The "unsprung" weight includes wheels and tires, brake assemblies and other structural members not supported by the springs. With that said here are my questions.

1. What are the advantages/disadvantages of "heavy/light" unsprung weight when setting up a car for the track?

2. When corner weighting your car do you calculate unsprung weight into it. Obvisously the scales are going to weight the unsprung portion of the car but how does that weight affect the car while cornering thru a turn?

I am just trying to understand the relationship between the suspension and the weight of the car.

TIA

 

im no genius on this but heres what i got:

1.Advantages of a light unsprung weight is ride quality, suspension control and prolonged shock life. since there is less weight stressing the shock when it compresses, it can do its job easier and less energy is used.
No disadvantages as far as i know.
Another thing is Sprung/unsprung ratio. this just basically determines ride quality. if the ratio is high (correct if im wrong), then the ride quality will be better due to the sprung mass having alot of weight to "push" onto bumps and ditches.

2.Ive never done it but it thibk its something to consider while cornerweighting but ill leave that to the pros to answer

 

One of my drag racing buddies told me yesterday that :

for every 1lb of rotational weight saved it equates to 7lbs of real weight.

Example would be: save 4lbs on a lighter wheel and it will equate to approx 112 lbs off the car.

4lbs x 4 wheels = 16 x7lbs = 112lbs

It would be nice if this is true......

 

Maxx4-im pretty sure that has to do with the ratio like i explained in my post so its different with every car because every car has a diffewrent ratio.

 

well that would make sense.........i'm no info guru...just thought that it was an interesting fact.....

who's say an old dog can't learn a few new tricks.....

 

so unsprung weight would be a major factor in the cornering ability of a car?

is rotation thru a turn affected more by unsprung weight or spring rates/sway bar?

the sway bar does support some of the unsprung weight, yes?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Maxx44 &raquo;</TD></TR><TR><TD CLASS="quote">for every 1lb of rotational weight saved it equates to 7lbs of real weight.

Example would be: save 4lbs on a lighter wheel and it will equate to approx 112 lbs off the car</TD></TR></TABLE>

Bunch o' BS...

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by .RJ &raquo;</TD></TR><TR><TD CLASS="quote">

Bunch o' BS... </TD></TR></TABLE>

is there any truth, i have heard several things.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by AK1200 &raquo;</TD></TR><TR><TD CLASS="quote"> i have heard several things.</TD></TR></TABLE>

Exactly.

The DC2 chassis has about 110 lbs of unsprung weight on each front corner and 60 lbs on each rear corner. Thats a lot...

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Maxx44 &raquo;</TD></TR><TR><TD CLASS="quote">One of my drag racing buddies told me yesterday that :

for every 1lb of rotational weight saved it equates to 7lbs of real weight.

Example would be: save 4lbs on a lighter wheel and it will equate to approx 112 lbs off the car.

4lbs x 4 wheels = 16 x7lbs = 112lbs

It would be nice if this is true......
</TD></TR></TABLE>

You're mixing up topics. Unsprung weight has nothing to do directly with rotational weight. Completely different topic.

 

The way I think of stuff like this is to take it to an extreme. Consider the unsprung weight, wheel/upright/brakes. Lets say you discover a magic material that weights practically nothing, and make all new parts. Now, when a bump is hit, there is practically no mass to displace up and over the bump. Since there's no weight to deflect, the tire can exactly follow the bumpy surface of the road and always maintaining full contact. It's because those parts of the suspension can now change direction so quickly, since they don't wiegh anything.

Now replace the same parts with lead (say...) Now when the same bump is hit, this big mass is given a huge push upward. Once moving, it's impossible for the shock (and the rest of the car for that matter) to stop it quickly. With that much mass moving, it will shove the entire chassis upward, hence a bumpy ride.

Another way to think of it is, instead of changing the unsprung weight riding on the road, load up your car with all you fat buddies. Now hit that same bump and notice the ride is much smoother? That's because, in *comparison*, you've increased the sprung-to-unsprung weight ratio.

You can get a good right by either lowering the unsprung wieght, or by raising the sprung weight (a full car.) Of course only the first is best for a sports car, but both will help the tire better follow the surface of the road.

Hope that helps.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by .RJ &raquo;</TD></TR><TR><TD CLASS="quote">

Bunch o' BS... </TD></TR></TABLE>


Sorry O Sir Professor R.J......

 

Well think about it - if you go from 13lb wheels to 17lb wheels - is it really the same as putting 112 lbs of lead in the pass floorboard?

 

Might be, if all the weight is on the outside edge. Think about the relative difficulty of spinning a 10lb flywheel vs. a 10lb metal rod (along its axis).

The total rotating weight (or unsprung weight for that matter) isn't the whole story; the distance from the axis of rotation makes a big difference. A few ounces saved at the wheel rim is worth more than the same mass saved at the center of the hub.

 

so what can you do to reduce the unsprong weight? You could get new parts made from aluminum but that would not be cheap, and i'm not sure it would last though an off track run. Maybe drill out some holes in the OEM steal parts? Has anyone actually tried reducing the unsprong wight of their track/race cars?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by .RJ &raquo;</TD></TR><TR><TD CLASS="quote">Well think about it - if you go from 13lb wheels to 17lb wheels - is it really the same as putting 112 lbs of lead in the pass floorboard?</TD></TR></TABLE>

I have no idea the validity of these equations of 7:1 or x:y or whatnot but given the choice of 13 lbs. wheel + 112 lead in the passenger floor well vs. 17 lbs. wheels in my 114ish whp CRX, I'm taking the lead. Not based on any hard science, just if given the choice, would be my preference.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 577HondaPrelude &raquo;</TD></TR><TR><TD CLASS="quote">so what can you do to reduce the unsprong weight? You could get new parts made from aluminum but that would not be cheap, and i'm not sure it would last though an off track run. Maybe drill out some holes in the OEM steal parts? Has anyone actually tried reducing the unsprong wight of their track/race cars?</TD></TR></TABLE>
Wheels and tires are about it.

 

we are talking rotating mass...it makes a difference

 

If you are scaling a car, do not worry about unsprung weight. Scales weigh static weight.

Figure all the suspension as 1/2 sprung, 1/2 unsprung since they are bolted to the frame. Keeping the usprung weight is valuable in cornering and braking. Less weight being controlled by the shock and creates less noticable bump steer. Less unsprung weight also helps keep the tire on the ground, you know, obvious stuff.

Don't pop a capulary in your brain trying to figure it out. Lighten up what you can, build lite so you can put the weight where you need. Some folks like to build heavy because the have a minimum weight requirement, but if you are lite, you can add the weight low to balance the car.

There are some good chassis set up books available online from Steve Smith, try Amazon or Borders.com

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Maxx44 &raquo;</TD></TR><TR><TD CLASS="quote">One of my drag racing buddies told me yesterday that :

for every 1lb of rotational weight saved it equates to 7lbs of real weight.
</TD></TR></TABLE>

People have to stop spreading these old wife's tales, as they are indeed a bunch of BS.
Here is some proof with some math attached.
https://honda-tech.com/zerothread?id=1130547

The same can be said for rotational mass and unsprung mass not being related. They are very connected to each other in a car, as virtually all the rotational mass is unsprung (flywheel and gearbox components excepted). You cannot separate the two. Their effects are of course different, but if you add 1 lb to your tire, your rotational mass has gone up by 1 lb as has your unsprung weight. You can change shocks or suspension arms for lighter components, but the total change is usually fairly small. An NSX has forged aluminum arms for the suspension to reduce unsprung weight, but you just can't get some of those for a Honda. Aside from shorter springs and aluminum bodied shocks, there is very little you can change to reduce the unsprung weight that does not also involve rotational mass.

It is very difficult and expensive to reduce unsprung mass by a significant % on a car if you already have relatively light weight wheels to begin with. It is very difficult and expensive to reduce rotational mass by a lot as well in a way that matters, since the weight of the tire is 4 times more important than the wheel due to the location of its mass. If you have 27 lb 18" wheels and can go to 8 lb 15" wheels, then you will have a large change, but this is usually not the case.

There are just no miracles to be had when trying to make a car go faster.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by CagedRuss &raquo;</TD></TR><TR><TD CLASS="quote">
...Figure all the suspension as 1/2 sprung, 1/2 unsprung since they are bolted to the frame....
... Less weight being controlled by the shock and creates less noticable bump steer....

"you know, obvious stuff."

</TD></TR></TABLE>

Obvious? I don't see how either are true.

In the first statement, it's whatever parts of the suspension are directly supported by the road. That means 100% of the wheel, tire, upright, 1/2 the arms, and some % of the shock, depending where it is. "Half" isn't right.

In the second statement, how does unsprung weight have anything to do with bump-steer? It's a little misleading to say if you have Problem A, it will show more of Problem B, thereby "proving" Problem A is an issue. Why is there any bump-steer in the first place?


Modified by kb58 at 12:48 AM 2/27/2005

 

So what exactly is bump steer?

 

AK1200 - GO READ SOME BOOKS - you are not going to get complete well structured introductory education on a board - and you aren't equipped to seperate fact from fiction and build a coherent understanding without one.

That said, what the hell, here's some simplified information:

There are two perspectives on chassis motion I currently like to think in terms of. There's the chassis' reaction to driver inputs (pitch, roll, and yaw - of these only pitch and roll typically concern us since they accompany/interrelate with dynamic cornerweight changes), and the wheels (and chassis') reaction to surface irregularities (bounce or heave, and warp).

If a race track was perfectly smooth, we'd have no bounce or warp. And we'd have no concern whatsoever about unsprung weight.

Now even the smoothest tracks have surface irregularities material to the topic. You can prove this to yourself by raising your spring rates till they reveal themselves to you.

So we do have surface irregularities. When you run a wheel into a bump you can consider it to have been turned into a projectile with a kintetic energy dependent on the unsprung mass and the amount of time it had to be displaced by the impulse. Clearly a lighter unsprung mass looks good to us. But as Claude has tried to point out, just because it's intuitively obvious that something is a good thing DOES NOT mean it's a comparitively good value in performance. The preferences and intuition of the ignorant are not good guides for your spending.

Back to the bump. Now you've got this 100 or so pounds of unsprung weight moving very fast. And you've got a much heavier chassis with it's rotational inertia about it's various axies. And in between you've got a spring rate and a compression curve. With race car rates and curves you are going to displace the chassis. The bound is going to be performance. Or maybe even driver preference if it has to be.

Then the wheel has to follow the back side of the bump and the whole thing come back to what we conversationally call the fiction of stasis or equilibrium. The stored energy of the spring has to push the unsprung weight back against it's linear inertia and the rebound curve less the instantaneous cornerweight. Lighter is bettter - duh. But how much, and how much is it worth again?

Maybe at this point you want to ask, "Hey, if we've got the chassis reacting to driver inputs with changes in wheel position and dynamic cornerweight, and the wheels and chassis reacting to surface irregularites, how can a given spring and damper be optimal for one mode without being sub-optimal for another?" Well, they can't. It's called compromise. Reduction in unsprung weight can reduce the magnitude of this compromise.

You do not want to find yourself chasing a solution for deficiencies in one mode while compromising another. For example - unneccessarily balancing the cars dynamic cornerweights in transient roll using dampers at the cost of grip in heave and warp.

Scott, who thinks that when you don't know how much you don't know you think that there is an answer and it's just beyond your grasp...later when you know more you realize there is no answer and that it's well within your grasp...

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by kb58 &raquo;</TD></TR><TR><TD CLASS="quote">The way I think of stuff like this is to take it to an extreme. Consider the unsprung weight, wheel/upright/brakes. Lets say you discover a magic material that weights practically nothing, and make all new parts. Now, when a bump is hit, there is practically no mass to displace up and over the bump. Since there's no weight to deflect, the tire can exactly follow the bumpy surface of the road and always maintaining full contact. It's because those parts of the suspension can now change direction so quickly, since they don't wiegh anything.

Now replace the same parts with lead (say...) Now when the same bump is hit, this big mass is given a huge push upward. Once moving, it's impossible for the shock (and the rest of the car for that matter) to stop it quickly. With that much mass moving, it will shove the entire chassis upward, hence a bumpy ride.

Another way to think of it is, instead of changing the unsprung weight riding on the road, load up your car with all you fat buddies. Now hit that same bump and notice the ride is much smoother? That's because, in *comparison*, you've increased the sprung-to-unsprung weight ratio.

You can get a good right by either lowering the unsprung wieght, or by raising the sprung weight (a full car.) Of course only the first is best for a sports car, but both will help the tire better follow the surface of the road.

Hope that helps.</TD></TR></TABLE>