Physics lesson for the week, suspension

nkaminar

nkaminar

Well-known member
Automotive Suspension Physics

The wheel suspension on a car is a damped, spring-supported rotating system. The spring supports the car but holding the car fixed the wheel can be thought of as being supported by the spring. The shock absorber supplies the dampening. Any unbalance in the wheel/tire is a rotating force that causes the wheel to not rotate around it’s geometric center (the axle).

There is one speed (rpm) where the displacement of the wheel is the maximum. This is called the critical speed. Without any dampening the displacement would be quite large and could tear the suspension apart. Gas turbine engines (jet engines) that are not correctly balanced and that have no dampening can explode at the critical speed.

Below the critical speed the wheel rotates about it’s geometric center but is displaced towards the heavy side. Above the critical speed the wheel rotates about it’s center of mass and is displaced towards it’s lighter side. This is why the balance beads work. They move towards the lighter side of the tire above the critical speed to balance the tire.

The critical speed is proportional to the square root of k/m, where k is the spring constant and m is the mass of the wheel and tire. So a stiffer spring will raise the critical frequency and a larger mass (heaver tire) will lower the critical frequency.

The amplitude (displacement) will depend mostly on the dampening factor. With high enough dampening the wheel will not be displaced about the geometric center at the critical speed. Most dampening (shock absorbers) are less than this amount so there is some displacement at the critical speed. Less shock absorbing means more displacement (bounce).

For any normal dampening factor (normal shock absorber) and rpm (car speed) the displacement will be proportional to (m * e) where m is the mass of the wheel and tire and e is the eccentricity. The factor (m * e) can be thought of as the unbalance in the tire. More unbalance means more displacement and more bouncing down the road. If the shock absorber is stiff enough the displacement will be small or non existent even if the unbalance is large.

There are two types of tire balancing. One is static balancing where counter weights are placed opposite the heavy side but only in one plane of the tire. This is the kind of balancing where you loosen the brake adjustment and bearing on a front wheel and see where the tire stops when rotated. It will stop with the heavy side down. This works OK on skinny tires like on a Model T or A. With wider tires the tire can be in static balance but have equal top and bottom heavy places at opposite faces of the tire. This tire will try to wobble when rotated and will cause vibration in the steering wheel. A dynamic balance breaks the tire into two sides that are balanced independently. This eliminates any wobble and bounce.

One other factor is at play. The displacement is proportional to the 1/M where M is the mass of the car at the wheel. So, given all other things equal, a lighter car will have more displacement (bounce). Or if the car is light on the front end there will be more bounce there.

If the tire or wheel are out of round or the tire has a flat spot then that will add to the displacement and cause more bouncing. Some tires will develop a flat spot after sitting on the garage floor for a while. This type of flat spot usually goes away after the tire warms up.

More details can be found at https://www.scribd.com/document/942812849/Topic-12-Rotating-Unbalance-1

As you bounce down life’s highway, keep things in balance.
 
So, you don't need to balance your wheels.

Just have stiffer shock absorbers and heavier springs, you'll are fine.
 

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Nice analysis, nk. Taken to the rediculous extreme, I believe that it could be read that with the proper spring and shock absorber, no amount of vibration will be transmitted to the steering wheel. But both would need to be "tuned" to the specific wheel/tire combo, which in practice is impractical.

The only real adjustment we have is to limit the unsprung, undamped displacement - that is to say, "tune" the wheel so it rotates around the center of mass - using your analysis, to drive e ("eccentricity") to zero. That makes m*e equal to zero, and no displacement. Voila!
 
nkaminar,

Thanks for posting the following There are two types of tire balancing. One is static balancing where counter weights are placed opposite the heavy side but only in one plane of the tire. This is the kind of balancing where you loosen the brake adjustment and bearing on a front wheel and see where the tire stops when rotated. It will stop with the heavy side down. This works OK on skinny tires like on a Model T or A. With wider tires the tire can be in static balance but have equal top and bottom heavy places at opposite faces of the tire. This tire will try to wobble when rotated and will cause vibration in the steering wheel. A dynamic balance breaks the tire into two sides that are balanced independently. This eliminates any wobble and bounce.

Engine builders often balance the flywheel and pressure plate as an assembly instead of separately, and this will cause vibration.
 
“ One other factor is at play. The displacement is proportional to the 1/M where M is the mass of the car at the wheel. So, given all other things equal, a lighter car will have more displacement (bounce). Or if the car is light on the front end there will be more bounce there.”

Q: What practical application does this have for Model As? I recall some old Porsches (?) had weights in the bumper assemblies for something like this.
 
My car currently bounces in the front at 60 mph. There is a huge weight in back of the car, the trunk full of tools and parts and the spare tire in back of that, which makes the front end light. I bought an adapter that will mount on a standard balancing machine to dynamic balance the front tires (and wheels). UPS will deliver it in a few days. The back end does not bounce. Note: I am running 6.50-19 tires that are wider than the normal 4.50-19 tires. I did the static balance but that is not good enough for these tires.
 
nkaminar! Your engineering science is excellent to me because I speak engineering. However, being the technical VP of my local club, I need to put engineering science into layman's analogies. Try that with your #1 Post.
I shutter every time I hear Henry is the brain behind his products. His engineering skills were not at the level of actual designing parts, systems and actual vehicles. He was good at establishing production & vehicle architecture, & criticizing existing designs for producibility & cost avoidance. His engineering staff was the genius in Ford products.
Did Ford engineers of the Model "A" era have an understanding of your #1 Post?
 
Bubble balancing is another form of static balancing. It is probably good enough for the standard size Model A tires and Model T tires.
 
Hello, somewhat off post , but the weight in Porsches ,early 911 s were to get the center of gravity more to front of car , this was corrected by extending the wheelbase on rear for better weight distribution,in 1969 .
 
Bob Bidonde said:
nkaminar!
I shutter every time I hear Henry is the brain behind his products. His engineering skills were not at the level of actual designing parts, systems and actual vehicles. He was good at establishing production & vehicle architecture, & criticizing existing designs for producibility & cost avoidance. His engineering staff was the genius in Ford products.
Did Ford engineers of the Model "A" era have an understanding of your #1 Post?
Click to expand...


Ford did employ some brilliant engineers in the Model A era. Joe Galamb, Lawrence Sheldrick and Eugene Farkas come to mind. I am sure there were others.
 
NK- have you thought about getting a toolbox that sits in front of the radiator to move a little weight to the front and calm the bounce?
 
I LOVE the level of knowledge that is exposed on this site! There is something for everyone at every level. I would still be taking my A to a "specialist" garage (at 290 euro per hour) ....that used a big hammer to pound the wrong side of a stainless steel extension into my cast iron exhaust manifold (and of course cracked it) and then tried to bill me 300 euro to weld the crack closed... if not for the knowledge and confidence gained from The Ford Barn.


Many thanks to NK and everyone who takes the time to expand the knowledge and make this hobby more interesting.
 
Ayers, I had the tool box in front but sold it. I did not like the way it looked and the fact that I could not crank the engine with the hand crank.

I have bought some boxes that mount under the running boards for tools. These are used for storage under the seats of older F250 Ford trucks. They are only about 4 inches high. I will report on how they work after they are installed. Older high end cars in the 1920's and 30's used tool boxes that were built into the the running boards. If the new tool boxes work out I will remove the heavy trunk in back and move the spare tire to the normal place instead of in back of the trunk. Putting my tools and spare parts under the running boards will lower my center of gravity.
 
I feel the same way about those tool boxes. I have a brand new one that came with a car I bought. Have no intention of using it. I figured it would accomplish a little of what you were looking for.
 
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