarpartsshop.com/cheap-3rd-link.htm

 

[Thayne14]

Thanks for the quick lesson on them! What do you think of that double action upper link?

What's your opinion on putting the LS tires even and stepping the RR tire outside of the RF by an inch? PCRC suggested doing that.

 

[Ranger Mike]

I would go with 1600# spring as more spring is for unlimited hp engines and super wide tacky slicks.

I always ran an axle damper ( shock absorber) at 1 to 2 ° up hill angle over the spring loaded torque link. Now we have double acting torque link. I like simplicity of the cheap spring loaded torque link simply for maintenance and cost. Static pinion angle should be 5 to 6 ° .Wide is always good and watch out for too much back space as it may impact on left side weight limits.

Personally…master the basics first.Wheel spacers are not going to win races…knowing if you have roll under steer will. Knowing where the front and rear roll centers are static and in dive and roll will. Knowing how much shock travel you have on all four corners will. Knowing how much stagger growth you have will. Knowing tire temps will.

From the communication so far I got no doubt you better pack a card board box in the tow vehicle!

 

[Thayne14]

I am extremely meticulous with everything. I blue print the car after every race weekend so I know where I am at before I change anything. So if there is any information about the car you want to know I will have it in my notes.

You have an amazing grasp on all concepts it seems, so if there is anything you need to know to make a suggestion to me feel free to ask. I have this weekend off and can test on multiple nights so always willing to try something new!

 

[Thayne14]

Okay, I just measured the car and have a 67" wheel centre to centre. Through that formula it works out to 37 3/4 inches from rr centreline. That is 4 1/4" left of centre. Mine is mounted exactly 36 3/4 right now.

Does this sound right? Haha.

 

[Ranger Mike]

Thane you are spot on. That additional one inch will Tighten up the car on exit. Its equalizes the loading of the rear tires. Optimum application of forces which is what we want.

The 7 degree angle will tighten the car as well. See attached pic. Its all about the Instant Center. The torque wants to twist the rear end under the race car. The steeper degree of the angle of the 3rd link is increased the more force to the instant center. This plants the tires more,
( relative to the initial 4 degrees you have) and adds " down force " to the tires , thus additional traction.
When a spring loaded 3rd link is used, you have about 1/2 inch movement as the rear end pulls and compresses the spring. Besides cushioning the hook up to the tires it stretches the wheel base by about 1/4" adding to rear under steer ( over steer).

Pic from Steve Smiths Paved Track Stock Car Technology.Get a copy.
one word of caution- if you put more than 10 degrees in the 3rd link your rear end will be too loose during braking so watch it.
You still need to determine rear steer ( under steer or over steer) as this is the final factor. Good job!

 

[Thayne14]

Awesome. Thank you!

What do I need to do to figure the under steer and oversteer? Is there a formula as well?

 

[Ranger Mike]

No formula Thane , just a little precision work with the old tape measure.

Roll Over steer- the right side wheel base gets longer in bump and roll relative to the left side wheel base
Roll under steer – the right side wheel base gets shorter in bump and roll relative to the left side wheel base
see post # 81 on page 5 for photo
air up all tires and get car race ready
measure left and right wheelbase
measure the 4 ride heights to the floor
measure each wheels hub center point to floor
these are your bench marks
jack up the race car enough so the rear end can swing free
measure the left rear ride height
find out the difference from benchmark to raised position
Example ( you measure left rear ride height at 4 inch in 1st step now it is 24.5 inch so
you have 20.5 inch incremental displacement. )
Adjust the other 3 ride heights so they are exactly the bench mark reading plus increment.
On front tires jack these up so you get the original hub center point to floor reading plus incremental.
Block the front tires at those readings
go to the rear end and jack these up so you get the original hub center point to floor reading plus incremental
readings on both rear wheel hub center points.
Now you have a “ level” car elevated above the floor at simulated ride height.
Measure both side wheel base and you should have exactly the same readings as benchmarks
take off the rear wheels and
take out the rear springs
I use 2 Carpenters L squares ( 2 ft. x 3 ft.) but tape measure will work too.
The drill is we are going to drop the left rear hub one inch and on right side hub we bump the center up one inch. Measure both side wheel bases. I use a floor jack with a large 1/2 drive socket under the rear end center section and bottle jack under the right rear trail arm. You get the idea.
Repeat this at 2 inch droop left side and 2 inch bump right side and measure wheel bases
go to 3 inch droop and bump repeat wheelbase measuring on both sides.

Now you know if you have rear roll steer. You may not have any rear steer. Now is when you can crank on the trail arm rod end and see how much 4 turns crank into the rear steer. Keep notes, write it down.

tip- I use two tape measures and two big magnets to affix them to each front hub. Mark each tire with a line pointing straight down so the tire / wheel does not move during this process. You do not have to have these “ centered” the hook can be at the front of the hub cover as long as both tape measures are placed exactly alike and are not moved from this first bench mark reading. I have also used small zip ties thru the center hub cover bolts. As long as you have consistency through the process.
Also make sure that both tape measures read the exact same at 15 feet. One time I had 1/8 inch difference between the two over 15 feet because one was cheap tape measure the other was a quality name brand.

 

[Thayne14]

Ranger, just did the above and I have 1/2 lead in RS wheelbase already with the rear steer at 1" it shortened 1/16 and at 2" it shortened up an 1/8th. At 3" it shortened 3/16" so everry inch was a 1/16th.

I'm not sure that I am getting enough. Every 2 turns on the trailing arm was an 1/8th or so.

What do u suggest for rear steer?

Thanks as always!

 

[Ranger Mike]

I did a lot of thinking about your last post. You could run the set up but it is not the winning set up. For ultimate handling i recommend changing the rear end squareness.

I am not a big fan on rear roll steer.
Large Static roll steer is NOT what you want. I like a race car that is straight and square and running 1/2 inch lead is like running 1/2 inch toe out and is a waste of tires and scrubs off speed.
Your car is loose off the turn so I would take out the 1/2 inch lead. You know you will pick up at least 3/16” DYNAMIC roll under steer with trail arm angle setting you now have. Adding a spring loaded 3rd link will help as you will add another 1/4” to 3/8” inch depending on spring selection.

As last resort you can even go with spring trailing arm that compresses under acceleration. The book says, on asphalt, to keep wheel base change to under 1/2” during cornering. If you have to exceed this then change rear springs. NASCAR limits wheel base change to 1/4” by the way.

We have not discussed the rear panhard bar and this is a huge tuning factor as well.

I was looking over my chassis notes on rear roll steer and a typical GM metric chassis will have 3/8” rear roll under steer with stock trail arm set up.Joes racing in Washington has some great chassis set up articles..good reading
http://www.joesracing.com/kb/category.php?catID=2

 

[drobbie]

Must be some heavy racing going on here as there doesn't seem to be much activity.
Any way I thought I would start it back up.
I have been looking around the pits at many of the tracks I visit and I am seeing many cars using a heavier LF spring versus RF spring. What is the advantage of this type of set up? They say it keeps the car free for the race but I can't see how this helps based on the fact weight is transferred to the RF first. In my mind is would shift too much weight to the RF from both the LR and the LF. Am I thinking about this correctly?

 

[Ranger Mike]

I see no advantage to this set up. Just to review, no weight is transfered. We have to deal with force of momentum which makes the springs feel like more weight is acting on them. You are correct in that a stiffer LF spring will make a constant loose condition causing " over steer". Some drivers like " dirt tracking" the car as opposed to running into a push condition. A lot of monkey see, monkey do going on there i imagine.

 

[01racer]

Any thoughts on adding Ackerman to a open wheel modifed with steering box and what is the best way of doing it. Going to Fab a center link to help bump steer.?

 

[Ranger Mike]

welcome Racer01
yes i believe in ackermann and this comes from the spindle arm mount location. get turn plate to find out how much you have at the start. you can rig up plastic protrators to measure things if the cost of turn plates is prohibitive. smear some grease on large sheet metal plates so they turn easier when tires are rolled on them. You can and scribe a line on the plates to mark zero. it is a pain but i have done it.

 

[01racer]

I measured the difference the wheels turn with 10' string and turning the wheel a 1/4 turn this car being 0.5 i have Measured other name brand cars around 2-3"

 

[6t5mustang]

Hey Ranger Mike,

I'm in the very early stages of design of converting my 65 mustang from a unibody to a full frame car "pro-touring" car. I want the car to be capable of street and track use. Using some of the guidelines you've talked about in this thread, I have gone through several iterations of the front suspension design. Can I either post or email you some of my schematics to see if you think I'm on the right track?

This thread has some awesome information! Thanks for sharing!

Joey

 

[Ranger Mike]

i thank you for the kind words. Post on here as this is a teaching experience. By the way, I sure do not have all the answers. There are a lot more smart people on here than I as i am reminded quote often when i Step in it! welcome racer and post away!

 

[6t5mustang]

OK, I'll give it a shot. My original goal with this car was to do a vintage racer. After I got to looking in depth, both rear frame rails are completely rusted through. So my mind got to churning if those are shot, what other structural issues am I going to run into on this car. So I started researching and found where some folks were converting their cars from a unibody design to a full frame. Unfortunately, I have not been able to find a lot of info on specific measurements and dimensions of chassis created by other folks. So being an engineer, I decided I would try it myself. I took hundreds of measurements, and with the help of this thread in particular have come up with the following design (so far).

I think I have the front roll center squared away. I did not label my CG on any of the drawings, its about 10" above RC (assuming cam CL = CG). I'm still 50/50 on the rear. Part of me wants to do IRS like my Cobra, and the other part wants to do a 3 link for simplicity. My goal right now is to make sure I'm on the right track so I can get the front frame horn dimensions locked down. From there I'm going to start work on designing both an IRS and 3 link rear setup. I apologize for not have everything dimensioned out! I will work on that before the next post.

Thanks for any feedback.

Joey

 

[badfat71]

Hi I have a question I have a dirt pro stock Camaro clip 3000 pounds 3 link and I was wondering if it would turn better with a wider front end putting on ford lowers it has factory Camaro lowers with a 60 inch rear end I come from asphalt racing where wider is better and I wondered if it applied to dirt also I do have a performance trands computer program for roll center and I know I have to compensate for the front roll center

 

[Ranger Mike]

6t5, having owned a 67 corvette and now flogging this rear engine formula car with independent rear suspension..go with the 3 link. Get the front end right. Go with good solid rear 3 link set up..my opinion

 

[Ranger Mike]

badfat, welcome...good to have you in with other racers here!

read these on this thread
post # 255 on page 13
# 264 on page 14
#322 on page 17
# 505 on page 26
go as wide as rules permit
but don't do anything until you benchmark the car and know your current set up

 

[6t5mustang]

6t5, having owned a 67 corvette and now flogging this rear engine formula car with independent rear suspension..go with the 3 link. Get the front end right. Go with good solid rear 3 link set up..my opinion

Ranger Mike - did you see anything from the attachments that I need to revise or am I on the right track? Let me know if I need to provide more info.