Login    Forum    Search    FAQ

Board index » Racing Forum » Drive Train, Suspension and Brakes




Post new topic Reply to topic  [ 150 posts ]  Go to page 1, 2, 3, 4, 5 ... 10  Next
Author Message
 Post subject: Suspension Dynamics.
 Post Posted: Wed Apr 01, 2009 4:16 pm 
Offline
User avatar

Joined: Tue Apr 18, 2006 9:54 pm
Posts: 1619
Location: Western Illinois
Shrinker touched on it some in another post, but maybe we should get it going and dispel some myths. My PC has been acting up, so I couldn't join in on the last go round. I did see checking springs with scales. This gives wheel rate not spring rate and it is measured from the second inch of compression to the third, then backed up with the 3rd to the forth inch of compression. The only way to get true spring rate is to remove them and compress them in a spring checker. That is also measured from the 2ond to 3rd inch and how many pounds increase is your real rate. The first inch is never used because it is irregular.

The difference between wheel rate and spring rate can be big. Take a front right spring on a circle track car. I have used both a 400lb and a 900lb spring and they where both correct, the difference being, one was ford type suspension and one was GM. Where the spring is located on the control arm makes a big difference on rate. You have differences in bar length and width of mounting in the rear also, plus where the spring is mounted in relationship to axle center line.

One of the myths I see a lot is bar/ ladder/ IC settings will or will not transfer weight. The setting will or will not add or subtract from traction at times, but they have nothing to do with weight transfer for the most part. That is the job of your center of gravity and power and traction. What little effect an IC has is because of compression or separation that changes center of gravity height and increased or decreased traction, and that may be smaller than you think. IC settings are not the same for every car.

Springs/shocks and IC and roll centers are partners. All of them connect the weight of the car to the track. When any one of them change, it will require changes in some of the others to be dead nuts on. Back to basics here. Each part of the car has a primary job. The IC setting is to control compression or separation. The spring is to maintain tire compliance,(rougher the ride=softer the spring). Shock is to regulate the speed the spring does it's job, mainly to avoid oscillation. Sure a shock can change the spring rate but that is not it's primary job, that is a tuning tool that wouldn't be needed most of the time if everything else was right. Don't forget the tire, It will turn into an unshocked spring faster than you can blink your eyes if you have too much anti squat, spring rate or stiff shocks.

After years and years of flogging my dog The most important part of any combination, drag or circle track is center of gravity location and roll centers. If this is right all the rest of your parts get a break. They don't have to do much. No tool works as good as one that does very little. This is why you will replace 10 13 MM sockets for every one in say 9mm. I am interested in everyone's views and always keep an open mind. I do believe that a balanced center of gravity to power and traction is the biggest speed secret you will ever learn. That is what separates the men from the boys. That is why the F1 guys build their car hundreds of pounds lighter than they run and Stock cars also and the top drag cars too. Then they put the weight where it needs to be and adjust it if necessary.

_________________
Older I get the less I know for sure Dennis


Top 
 Profile  
 
 Post subject: Re: Suspension Dynamics.
 Post Posted: Wed Apr 01, 2009 5:49 pm 
Offline
User avatar

Joined: Sat Feb 07, 2009 4:12 pm
Posts: 1215
Location: Adelaide Australia
Im Glad someone has started this discussion because I have been trying to figure out where to start!!. There are so many things that this thread could encompass that I couldnt work out a start point. So I would like to comment on what I have been thinking about the actions of a ladder bar or fourlink etc. When you place a speedcar on a chassis dyno as I have done you instantly observe the effects of the torque tube. The torque tube in a speedcar is the same as the ladderbar in a drag car. What happens to a speedcar is the front wheels try to come off the ground very easily. There isnt enough axle torque being produced to lift the front wheels, its the upward force at the torque ball point that releases load in all the springs, thats all four wheel springs, not just the front ones.

Now lets look at the physics of that. Lets look at a ladder bar car.
The equal and opposite force to the axle torque is resisted by the front Heim (maybe you guys call them rose joints or spherical bearings or clevis). If the front Heim is located directly under the Center of gravity of the body then the force upward tries to lift the whole body evenly. If it is sufficient force the whole body will relax the springs and the weights exerted upon the wheels will be the same as if the rear axle was located under the center of gravity and the car was balancing on the rear axle only. In other words the rear axle will have all the weight of the body upon it. When the torque reactive force is insufficient to do this and the front heim is behind the CG then its not going to lift the front wheels is it but it still acts like the rear axle was further forward that where it actually is, there is an equivalent mounting position for the rear axle that you could install it at that would give you the same front wheel weight when the car is sitting in the shed. That is what the ladder bar does, it moves the rear axle imaginary distance forward in the car. (Im saying it this way in the hope that it creates a different way to view the same problem). Of course the car is accelerating forward in response to the axle torque so in the real world forward inertia adds to the rear axle weight. The CG height does indeed alter due to all these forces and balances but it’s not the principle reason for weight transfer. The weight transfer of a solid rear car is much less than one with a ladder bar. Of course if either design has enough traction to achieve front lift then there is no issue, the principle reason for a ladder bar design is to add downward force to an under tired car, in order to get front lift and ensure all available weight on the driving wheels.

Now comes the four link, boy what a mine field this is going to be, I argue that the books are wrong (well at least all the explanations I have seen are). There I have said it. Consider this. 2 different fourlink designs, one is conventional and the other is stupid. But both have the same IC. This is how its done. Design 1 is normal say like your car and design 2 is very very short bars. The bars are on the same bar angle but they are only 2” long and the chassis is brought right up to the axle to accommodate this sillyness. So both designs have the same instant center don’t they, obviously suspension movement will alter the IC location of the 2 designs but lets just consider a force at a static position before anything moves or reacts the suspension. Now consider that the lower bar is level with the ground so no up or down motion is cause by its angle and the upper bar is the only one pulling up on the chassis. Now it’s easy to see that the normal design will lift the chassis at the mounting point of the upper bar and the silly design will lift it 2” from the axle. SO the reaction of the chassis is going to be totally different between the 2 designs. So Ill leave it at that for now and see what others have to say.


Top 
 Profile  
 
 Post subject: Re: Suspension Dynamics.
 Post Posted: Thu Apr 02, 2009 9:52 am 
Offline
User avatar

Joined: Tue Apr 18, 2006 9:54 pm
Posts: 1619
Location: Western Illinois
That story sounds familiar, Mel. I switched back to Late Model dirt in the 70s and got an expensive education. We had made money drag racing for a couple years in the Street and Comp eliminator circuits. Stock car racing should be easy. Right he he. Not in The Midwest at that time. The line that comes to mind is, No one knows how much hard work it takes to suck. I got a lot of advice. The best advice I ever got was from A crew chief of one of the top cars. If you asked a direct question he wouldn't lie to you but he wasn't giving out speed secrets. He said you will get all kinds of advice, It's up to you to sort out the BS. If it doesn't follow the physical laws of nature, it's BS. Physics is hardly ever wrong. I thought it was wrong once but I was wrong. These guys was very closed lipped then and now.

Back to torque tube or ladder bars or 4 bars or 4 link or leaf springs the lift from positive anti squat or compression from negative anti squat is not really caused by the bars front end lifting the car from torque of pinion trying to climb the ring gear. It is caused by forward force applied at the tire contact patch and the inability of the center of gravity to move straight rearward. Remember for every force there is an equal and opposite reaction. It cant go straight back, so it goes over the top or under the IC depending on it's height compared to the center of gravity. This also applies to weight transfer, The difference being the same force at the tire and it's relationship to the center of gravity. We know it is above the line of force, it is how much above (the higher the easier it will transfer) and how far forward (more to the rear easier to transfer) These are examples of moment of inertia or polar moment. These are the reasons I think this way. For an IC to lift a car, would be like lifting a 8 lb sledge with two fingers at the end of the long handle. For anyone that has tried to hold that hammer at arms length and tried to touch their nose gently with business end, will get the idea of the leverage involved. There may be some force there, but just not enough to lift a car or transfer weight because of leverage, that job is for center of gravity trying to get to the rear of the car the only way it can.

Any thoughts?

_________________
Older I get the less I know for sure Dennis


Top 
 Profile  
 
 Post subject: Re: Suspension Dynamics.
 Post Posted: Thu Apr 02, 2009 5:04 pm 
Offline
User avatar

Joined: Sat Feb 07, 2009 4:12 pm
Posts: 1215
Location: Adelaide Australia
Interesting how the comments so far are basically the same, A lot of unknown physics and also different ways that people look at the same problem. First off we have to get the terminology correct and synchronized. One point comes to mind from Bubstr, if you place a car on a chassis dyno and strap the diff down and then power it up the front end lifts. On a supercharged street car I work on the front wheels nearly come off the ground, They dont because we dont power it that hard. But the car could lift the front wheels off the ground. There is no forward acceleration on a dyno so the thing lifting the front wheels is the physics of torque reaction. What Im getting at is Bubstr was talking about weight transfer and that's absolutely correct, weight transfer is reactive force distribution of the CG location. Of course the CG doesn't actually move but the acceleration of the car alters the weight upon the wheels from front to back. Springs transmit this weight and compress but the torque reaction of the ladder bar for instance removes load from the spring. So In the end the rear spring may not even move if the physics works out that way. But the tyre will still accept the weight transferred. It still squashes out due to the weight increase.
So perhaps we need to distinguish between weight transfer due to acceleration and force addition to the rear tyre from torque reaction.

There is one thing that I can see coming, that is people will confuse the issue of where the forces occur from. What I mean is this example; the tyre contact patch is indeed the force exertion point upon the ground BUT it is not the force exertion point into the chassis. The axle centerline is actually the place where the reaction of the tyre contact patch is distributed into chassis. The chassis does not react about the tyre contact patch it reacts about the axle. This is my biggest bone of contention to the books written on the subject. There are other things I think are wrong in the books too but that's a good one to start discussion with.


Top 
 Profile  
 
 Post subject: Re: Suspension Dynamics.
 Post Posted: Fri Apr 03, 2009 12:02 am 
Offline
User avatar

Joined: Tue Apr 18, 2006 9:54 pm
Posts: 1619
Location: Western Illinois
Weight transfer is dependent on power, traction and center of gravity location. It's kind of on a sliding scale. Unless you have no power or no traction or the center of gravity is on the ground, you will have some weight transfer. Example= 46 Dodge one ton car hauler. It would pop a wheelie with race car on it, but wouldn't with out the extra car. Kids don't try this at home. With a two piece drive shaft it was very hard on U joints.

_________________
Older I get the less I know for sure Dennis


Top 
 Profile  
 
 Post subject: Re: Suspension Dynamics.
 Post Posted: Fri Apr 03, 2009 6:39 pm 
Offline
User avatar

Joined: Tue Apr 18, 2006 9:54 pm
Posts: 1619
Location: Western Illinois
Had to sit back and think about Shrinkers Chassis dyno wheelie. We are still getting the same forces, and it is about forces, not action. Really we have replaced the forward movement of the car by turning the rollers with the tire contact patch. This is a little hard to get a hold of but it is the same reason Billy Shopes Traction dyno should work. It's about force not movement. Movement is the result of the force.

For Mel's car, changing to a SBC from a BBC and dumping a bunch of weight, There should be a significant change in center of gravity locations. Even a couple inches is a big difference. When you change COG location, it automatically changes IC . I see the professional build dirt chassis ave two different motor mounts, one for an aluminum block and one for a steel block. The steel is about 2 inches lower and to the left. Your combination should want a higher and more rear COG location if the chassis was working great before. The SBC and Aluminum heads will lower COG, but will also move it rearward. If you removed ballast from the rear that moved COG forward. If it was low ballast it moved the COG higher. There will be another difference. That will be maintaining correct slip angle on tires. Amount of spin. All tires need to spin some, (not much) (a churn just short of tire shake)to get best traction. To get this right, you may have to change the gear, leave with more RPMs, looser converter, higher tire pressure or last resort lower IC till you give up traction. The last one opens a new can of worms. It makes body roll or drive shaft torque harder to manage. It most times requires a stiffer spring and shock rate to make up for the help you used to get from your IC. I wouldn't change springs and shocks till you see what the car asks for.

I've always found that there are no easy ways to tune a chassis, even with help from professional builders. It always comes down to the car telling you what it wants to squeeze that last little bit out of it. Remember one thing, Its like sex, you think you had the best till you have something to compare it to.

_________________
Older I get the less I know for sure Dennis


Top 
 Profile  
 
 Post subject: Re: Suspension Dynamics.
 Post Posted: Sun Apr 05, 2009 1:13 pm 
Offline
User avatar

Joined: Wed Apr 19, 2006 8:20 pm
Posts: 5852
Location: N.J.
I've always found that there are no easy ways to tune a chassis, even with help from professional builders. It always comes down to the car telling you what it wants to squeeze that last little bit out of it. Remember one thing, Its like sex, you think you had the best till you have something to compare it to.[/quote]





Ok guy's I can't give you an exact measurement of how much they are compressed but here is a question, and this is my car that is not right yet and im just at the end of my rope ](*,) ....If you have a 110 lbs spring that is compressed enough to get the rear of the car at the proper shock length of 16 inchs......Now a 125 lb spring is compressed a little to get the same 16 inchs of shock length....They should react different correct??? But will the heaver spring compress more than the lighter spring and stay compressed for a longer period of time while going down track????............

We only ran 1/8 mile yesterday cause of the wind so I didn't get a time to tune my carb but went after the 4 link/shock setting...So after lowering the lower bar to 3* from 2* and getting it out a touch longer 53" from 50" there was just about no difference O:) in the graph on my logger!!! I did tighten up the extension to keep the suggested extension to about 1/2 inch and totally loosen up the compression all the way loose and was only a touch better but still stays above the zero ride height...I then loosened the front struts (extension) all the way loose and this had no affect also!!! Then I removed the front springs (300lb) and put on 275 lb in there place and tried to store some engery up front and made the last pass of the day and there was no difference!!!!!!!

HELP!!!!!!!!anybody???? Please plain English as I am reading above and still scratching my head #-o #-o #-o Thanks

_________________
Image
running E85
Best ET 8.07
Best MPH 170.71
Barry


Top 
 Profile  
 
 Post subject: Re: Suspension Dynamics.
 Post Posted: Sun Apr 05, 2009 3:22 pm 
Offline
User avatar

Joined: Tue Apr 18, 2006 9:54 pm
Posts: 1619
Location: Western Illinois
Ok Barry, Lets see what your car is telling you. Not responsive to changes first tells me there is something off in the balance of Power, COG and traction. These are the key that make changes noticeable.

I followed your build when you did it. Nice work by the way. That thing is a thing to be proud of. The think that haunts me is Possible too low a COG for the power. This is not the problem usually but you did an exceptional job of getting weight off and what was left is low. So now how do we test it with out a ton of work? One of two ways come to mind. More power,(possibly a small shot of NOS). or add a little high weight for a bit. Not much 50lbs about head high, mounted to the cage main hoop should tell you something. Mount it securely , no muffler clamps, If it works you can look at ways to juggle weight. If you have access to scales, Billy Shope has the math on his blog to tell you exactly where your COG is. The math looks harder than it is. just multiplication and division dressed up as algebra. I'm sure someone with math skills could show me and you how, in simple terms. Then, I would set IC a tad lower than the neutral line around 2/3 to 3/4 the wheelbase forward, for a starting spot. With out scales bring the IC up till you get very little to no compression in the rear. This may look like your above the Neutral line, but the car don't lie, the guess where the COG is, does. With out compression, it won't matter what springs are in the rear. The IC and the spring are partners in removing compression. The IC can do the job by it's self and leave the spring to do it's primary job, maintain tire compliance. If the IC is too high, it will give the springs main job to the tire side wall. That is why I say almost no compression. If the IC is too low the spring will not have a high enough rate to do it's job. If you have 1,100 lbs on the front and transfer it to the rear, you have to expect a little compression. If you are getting compression in the rear, that is a spring storing weight/ energy and not putting it at the tire contact patch. It is going to give it up but most likely back to the front when the front settles down. Can you see what compression and separation tell you about contact pressure? You want to watch these, but also tire sidewalls

_________________
Older I get the less I know for sure Dennis


Top 
 Profile  
 
 Post subject: Re: Suspension Dynamics.
 Post Posted: Sun Apr 05, 2009 5:34 pm 
Offline
User avatar

Joined: Sat Feb 07, 2009 4:12 pm
Posts: 1215
Location: Adelaide Australia
Yes the two different spring rates will react differently. Its because of the range of movement of the angles of the 4 link bars. I know you guys like to talk in IC terms etc and I dont agree with that concept but as long as we get the same results who cares. In IC terms the harder spring rate suspension will alter the IC less; It wont move as far during the launch etc. I dont have much time at the moment but I wish to assemble some words on 4 link reaction physics.
But something I dont understand here is why you are bothered about compression of the spring.


Top 
 Profile  
 
 Post subject: Re: Suspension Dynamics.
 Post Posted: Sun Apr 05, 2009 7:00 pm 
Offline
User avatar

Joined: Wed Apr 19, 2006 8:20 pm
Posts: 5852
Location: N.J.
shrinker wrote:
Yes the two different spring rates will react differently. Its because of the range of movement of the angles of the 4 link bars. I know you guys like to talk in IC terms etc and I dont agree with that concept but as long as we get the same results who cares. In IC terms the harder spring rate suspension will alter the IC less; It wont move as far during the launch etc. I dont have much time at the moment but I wish to assemble some words on 4 link reaction physics.
But something I dont understand here is why you are bothered about compression of the spring.



Dennis I have a set of scales and maybe it does need more power to work but not to excited about strapping 50lbs of weight to the roof of the car....






Spring compression???????In my 1st post I guess you didn't understand :-k ..So Ill try again.......The 125 lb 14"spring...Say it was compressed to say 12 and 1/2 inches to give me the correct shock length of 16 inches...Now that shock I would think will compress more than the 110lb spring that was compressed to 9 1/2 inches also to the correct shock length of 16 inches...Right???Wrong???

_________________
Image
running E85
Best ET 8.07
Best MPH 170.71
Barry


Top 
 Profile  
 
 Post subject: Re: Suspension Dynamics.
 Post Posted: Sun Apr 05, 2009 8:22 pm 
Offline
User avatar

Joined: Wed Apr 19, 2006 4:11 pm
Posts: 2749
Location: Shepherdsville, KY
Beretta wrote:

Spring compression???????In my 1st post I guess you didn't understand :-k ..So Ill try again.......The 125 lb 14"spring...Say it was compressed to say 12 and 1/2 inches to give me the correct shock length of 16 inches...Now that shock I would think will compress more than the 110lb spring that was compressed to 9 1/2 inches also to the correct shock length of 16 inches...Right???Wrong???


I had to read that a couple of times to understand at what you're getting at. The heavier spring will take more weight to compress it the same distance as the lighter spring. I think what you're referring to is the amount of travel the shock has before the spring coil binds. If you have to crank the adjuster up with the lighter spring to get the center to center of the shock mounts where it belongs the lighter spring leaves a shorter amount of travel before the spring coil binds.

_________________
Chuck Woloch

Chuck's Automotive
Full line Computech dealer
chucksautomotive@yahoo.com
Image


Top 
 Profile  
 
 Post subject: Re: Suspension Dynamics.
 Post Posted: Sun Apr 05, 2009 10:20 pm 
Offline
Moderator
User avatar

Joined: Tue Apr 18, 2006 3:20 pm
Posts: 1457
Location: Missouri
Some interesting thoughts here by shrinker. I like it when someone thinks different than the "accepted" way of doing things. I haven't quite processed it all yet ... it takes my brain a while to run a new thought around enough to have an opinion, but its starting to bang around in there.

How does a car with no front suspension respond to a 4-link instant center (IC)? The % rise or % anti-squat (whichever you call it) is related to how the 4-link tries to lift the front vs rear suspension, but if the front has no suspension it can only separate the rear. Does this mean the IC has a different meaning than with front suspension cars?

My roadster has a front suspension with basically no travel, just allows some chassis roll w/o a front tire coming off the ground. Some alterds are completely solid on fronts as are dragsters.

Rick


Top 
 Profile  
 
 Post subject: Re: Suspension Dynamics.
 Post Posted: Mon Apr 06, 2009 2:25 am 
Offline
User avatar

Joined: Sat Feb 07, 2009 4:12 pm
Posts: 1215
Location: Adelaide Australia
Rick you are starting to think about it which is good. Think about this; think only about a ladder bar car, Now make this car weigh 1000 lbs with a wheelbase of 120 inches and a CG located in the middle, make the ladder bar so that the front mounting heim is directly at the CG of the car, so the distance from the axle centerline to the front heim is 60". Now apply considerable axle torque to the rear axle, while the car is on a dyno say, The equal and opposite reaction to the axe torque has to be resisted somewhere and its done by the front heim of the ladder bar. If this is 60" away from the axle centerline then it will require a resistance force of 1/5th of the axle torque. So if this silly car weighs 1000lbs it can resist a total of 5000 foot lbs of axle torque before the whole weight of the car is suspended on the ladder bar. What will happen to this car is it will lift the whole car off the ground by the torque reaction of the ladder bar at 5000 lbs of axle torque. If you were to place some scales under the front wheels while your loading the car on the dyno then the front wheel weights would get less and less as the torque increased. In this example you should be able to see that the torque reaction of the axle does indeed add downward force to the rear wheels.
Now look at a fourlink, Lets visualize the lower bar as being horizontal to the ground. Now this car is still on a dyno. the reason for doing this is to eliminate issues with forward acceleration etc. Just break the problem down into a simple test of one aspect. So this car is on a dyno Ok-- and because the axle torque is resisted by the four link arrangement there is force in the bars. The force in the lower bar is horizontal to the ground so the force applied into the chassis is horizontal to the ground so the force in the lower bar doesnt lift anything. Now the upper bar is at an angle to the ground and its angled down toward the front of the car so its going to lift the car as the torque is resisted. Now the point I am trying to make is this, the fourlink has a bar thats say 18" long and the center of gravity of the car is probably 50" forward so the top bar cant lift the car under the CG can it! All the top bar can do is lift the car at the 18" forward mounting point. This is the big difference between four links and ladder bars. the four link does not work like a ladder bar, the fourlink's so called IC is not the point that the lift force occurs at, what actually happens is a torque is applied between the forward mounts of the fourlink
The ladder bar pushes the car and reacts the axle torque at the front heim the four link pushes the car near the rear of the chassis and reacts the axle torque as a torque upon that mounting system you have for the fourlink front mounts.


Top 
 Profile  
 
 Post subject: Re: Suspension Dynamics.
 Post Posted: Mon Apr 06, 2009 6:14 am 
Offline
User avatar

Joined: Wed Apr 19, 2006 8:20 pm
Posts: 5852
Location: N.J.
Mel the short times have been up and down...Can't seem to get them consistent..
It will hook good and have 1.22 short time maybe once and then go to 1.24/1.23/1.25 then back to 1.22 O:) O:) O:) ...Dialing the car is a crap shoot..

Some said it was the tire compound so have new tires now $600 bucks....then it was rear springs so got lighter ones.... Some think it need more power working on that but not done yet =; .....Im almost ready to throw in the ](*,) ](*,) ](*,)

_________________
Image
running E85
Best ET 8.07
Best MPH 170.71
Barry


Top 
 Profile  
 
 Post subject: Re: Suspension Dynamics.
 Post Posted: Mon Apr 06, 2009 6:49 am 
Offline
User avatar

Joined: Sat Feb 07, 2009 4:12 pm
Posts: 1215
Location: Adelaide Australia
can you post some logs of when its fast and slow?


Top 
 Profile  
 
Display posts from previous:  Sort by  
 
Post new topic Reply to topic  [ 150 posts ]  Go to page 1, 2, 3, 4, 5 ... 10  Next

Board index » Racing Forum » Drive Train, Suspension and Brakes


Who is online

Users browsing this forum: No registered users and 4 guests

 
 

 
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: