Automotive Race car suspension Class

[Bandit21]

Unless i missed something in your post, is sure sounds like your friend took an ill handling metric car and made it into a solid suspension go kart. Any time you have solid stops and limit the suspensions ability to suspend, you have a cornering situation that goes to snap roll. The car rolls over during cornering and hits the stops, goes to a solid contact and slams the car to the right side. When your hit the stops or teathers, all other spring action is null and void. You continue to slide until you straighten the car out and now since the left rear is so soft a large percent of the weight settles on the left rear instead of both rear wheels. Now you are driving off the corner with the left rear tire that is over loaded. All this is masked because you are on a dirt track and and can can get away with a lot of chassis probelms with a good driver.

I am curious about this set up. exactly how much wedge is this guy running? You know there is a reason we run cross weight on left turn cars. You go buy Steve Smiths book " Street Stock Chassis Technology" .
Base all your chassis adjust on known results and know how and why these work, Keep asking questions..only way you will learn,,and win!

Thanks for the reply, after talking some more it sounds like the tether is just barely making contact, maybe not in the snapping fashion. I think he is basing this particular setup along the lines of dirt late models and spring stacking. His theory is that when the car reaching maximum hike on the left rear, there is still energy left in the spring, which will then push back down on the left rear axle and keep a constant load to the ground. I just feel this extra energy would result in the weight trying to transfer more instead of having a standard stiffer spring that may lose energy at the end of the travel. It sounds like a radical setup and maybe it does indeed work, but my thinking was it was adding more grip to the left rear on exit with the extended compression. I understand there is a lot more to the overall chassis dynamic that comes into play than just this spring.

I have read that dirt late models will use taller softer springs in the rear, but this is to Keep the left rear higher, I would assume to take advantage of the aero benefits those cars have. But with a Street Stock I know aero comes into play somewhat,but if we weren’t focusing on that wouldn’t that indeed take more weight off when transitioning. My thoughts may be flawed as well

Thanks I will definitely be purchasing the Steve smith books, I have read bob bolles theory, I know how he stresses the balanced setup but it would be good to learn some more. Thanks again

 

[Ranger Mike]

there is a big difference between the metric 4 link stock suspension and a full floating adjustable 4 link bird cage set up. Those wild 4 link have huge roll over steer and insane body roll. concetrate on your tire temperatures and get proper wedge and stagger.

 

[Hillbillyd]

Well Id just like to say its taken me quite a few days of reading this from the start to finish and would like to thank all those involved I am overwhelmed with information but I've now got a great opportunity to learn so far I've been reading Steve Smith IMCA Modified and its pretty much covers what i need to know but being on here helps me understand it easier. Thank Ill be looking and learning

 

[Hillbillyd]

How do I determine the ideal scrub radius for a race car running 1/4 mile dirt oval , double A arm front suspension, 9 1/4”wide radial tyres, power steering and 3 link rear, 95.5” wheel base” 61” wheel track. I’m currently in the designing stage. Thanks

 

[Ranger Mike]

ideal scrub radius for our stock cars racing left turns is 5 inch. They typically have between 3.65" to 6 inch. Below 3.6 does not provide enough feedback to driver, above 6 inch adds too much scrub wear on the tires.

 

[Hillbillyd]

Thanks Mike I’ll work on 5” and also what height would you recommend for my front roll center and Rc off set

 

[Ranger Mike]

read post 251 on
page 11 and post 691 on page 28
if you are stock front clip on flat track you want it about 2.5" up and 4 inch offset to right for flat to 10 degree dirt

 

[Hillbillyd]

thanks again Mike that's clearer now as i had read that earlier on and now can understand it better.
Is there away to private message you with some brief details on my build or detailed details I'm sure it will be easier for you to advise on what's best suited to my needs. Thanks again for your time

 

[Ranger Mike]

do you turn left or right down there?

 

[Hillbillyd]

counter clockwise so yes left

 

[Hillbillyd]

What affect will it have running 2" less scrub radius on the front left compared to the right front?
I have been researching this and my understanding left scrub wedges the right rear on turn in and tightens up the car.
I have seen a lot of cars running a 2" spacer on the right front and and 1" less off set on the left front rim to keep the track width with in the rules and when I've asked them about it they've said they've done it to gain more left side weight. I was seeing it as a scrub radius gain as many of these newer stock cars run very little scrub radius.

 

[Ranger Mike]

Read post 325 on page 13. Kingpin Inclination Angle (KIA) when installed is a compromise between scrub radius and weight jacking. The KAI and caster combine to weight jack the car in a turn. The amount of KIA will vary and you can change the KIA when you are building the race car. Caster can be dialed in at the track. In a left hand turn Positive caster will cause the left front corner to rise and add weight to that corner and the right rear corner. Adding positive caster to the left front will take out wedge or cross weight and loosen up the car. Most drivers run a split caster so the car will naturally turn left. Wheel spacers are a tuning trick I never much believed in as the car should have been built with maximum left side weight to be gin with. So if you build I right, you will have +1° left side caster and +3° right side caster (+4° caster for power steering) with the proper cross weight, left side weight camber build stagger etc.…
If the scrub radius is correct, the tire rotates left and right directly on it's contact area with the ground. It turns freely and doesn't fight the geometry of the steering or affect handling adversely.
If the scrub radius is not correct, the tire is being moved through an arc across the ground. It's harder to steer and very much fights the steering geometry.

Also, the greater the scrub radius, the more the bumps, pot holes and other pavement variations cause the wheel to be deflected from it’s desired path. The steering wheel will be rotated by the wheel deflection...kind of “the tail wagging the dog”...
Also tends to increase loading on suspension components and wheel bearings, especially the distribution of weight between the large inner bearing and the smaller outer bearing. All in all, no good comes from too much scrub radius

Keeping the center of the tire as close as possible to the center of the king pin/ball joint center line, the happier the suspension will be and we have the least amount of scrubbing.

Positive scrub radius (KPI line or SAI line hits the ground inside the center of the contact patch) imparts a toe out force on the tires. This is typical of RWD cars. Negative scrub radius (KPI/SAI line hits the ground outside of the center of the contact patch) imparts a toe in force on the tires. This is typical of FWD cars. The selection of tire diameter and rim offset can radically change scrub radius. Sometimes changing it from + to - or - to +. All cars I have worked on have a little bit of scrub radius designed in and it helps with road feel. The reason for 'toe out on turns' is because the inner front wheel turns a tighter radius than the outer front during cornering. Without it the inner wheel will be dragged slightly sideways. If you draw a perpendicular line from the center each wheel of a during a turn, the lines should intersect at a common point. This is the point about which the car is rotating during a turn. Because of slip angle, dynamically that point is slightly ahead of its theoretical location. Slip angle is how far you have to turn the wheels compared to how far you would have to theoretically turn them to make a corner. The heavier the car, the faster you are going and the tighter the corner, the greater the slip angle.

 

[Hillbillyd]

Excellent!
Thats it i didn't take into consideration the positive camber and my reduced KAI will reduce the left side scrub Radius
Starting to understand this Thanks again

 

[Ranger Mike]

Why are coil overs “ better” than the separate coil spring / damper arrangement. Well, a lot of times they may not be. But if we look into this question a little deeper, the real question is – What is the best set up for my class to WIN.
If your track rules permit, I would say coil overs are the best way to go if the budget permits. You cannot beat a refined concept and the evolution of coil overs has reached this point. Most high-end coil overs are lighter than the factory components they replace to reduce unsprung weight and further increase handling. They are easier to tune in and adjust.
Penske 3 way adjustable shock weight 8#

Coil over spring 4#
Stock coil rear spring 11#

Stock type tube shock mounted up side down – 1# sprung 3 # unsprung
Stock type rear spring / shock weight total – 15#

Coil over rear spring / shock weight total – 12 #
But now look at Unsprung weight – coil over sprung weight is 6# and stock type coil set up is 14# per side so we are talking about 12# unsprung with coil overs versus 28# unsprung with stock coils/shocks.
Now the real inside baseball stuff begins to make sense. ALL Coil springs will have side loading.
From http://www.pawangroup.co.in/archives/10531
Definition of Eccentric Load: When the load applied to a portion of the column (spring) is not symmetric with the central axis of the column (spring), the load applied is called eccentric load
see spring 1 pic belowExplanation: In above figure (Diagram 1) P is applied load, L is length of the object under load. Here load P is not symmetric with the central axis of the object.

Effect of Eccentric Load on Spring: Spring is an elastic body which when subjected under any type of load starts to deform. If the applied load is in allowable physical tolerances the spring gets back to the initial position due to its elasticity.

It is very important that the applied load is not just in the given physical tolerances but, is also applied properly. If a spring is applied be load P of which the axis is not symmetric with the central axis there is a chance of spring buckling, which can lead to spring breakage.
see spring # 2 pic belowWhen eccentric load acts on the spring the stresses on spring coil increases and load parameter also varies. Due to eccentric loading the stresses on the farthest side (ref. Diagram 2) increases i.e. side A. The stresses on the nearest side of eccentric load decreases due to extra compression. The effect of eccentricity on spring varies considerably with the shape of end coil and also the number of coils in the spring. Stress increase due to eccentric loading varies directly with the distance between central axis and load axis (e).

In plain hillbilly tech – when you compress any coil spring you will have side loading. This side force loading is caused by the springs ends not being able to distribute the compression forces equally to the spring perches. This is magnified if you got the spring ending in a non parallel ground seat. You have at best about one half a coil displacing the load. For the typical grocery getter you would never care about spring eccentric loading.
see coil spring # 3 pic belowParallel ground coil springs used on coil overs are a lot better but inherent characteristic of a coil spring when mounted co-axially around the damper of a vehicle with fixed position spring perches is that it will generate a lateral load due to its not being able to distribute load forces evenly around the face of its end coils. This lateral loading manifests itself as a bending load in such coil over applications, and results in greatly increased frictional forces within the damper. The answer is the hydraulic device supplied by Hyperco. Their ultra high quality spring rate and load test scale, which charts axial load as well as longitudinal load found with some configurations, 98% of the lateral forces were eliminated. You can run softer spring set up since a portion of the old spring rate was going to side loading and not compression. You get better shock life and better tire grip than the other guy running coil overs. You add ½ pound to sprung rate with these coil perches.

 

[Ranger Mike]

thank you all for the past ten years. been fun...hope next 10 are as fun!
rm

 

[LogicIndustries]

I'm working on a design for a pavement modified, more or less an IMCA style car, but not running under that sanction. Track used to be NASCAR but has since gone outlaw, rules remain more or less the same though.

I've been doing my front end geometry calculations and layouts by hand in AutoCAD, but that is time consuming and tedious, so I was thinking of buying a geometry program (or two, if needs be), but I cannot find any real feedback from people I'd consider knowledgeable, so here I am.

The two I've looked at most heavily are the ones from Chassis R&D (which seems to have disappeared off the face of the Earth as a company, their website just goes to a generic Network Solutions Under Construction landing page) and Performance Trends Circle Track Analyzer. Price is about the same for either one, but I can't even figure out if the Chassis R&D stuff will run on a Win10 machine. Literally no one who sells the stuff lists the system requirements for it (not a good sign).

The only feature I like better from CR&D that the PT-CTA doesn't explicitly have is the balanced front/rear roll angle thing that Bolles is always on about, though maybe similar information can be teased out of CTA as well and it's just called something else. I've never used either one, so I'm just going by what their sales literature says.

Any advice you can render would be most helpful.

Thank you for your time.

 

[Ranger Mike]

All you need to do is look at page 1 written 10 years ago. it recommends performance trend software. I began using it in 1995. I have circle track analyzer and suspension analyzer. no endorsement just my opinion.
the suspension analyzer is way more detailed and takes a lot more time to get the measurements but will be better than the other when you are scratch building a front end. You want quick and dirty feed back on already existing front end go with circle track analyzer,

you can down load free version to test drive too

 

[LogicIndustries]

So, that was sort of like not reading the FAQ, wasn't it? Sorry about that.

Is the Full Vehicle version worth the price? It's nearly twice as much money as the standard version, and close to three times the price of the CTA.

My project is kind of half way between a scratch build and salvaging an existing system. My rule set dictates that I use the stock lower control arms, in the stock location, on the stock front stub, but everything else can be scratch built to suit, with a maximum track width of 80" outside side wall to outside sidewall (at spindle height). That stock lower arm thing is going to lead to a few funky compromises with the spindles and scrub radius in order to maximize the track width, but shouldn't cause too many problems otherwise.

Steering wise, I have to use the stock style of steering box (but NOT the actual stock box, just one that will bolt up in the same place) in the stock location with the stock idler arm, but then everything else can be scratch built there too.

I like the rule set. It leaves me plenty of room to play without turning the class into Late Models that are missing the front fenders. The stock stub and the 8" tires keep the Modifieds a handful to drive without the speeds getting way out of hand.

Thanks again for the advice, I appreciate it.

 

[Ranger Mike]

CTA is all you need..look at street stock and metric topics in the index..you can use stock impala lowers and swap out stock mustang and pinto spindles and mix a lot of stock stuff to get some good camber build..all stock appearing stuff...good luck

 

[LogicIndustries]

The stub I'm working on is a 68-72 Chevelle, so the big Impala lowers won't fit. Wouldn't gain me anything width wise anyway.

Building the spindles from scratch, that's not a big deal, I've done that before. Think I'm going to go with the big bearing GN hubs and bolt-on rotors. More rotor and bigger bearings at about half or less the weight of the stock big Impala hubs/rotors, plus that let's me use one hub both front and rear. May be awesome, may go terribly, we'll see how it turns out.

Appreciate the advice.

 

[Vintageracer13]

Hi All. I had a good season this year and learned a lot including the value of less crossweight in my setup. I was wondering if anyone has used the Computerized Chassis Setup Software from Steve Smith. I have a straight axle car and this one has capability to work with my setup

 

[John LNZ]

Hi Mike, I have been working with the Performance Trends programme to assist in re-design of front suspension on our dirt cars. I am particularly looking at my front roll-centre migration. We run a stacked right front spring and as such have 2.5 deg of roll. I seem to have quite a lot of rollcentre migration - up to 12 inches - say 10.5inches left of centre line to 2 inches right of centre line. We are trying to maximise our moment arm length as we run on flat slick tracks predominantly (1/4 mile). Is that much migration expected in your opinion?
Thanks John Lovelady

 

[Ranger Mike]

Racing and winning is about tires, Tires, TIRES..
It is all about the best tire contact patch. He who has it – wins.If you do not have access to the software programming to find your Roll Centers, I think the racer who can adjust track side and can drive the best will win the race. If you can measure the car and find the roll centers and the software can show the roll center migration in roll and dive, you have a significant advantage over the no software racers. The software will get you close to the dynamic measurements. It assumes things move dynamically in true arcs and straight up and down moves.BTW - Those non-believers of roll center logic are lost causes. You can not see gravity but drop that sledge hammer on your toe once. RC matter and understanding it is very important.
The ultimate is to have your car and best set up on a PULL DOWN RIG. This rig shows the effects of suspension components in a dynamic state. Basically the race car is mounted on the stands where the tires are at proper pressure and stagger. Hydraulics are attached to the chassis and the car is forcibly moved to a roll and dive position to replicate the cars cornering attitude under speed. Actual measurements show all the sins of track abuse, bent A-arms, worn bushings, bent tie rod arms. Control arms , bent rear ends, bent Ball joints you don’t know about until post season tear down etc.…
This is the true movement condition of your car in a turn. It is the closest to track side as you can replicate. We used to do this on a chassis puller used to straighten frames. Was a bunch of slotted rails placed in concrete and we would hook a come along or cable rachet tightener on the corner of the race car and read the camber gage as the car was pulled toward the right side down under load. Today’s pull down rigs have large displacement transducers that measure movement and the input goes to a slick computer program to show toe, camber, castor, roll steer. Pretty tech!
So if you can change out your suspension components, you want to get the best tire contact patch you can get in a dynamic state. Also rear roll steer will help on dirt. The roll center migration should orbit around 1 to 2 inch from static. The less migration the better but we are talking real world so do your best.

 

[John LNZ]

Thanks Mike - what are your thoughts on the moment arm theory which Bob Bolles talks about regularly. Whilst working on our roll centre migration we are also trying to keep our right front camber change to the minimum. We run about 3.5 deg negative with about 0.7 loss through 2.5 deg of roll and 1inch dive. I have noticed that the late models seem to run quite a bit more than that even when the right front is locked down. What do you feel is the right number of degrees to get the best contact patch? Is this something you would use the pull down rig to quantify? We have a datalogger in the car and we have used the corner entry measurements from the travel indicators to set the car on the workshop floor at its dynamic state by changing the coil over heights. This verifies our camber change but I have not plotted all my suspension points with the car at that state - perhaps that would be a good exercise? Thanks

 

[Ranger Mike]

Read post 1433 page 58. My opinion on moment arm theory.Why would you copy what an entire different class is doing with camber settings?

Would you replicate how the formula 1 cars are set up?

Money see monkey do racing is very expensive and provides much frustration as the money doing it does not understand what is done and why.The point about adding static camber is to lean the tire when going down the straight away knowing full well the tire is only making contact with the track half the time. This set up is compromise so when the car does go into the turn the cornering force jacks the suspension to straighten up the tire ( zero camber) for maximum contact when most needed.

You want ZERO CAMBER for maximum contact patch in the turn.The best tool you have is a tire pyrometer to take tire temperatures. And the track will change so you need to constantly monitor the temps and adjust accordingly.

So your current set up is neg. 3.5 ° camber going to 2.8° in dive. Why not make it go to ZERO if you can?

You want ZERO CAMBER for maximum contact patch in the turn.

 

[Vintageracer13]

That is the tough part with a straight axle. The camber you have doesn't change. I have been experimenting with different angle kingpin snouts at different tracks and checking tire temps

 

[Blue Foxy]

Hello Mike,

What an interesting thread.
As an engineer working in motorsport in France. Most of the people in EU did not carry so much about roll center migration.

Many author suppose in their calculation, roll axis is constant during chassis roll, which is no often the case, thus their calculation might be wrong.

My though are, if the roll center is the instant point toward the chassis roll in dynamic, it make sense to making it not moving regarding center of gravity, in order to get the driver having a constant feeling.

On your experience on the track, what create on a car a side ways movement of roll center?

On the other hand, design a suspension with upper and lower wishbone, with a minimum movement of the roll center in heigh and laterally, is solving a three bars compatibility problem, this can be done graphically (15mn work on my side, if the wishbone length are not crazy short).

From my experience, roll center not far from the ground, give minimum track variation under bump and rebound, because IC is near to the ground.

Jacking, I would say, let's consider suspension is mechanical system, which spring are the only element which can be deformed under load. If you calculate the vertical and lateral effort on each tire (inner and outer) due to weight transfer and grip tire, and then calculate effort on all suspension element (wishbones, springs) on each side , it shows that too low or too high roll center generate asymetric deflection on springs, thus moving the COG in height during roll.

English is not my native tongue, so I beg your pardon if I make mystake while writing.

 

[Ranger Mike]

Unless the race car is works built from the ground up, you will have roll center migration. You should try to have it orbit within 1 to 2 inch of the static location. The less movement the better but it will move. The suspension links will make this so because of the original design ( usually passenger car engineering). You should read posts regarding location ( see index ). Weigh Jacking is a result of roll center location and tire contact patch. See Jacking effect in index. Tires defelct as well as the springs and you should understand this. There is no such thing as weigh transfer. How the tires and suspension handle momentum should be understood.

You have good language skills and welcome to the forum.

 

[Blue Foxy]

May be, I did well explain what I meant.

Since a suspension is a mechanical system, Newton physics rule are right. That 's why I prefer calculate the force on the tires due to vertical and lateral acceleration, then calculated all the force in upper and lower wishbone and on spring (inner and outer wheel). Since I have the effort on spring and know their rate, I know their deflection font static position, I redrawn the suspension with those deflection and itirate the calculation.
Then I have a whole picture of the suspension in roll, movement of RC, movement of COG, roll angle, etc.
It gives also me a clear wiew of jacking effect.

It is the solution I use, and I made my own software on CAD.

I prefer to use this method, because less approximation that moment calculation.

Regarding roll center migration, It exists a construction methods that allow to minimize his displacement. it is the same construction problem as having no bumpsteer ot having no CVD plunge. If you have a example to submit, I will be happy to show the method.

 

[Ranger Mike]

that may work good for left and right turn racing but not for left turn only round track racing in USA.