Motorsports Village

Suspension Dynamics.
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Author:  rick360 [ Fri Apr 24, 2009 10:36 pm ]
Post subject:  Re: Suspension Dynamics.

shrinker wrote:
The CG of the sprung mass of the car is indeed that low. The CG of the whole car is not. The car is very low construction with dry sump and the lowest possible crank height. The car has aluminium panels and a light weight driver. The gearbox and everything else is below 13" off the ground. The rear axle is not part of the reaction that causes a body to lift or to go into the air. The rear axle is the thing that pushes the body.

How did you determine the CG height of JUST the sprung mass? Still can't imagine how it could be that low. I'd be interested in seeing the math (with actual weights) used to determine this.

None of the online calculators do this correctly, as far as I can tell. The different online calculators show my forward tilted vs rearward tilted car to have significantly different CG hgt (~5" difference). Are the scales that far off?? .. NOT! When I completed my spreadsheet to calculate this correctly (IMO) I plugged my weights and lift amounts (and BOTH tire diameters=; ) and presto, the CG hgts calculated < 1/4" difference. That made me pretty sure the trig/geometry is correct.


Author:  shrinker [ Sun Apr 26, 2009 6:28 am ]
Post subject:  Re: Suspension Dynamics.

I didn't use an online calculator, I did it back in 1998 on a calculator and paper before I had the internet. I worked out how to do it myself like you did probably. Anyway the unsprung weight of the diff and the half weight of the bars is 328 lbs and the diff weight is on the axle centerline so you can calculate the sprung mass of the car by subtracting that at that position. I agree the online calcs are suspect some times. When you rotate the car by lifting the front for instance you actually rotate it about the rear axle C/L not the ground. I don't have the data from the CG stuff it was too many computers ago and its all lost now but the answers live on, that's all I need.
There is obviously many ways people arrive at answers; Billyshopes way is valid and so is mine but I'm happy to do it my way as it enables me to control what is happening better. Our car has a very good reputation for track consistancy and safety in all conditions of traction encountered so far. We have impressed on days when the track bitumen is melting it still goes straight and runs the numbers. What I do works for us. It is a nationally recognised car.
But the way I do it goes against the conventional methods and explanations and that is precisely why I have never spoken about it in the past. I dont even set up the weights the normal way, and we use abnormal drag racing components. I have done just about every form of motorsport in my life and I combine skills from all disciplines. And you know what skill is the hardest and the most complicated --- Speedway Sprintcars. I think if you can succeed at that and understand it correctly then you can do most other forms.

Author:  Bubstr [ Sun Apr 26, 2009 8:31 am ]
Post subject:  Re: Suspension Dynamics.

Shrinker, I'm having trouble following your train of thoughts. I definitely am a slow learner. It took 60 years to learn what I have. I understand Sprints, Midgets and Late Model stock cars and had some success at it. Right now I have trouble seeing how you can lower Center of Gravity by eliminating unsprung weight, as most all of it is below the COG to start with. Wouldn't this raise unsprung weight if ride height was the same?

I realize that not every great chassis tuner uses a paper and pencil to get his set ups. The great Carl Kinzer is a prime example. He can go out on a track, dig his heel into the dirt and tell you what gear, stagger, tread and suspension settings should be an hour later at the end of a 50 lap feature race on any track in the world. Steve don't have the winning percentages now, that he did when Uncle Carl was around. What I'm saying is that even if they didn't go around with a paper and pencil in their hand, the things they did to make a car work did agree with the physical laws that govern everything. The car and track, talked to them. They put that together with the experience of making many mistakes for their settings. For guys that don't hear it, need the math and physical laws. Since there is only one set of physical laws and math. Confusion here condemns them to making mistakes to gain experience.

Author:  Beretta [ Sun Apr 26, 2009 9:25 am ]
Post subject:  Re: Suspension Dynamics.

Beretta wrote:
shrinker wrote:
This is the info i need to calculate it if you wish too. With some example numbers.

diff ratio= 4.57
rear tyre circ= 104

gear box first gear ratio = 1.76
rear spring rate 110
front spring rate at wheel 275
tyre press psi 6

The four link geometry measured with car sitting on ground.
chassis end attachment upper forward of axle C/L x=19.375
chassis end upper above ground y= 13.75
chassis end lower forward of axle C/L x=20.25
chassis end lower above ground y= 6.75
axle end upper forward of axle C/L x=2.75
axle end upper above ground y= 18.00
axle end lower forward of axle C/L x=.50
axle end lower above ground y= 7.75
scaled weight's
left rear= 526
right rear= 527
left front= 572
right front= 536
static cgh= ??
rear axle assembly weight= ford 9" with a Iron housing
wheelbase= 110
FRONT tyre dia= 25
rear tyre press= 7.0
front tyre press= 30
rear tyre tread width in contact with ground when static on start line 17
max tread width= ???

This is about the best I can answer these question and some I can't?????? O:) O:)

Sorry for the delay on these new measurement buy its spring and the honey do list has grown as there is a lot of yard work to do and trying to get the car set and race on Saturday's well you guys know what Im talking about ](*,) =;Hope this looks better...If not tell me what you need in plain english please...I have also listened to Don's,Scott's and Chucks advise and now it's starting to work better..........

Author:  Bubstr [ Sun Apr 26, 2009 3:54 pm ]
Post subject:  Re: Suspension Dynamics.

Barry, Are you going to share what adjustments you made to see improvement and what kind of ET did you gain?

Author:  BillyShope [ Sun Apr 26, 2009 4:50 pm ]
Post subject:  Re: Suspension Dynamics.

Shrinker, using your 4link information (quoted above by Beretta), the percent antisquat comes out to only 48 with a 22 inch high CG. Even inputting a 13 inch CG height raises it to only 81%. So, you're a long, LOOONG way from achieving any kind of hit.

Which only goes to prove, I suppose, that decent performance can be achieved with a lot of squat.
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Author:  shrinker [ Sun Apr 26, 2009 5:15 pm ]
Post subject:  Re: Suspension Dynamics.

Bustr; our car has the crank at about 11 to 12" above the ground. The diff weight is 17" above the ground. The diff weighs 328 lbs the total rear weight of the car is 1178 so the rear of the car only weighs 850lbs of body weight. The fuel tank, sump tank etc is all below the 13" CGH The lower chassis rails are 6" off the ground. Its a low built car. There is hardly any components above the CGH. Even the electronics is 6" off the ground. So when you add the weight of the diff the CG location of the whole car moves upward and rearward from its sprung mass location. BUT the rear axle weight isnt a transfered weight.
The rear axle has inertia of course but it cant press downward as it doesnt have a base with a length. If you ride around on a unicycle you cant alter the force of the wheel upon the ground unless you jump up and down etc. The only method you can employ when riding a unicycle to maintain your upright state is to accelerate and decelerate so as to move the wheel under you. Now that action of acceleration and deceleration has inertia and the inertia is at an angle other than horizontal so that alters the force of the wheel on the ground but a car diff doesn't move. The car diff is the wheel of the unicycle Its mass isnt what varies in downward force. The body inertia is transferred to the ground via the diff but the diff inertia is horizontal to the ground.
The diff torque is imparted into the body as an upward force at the front mounts etc and that lift is a downward force at the wheel. The diff torque upward force removes weight from the body that would normally go though the rear springs. You can see this on any data logger equipped car, If you run the IC short enough the rear spring will do nothing so where is the weight being held? Its on the front mount being held by the torque. And because that position is further forward than the diff, the weight going through the diff is greater. This is the effect you see on dynos etc.

If you were to mount wheels onto the shaft of an electric motor and then power the electric motor the assembly will accelerate but the inertia cant be a downward force can it. It can only be in a parallel direction to the ground and because our tracks are flat thats what the diff does in your car. For the sake of physics the electric motor would react with reverse rotation and severely limit the acceleration but if we hung the battery on the electric motor so it was below it the torque could be effectively resisted and the battery would swing forward as the assembly accelerated. This forward location of the battery would cause a weight increase upon the ground but its only the weight of the sprung CG of the motor and battery NOT the weight of the wheel and shaft.

I understand that the CGH and the wheelbase form a limit of acceleration possible before blowover etc but what I say a linkage system can do is it increases the weight upon the rear wheels to a higher value than would be achieved by acceleration forces prior to the blowover point. If you dont have enough tyre to achieve front wheels in the air then you need a four link etc to force the tyre onto the ground while the front wheels are still on the ground.
Parallel bars reduce the lift.

What Billyshope is saying is true if the tyre can be managed to maximum traction for the weight available the performance of the car will be quite good but its the little bit extra weight you can create that makes the difference. Most cars like Berreta's can pull the front wheels in the air with or without suspension. They have enough tyre to do it. But its how quickly you pull the front wheels. Do you do it by slowly applying torque and build the force and acceleration until wheels up or do you snap them up just off the line and enable a slightly longer wheels up phase for more force application to the ground?
With low squat like Billy is pointing out there is a loss of distance and time opportunity to apply max power .

Author:  BillyShope [ Sun Apr 26, 2009 6:23 pm ]
Post subject:  Re: Suspension Dynamics.

shrinker wrote:
BUT the rear axle weight isnt a transfered weight.

While the rear axle assembly is always over the rear tire patch, its presence affects the weight transfer. Imagine a very strange car that weighs 3500 pounds, but 3400 pounds is rear axle assembly weight with its CG directly above the rear tire patch. The remaining 100 pounds is at ground level. Although the rear tire tractive effort is passing directly through that 100 pounds, that 100 pounds would be pitched upward as the car accelerates forward. This is because the rear axle assembly weight, as it is accelerated forward, generates an inertial force in the opposite direction, located at its CG. This inertial force, when multiplied by the CG height, produces a torque to act on the 100 pounds.

When considering weight transfer, component location, etc., it is necessary to use a free body diagram of the ENTIRE car, which means including both sprung and unsprung weight.
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Author:  shrinker [ Sun Apr 26, 2009 6:32 pm ]
Post subject:  Re: Suspension Dynamics.

Yes thats right the rear axle weight affects the weight transfer by torquing the CG But the rear axle weight isnt transfer in its own right. I have to go work now so later guys Bye.

Author:  Beretta [ Sun Apr 26, 2009 8:44 pm ]
Post subject:  Re: Suspension Dynamics.

Bubstr wrote:
Barry, Are you going to share what adjustments you made to see improvement and what kind of ET did you gain?

Dennis, Last week it had the 4.30 gears in the rear and did work well as it just touched the wheelee bars and short times were better. So this week I put the 4.57 gears in and short times were better and tightened up the extension and the compression up on the rear shocks and both were run with a IC of 55" and 4" off the ground.....

The chassis/motor likes the lower bar a 4 1/2* down and 6psi of air any less air and I got tire shake....The air was in the 2500 range as last week it was 1800 range...Last week the short times were in the 1.25 range but this week in two rounds they were 1.239 and 1.242.........With the 4.57 gears....I am going to try a longer and shorter IC with trying to keep the height the same....

Author:  want-a-be [ Sun Apr 26, 2009 9:03 pm ]
Post subject:  Re: Suspension Dynamics.

looks like you're creeping up on the combination pretty good. It's good to see your changes are showing you some progress. I think your next weekend ideas ought to show which direction you'll need to go. Glad to your car is responding to the changes.


Author:  shrinker [ Mon Apr 27, 2009 4:18 am ]
Post subject:  Re: Suspension Dynamics.

I cant seem to get my excel clipboard to post the list of numbers for you. Anyone know how to do that?
So Ill do it this way.
clutch torque axle lift weight trans
100 138.3 202.7
125 176.2 259.4
150 215.4 316.0
175 256.1 374.3
200 298.7 434.6
225 343.1 497.1
250 389.5 562.0
275 436.6 622.7
300 487.5 692.1
325 541.0 764.6
350 597.6 840.6
375 657.6 920.4
400 721.5 1004.6
425 789.8 1093.8
450 737.6 1077.2
475 799.0 1108.0
500 854.4 1108.0
525 911.7 1108.0
550 970.9 1108.0
575 1032.2 1108.0

Author:  shrinker [ Mon Apr 27, 2009 4:19 am ]
Post subject:  Re: Suspension Dynamics.

Is it possible to post a column of numbers etc? There is a lot more other factors to see than this but it wont make sense if it just comes out the way it is for me at the moment.

Author:  BillyShope [ Mon Apr 27, 2009 6:36 am ]
Post subject:  Re: Suspension Dynamics.

Shrinker, I need a little help in understanding those columns of numbers. I assumed that the first column was the driveshaft torque in foot pounds. I have no idea what the second column means. The third column I assumed to be the dynamic weight transfer.

But, the forward thrust, at the rear tire patch, is equal to the division, by the tire radius, of the product of the driveshaft torque and axle ratio. So, if you multiply that forward thrust by the ratio of CG height to wheelbase, you get the dynamic weight transfer.

The numbers you've provided, then, should yield the opportunity to solve for the CG height. But, not only is the calculated CG height very high (a little over 5 feet), it also does not remain constant for all the rows.

Obviously, we're not talking the same language.
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Author:  BillyShope [ Mon Apr 27, 2009 7:02 am ]
Post subject:  Re: Suspension Dynamics.

shrinker wrote:
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.

No, within your stated constraints, the chassis reaction will remain unchanged, whether the link lengths are typical, 2 inches, or 2 thousandths of an inch. So long as the IC location remains unchanged, the force vector at the rear tire patch will act through the IC and that's all the chassis understands.

A common analysis mistake is to picture the links "lifting" the chassis. It must be remembered that the links can only transmit forces which act on a line through the 2 pivot points. With that understanding, it becomes apparent that the lengths of the links are totally inconsequential within the constraints you've imposed.
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