Mazda 6 Forums banner

RB Rear Sway

61 - 80 of 90 Posts

· Registered
Joined
·
2,852 Posts
Discussion Starter · #61 ·
If fitment or space is an issue than maybe we are barking up the wrong tree going after Racing Beat. Hotchkis is the only one I know that makes hollow bars. So here is a way to save weight and increase roll stiffness because a hollow tube ends up being twice the rate as a solid bar.

But who cares what size the bar is, as long as it performs better. Bigger is not always better.
[/b]
where did you get that information? increase roll stiffness? a hollow tube is more susceptible to failure. for the same size diameter solid vs. a tube and the same amount of torque applied, the tube will have more torque applied to less material, giving the overall shear stress to be greater throughout the bar. the only thing beneficial of a tube is weight savings.
 

· Registered
Joined
·
2,852 Posts
Discussion Starter · #62 ·
At the front, improvements designed to enhance handling include increased dampening force of the double-wishbone suspension, a 25 percent higher spring rate for the coil springs and an increase in the diameter of the stabilizer bar from 23mm to 24mm. The multi-link rear suspension changes began with a switch to mono-tube dampers for superior response. The spring rate of the rear coils has been increased by 37 percent, the stabilizer bar diameter increased from 21mm to 23mm. [/b]
from supercars.net. at least everyone seems to agree that the increase was 2mm. better go home and measure the damn thing.
 

· Registered
Joined
·
2,890 Posts
Here is my "new" concern...

2 companies now have made a new rear sway bar for the car.... if you don't know them do some research :)

They are both 24MM, which is a mere 1MM larger than stock...

My confusion is growing, till I remember talking to Mazdaspeed.... Click here to see the thread...

According to one of the higher-ups at Mazdaspeed Accessories, its not actually possible to put a LARGE rear bar on the Speed6... because it simply "won't fit"....

Now we know that a large rear bar will fit the endlinks (They are the same endlinks as the other 6's)... but with there being a rear differential all of its neccessities back there... does anyone else think maybe these companies are being honest? Or do you guys think they just don't make insanely large bars like Racing Beat...
[/b]
last first .....

The 27mm RB rear for the plain M6 does not fit the endlinks properly. It needs to be about as thin as oem there so the nut is on the solid part of the stud.

The "no room" issue is likely when both wheels are at full up (or at full droop) and the elevated hoop over the diff gets close to frame or gas tank.

If true, a 27mm bar could be done, with the central high section machined to 24mm wide to avoid contact. Based on Roark formula, this would give a rate somewhere between a 26.2 and 27mm bar, vs 23 oem. Less "stressfull" would be a 25mm press-flattened to 24, which would stay close to 25mm stiffness.

--------

Looking at the earlier pics, rear endlinks are about 1/3 out on lower control arms. The motion ratio to the "wheels" is squared to get bar rate at wheels. Things like heim-joint links, and poly pivots, will get more out of oem rear bar. Also, for DIY's, a very cheap oem 23mm mazda6 front bar could be used vs MS6 24mm front bar. The combo would reduce understeer, with very little extra body roll.
 

· Registered
Joined
·
453 Posts
if racing beat makes one, i will buy one... i have an rb full exhaust, springs, front and rear sways on my rx7, and lovin' it
 

· Registered
Joined
·
210 Posts
A hollow tube is stronger than a solid bar, it is just more expensive to produce. I read an acrticle in a SAE magazine about this, and that is what lead me to find Hotchkis. I am just trying to find the info again. I may have gotten the size ratio wrong. A lack of sleep can do that.

*edit* Okay hollow bars can exhibit less stress over a solid bar, weigh less, and be stiffer, just need to vary tubbing wall thickness. I think I am reading it right. It also mentions that a variance less than 10% in rollstiffness cannot be felt.
But I was wrong when I thoght that a hollow bar could be smaller and stiffer, it can be alot lighter and stiffer, just not smaller.
 

· Registered
Joined
·
479 Posts
I would buy one for sure. I would really hope they make it adjustable though so we can dial in the real roll stiffness to our needs. And PLEASE ASK THEM TO MAKE SPRINGS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

One major question though. Are they going to do any ACTUAL TESTING or just whip up a bar and sell it?
 

· Registered
Joined
·
2,852 Posts
Discussion Starter · #68 ·
A hollow tube is stronger than a solid bar, it is just more expensive to produce. I read an acrticle in a SAE magazine about this, and that is what lead me to find Hotchkis. I am just trying to find the info again. I may have gotten the size ratio wrong. A lack of sleep can do that.

*edit* Okay hollow bars can exhibit less stress over a solid bar, weigh less, and be stiffer, just need to vary tubbing wall thickness. I think I am reading it right. It also mentions that a variance less than 10% in rollstiffness cannot be felt.
But I was wrong when I thoght that a hollow bar could be smaller and stiffer, it can be alot lighter and stiffer, just not smaller.
[/b]
sorry, but im not buying it. the polar moment of inertia for a tube is less than that of a solid bar. the formula is as follows:

I=(pi*(Do^4 - Di^4))/32 where Do is outside diameter and Di is inside.

since a solid bar has no inside diameter, then Di = 0. thus, I is greater for a solid bar.

now we plug that into the equation for shear stress:

tau = T*R/I where T is the torque applied and R is the radius from the center of the bar.

since I is greater for the solid bar, that means that the shear stress is smaller. Tada! it would be different if the loads on the bar were planar and not rotational. in that case, the tube would be better. but since a sway bar is loaded via torque (rotation force), then a solid bar trumps.


I would buy one for sure. I would really hope they make it adjustable though so we can dial in the real roll stiffness to our needs. And PLEASE ASK THEM TO MAKE SPRINGS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

One major question though. Are they going to do any ACTUAL TESTING or just whip up a bar and sell it?
[/b]
im sure they will be doing testing. this is a reputable company, not my backyard tool shed.
 

· Registered
Joined
·
360 Posts
sorry, but im not buying it. the polar moment of inertia for a tube is less than that of a solid bar. the formula is as follows:

I=(pi*(Do^4 - Di^4))/32 where Do is outside diameter and Di is inside.

since a solid bar has no inside diameter, then Di = 0. thus, I is greater for a solid bar.

now we plug that into the equation for shear stress:

tau = T*R/I where T is the torque applied and R is the radius from the center of the bar.

since I is greater for the solid bar, that means that the shear stress is smaller. Tada! it would be different if the loads on the bar were planar and not rotational. in that case, the tube would be better. but since a sway bar is loaded via torque (rotation force), then a solid bar trumps.
im sure they will be doing testing. this is a reputable company, not my backyard tool shed.
[/b]
that may be true, but what you may not understand is, having a sold bar over a hollow will not give that much more of a benefit. The inner most part of a bar has very little stress in it. The farther from the center of a bar or tube you go, the higher the stresses are. The middle of a bar does jack. All of the stresses are highest at the farthest point away from the centerline. This is why many times hollow tubes are used because the strength benefit does not out way the weight benefit. The larger the bar gets, the more "hollow" you can make it for the same amount of stiffness. This is where you will get weight savings. You could make a 28mm bar and drill a 1mm hole through it. When compared to a solid bar, the difference in stiffness is not significant enough.

I got to spend many weeks on a project in my Machine analsis class designing a shaft for a wind turbine. A hollow tube is the best choice when based on weight and material. Using a solid bar for a sway bar would be adding more weight than doing more good.

Also, a sway bar is not a cylinder but rather a bent bar (most of the time). This means there are more forces than just torque acting on the bar. The more bends in it, the more forces in different directions there will be.
 

· Registered
Joined
·
3,137 Posts
hey im with ryceboi.


the equation for the angular displacement of a bar being torqued, is:

Phi( angular displacement)= Torque*(length)/ (Youngs modulus*cross sectional area)


its either youngs modulus or the elastic coefficient. or such.

anyway, IN NO WAY ( besides being lighter, but weaker) is a hollow bar better than a solid bar.


if you still believe so, ask yourself why they dont make axles and drive-shafts hollow.


and weight isnt the factor, even if we increase weight a little bit, we're trying to get as much of an increase in rate over stock as we can per the size of the bar. it NEEDS to be solid. ALL racing teams use SOLID MASSIVE sway bars. want me to break the pics out from mid ohio?
 

· Registered
Joined
·
4,371 Posts
I don't have much to say on this, but I know for some reason the RX8 crowd loves to drill holes in their stock sways???
 

· Registered
Joined
·
780 Posts
Racing beat has made bars for years. Rather than continue argueing it maybe someone should give them a call and post what they find out.

I'm not interested in it right now at this point but maybe sometime in the future. I will let the interested people look into it for now.
 

· Registered
Joined
·
2,852 Posts
Discussion Starter · #73 ·
that may be true, but what you may not understand is, having a sold bar over a hollow will not give that much more of a benefit. The inner most part of a bar has very little stress in it. The farther from the center of a bar or tube you go, the higher the stresses are. The middle of a bar does jack. All of the stresses are highest at the farthest point away from the centerline. This is why many times hollow tubes are used because the strength benefit does not out way the weight benefit. The larger the bar gets, the more "hollow" you can make it for the same amount of stiffness. This is where you will get weight savings. You could make a 28mm bar and drill a 1mm hole through it. When compared to a solid bar, the difference in stiffness is not significant enough.

I got to spend many weeks on a project in my Machine analsis class designing a shaft for a wind turbine. A hollow tube is the best choice when based on weight and material. Using a solid bar for a sway bar would be adding more weight than doing more good.

Also, a sway bar is not a cylinder but rather a bent bar (most of the time). This means there are more forces than just torque acting on the bar. The more bends in it, the more forces in different directions there will be.
[/b]
as much as you would like to argue, the physics equations are what is ultimately correct. the middle of the bar is taking the torque load because you are essentially twisting the bar when you hit a corner. this is what a sway bar is designed to do. bends do not change the direction of the force being applied, but rather change the distance away from the bar's rotational center that the force is being applied giving it a larger or smaller moment. sway bars are not fixed in position. they are not bolted the frame, but rather held by bearings which allow the bar to twist.

i think you are misunderstanding how a sway bar works.

the sway bar does see some compression/tension, but it is not important to the purpose of the sway bar and is not the failure point under load.
 

· Registered
Joined
·
3,137 Posts
just to clarify ryceboi, you do understand however that most of the resistance to rotational deformation comes in the outer edge of the bar, rather than the inner ( although having a solid bar provides more resistance than a hollow one, i agree)

thats what they are arguing about here, if the center of the bar is just dead weight, because in reality the absolute center axis of the bar undergoes 0 deformation under load.
 

· Registered
Joined
·
2,852 Posts
Discussion Starter · #75 ·
just to clarify ryceboi, you do understand however that most of the resistance to rotational deformation comes in the outer edge of the bar, rather than the inner ( although having a solid bar provides more resistance than a hollow one, i agree)

thats what they are arguing about here, if the center of the bar is just dead weight, because in reality the absolute center axis of the bar undergoes 0 deformation under load.
[/b]
yes, because the forces cancel each other out. my previous post is probably all over the place. i was watching football and typing at the same time.
 

· Registered
Joined
·
360 Posts
I didn’t say it saw compression or tension. I do understand how sway bars work. Yes a solid bar is better than a hollow bar because of the more material, I'm simply saying, the difference between a solid bar and a bar with a hole in it is minimal (obviously this depends on the hole, I'm not saying there is a 28mm bar with a 26mm hole in it.)

This is kind of long so bear with me here.

Also "ask yourself why they don’t make axles and drive-shafts hollow"

Um, drive shafts and axles are hollow. Otherwise you have a lot of rotating mass that is not doing you any good, at least not enough to make a difference.

And to ryceboi, the center of the bar has zero shear stress; it does not take most of the load. Here is some engineering for everyone.

Angle of twist (or theta in radians) = [Torque applied x length (length from point of applied torque)]/[G (modulus of rigidity for whoever was asking) x J (polar second moment of area)]

For a solid round bar J = (pi x d^4)/32
For a hollow bar J = (pi/32) x (do^4 - di^4)

Tau was mentioned earlier but just to put it with the rest up here, tau = Tr/J. When the radius is equal to zero, the shear stress is zero. No stress at the center of the bar. The greatest stress is when r = radius of the bar, meaning, the outside has the greatest shear stress.

We will use G for carbon steel which is 79.3 GPa.

So we'll test a 28mm solid bar and a 28mm solid bar with an id of 2mm. Both 1,219mm long (4 ft).

Solid bar J = (pi x 0.028m^4)/32 = 6.034371 x 10^-8
hollow bar J = (pi/32) x (0.028m^4 - 0.002m^4) = 6.034214 x 10^-8

I'm merely using more decimal places merely to show the difference in the polar second moment of area. If using significant digits, the numbers would be the same.

We will use a torque of 2000 N-m or 1475.122 lbf-ft.

Tau will not change much because J is very similar between the two, however, just for poops and giggles.

Tau at outer edge for solid bar = (2000N-m)(0.014m)/6.034371x10^-8 = 464.0085804 GPa
tau at outer edge for hollow bar = (2000N-m)(0.014m)/6.034214x10^-8 = 464.0206593 GPa

Hmm, not much more stress at the outer edge.

Now let’s look at the angle of twist from one end to the other.

Solid bar theta = (2000 N-m x 1.219m)/(79.3x10^9 Pa x 6.034371x10^-8) = .50948 rads = 29.19 degrees
hollow bar theta = (2000 N-m x 1.219m)/(79.3x10^9 Pa x 6.034214x10^-8) = .50949 rads = 29.19 degrees

Hmm, solid bar doesn’t appear to be any better than the hollow bar. So let’s try a 5mm hole.

Tau for hollow bar with a 5mm id = 464.480 MPa
Theta = 29.22 degrees

Tau for hollow bar with a 10mm id = 471.682 MPa
theta = 29.67 degrees

hmm, starting to see a difference now.

again, just for poops and giggles a 28mm bar with a 20mm id.

tau for hollow bar with a 20mm id = 627.299 Mpa
theta = 39.46 degrees. Now there’s a significant difference.

The difference in twist does not really begin to make a big difference until the hole gets to be about 11-13 mm. Then from there as the hole gets bigger, the angle increases greatly.

So, what does this mean? Is a solid bar better than a hollow bar? Technically according to the data shown, sure. Is there a noticeable difference between a 28mm solid bar and a 28mm bar with a 10mm hole? Not enough that you would notice. Now some of these numbers will change as it depends on torque applied, length of sway bar etc. But is a solid bar that much better than a hollow bar? Not unless there is a big hole in it. Why solid then? Well, much easier to manufacture a solid bar than it is a hollow bar. how much weight savings would there be between the solid bar and hollow bar? Well, here’s a quick glimpse.

Density of steel is 7.85 g/cm^3

Volume of solid bar is 121.9cm x pi() x 2.8cm^2/ = 750.60cm^3. Mass = 7.85 x 750.60= 5892 g = 12.96 lbm.
Volume of hollow bar is 121.9cm x pi() x (2.8cm^2 - 1cm^2)/4 = 310.19cm^3. Mass = 7.85 x 310.19 = 2435 g = 5.36 lbm.

So about a 7lb weight savings. So is it worth it?

The argument of whether a solid bar is better than a hollow bar should stop. Is a solid bar better, yes. Will you notice the difference between a solid and hollow bar, unless there is a big hole, no. Therefore, whether its a solid or hollow bar makes no difference (unless, its a big hole) There is basically 7 lbs of dead weight in the example just shown. Notice this was an example. If anyone can give me real world numbers for torque, length etc I would happily plug them in. This is just to show that a solid bar is not much better than a hollow bar. Just adds more weight.
 

· Registered
Joined
·
3,137 Posts
hey, umm... im an engineer, and we're doing the angle of twist in class... you sure j is polar moment?

i swear to god it was area..... perhaps it was area *d or somthing....
 

· Registered
Joined
·
360 Posts
hey, umm... im an engineer, and we're doing the angle of twist in class... you sure j is polar moment?

i swear to god it was area..... perhaps it was area *d or somthing....
[/b]
J is the polar second moment of area. Ill fix it in my long post, I put inertia instead of area. Thanks for catching that. Its late, i need sleep. I hate working early.
 
61 - 80 of 90 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top