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which of these tubes is best

smart51 - 18/11/09 at 03:53 PM

I'm making a strut brace for my cappuccino but I'm struggling to find the oval tube I want. Everywhere I've tried doesn't stock it or is out of stock or wants Ł115 minimum order. What I really want is 36 x 16 elliptical tube. What I can get is:

flat sided oval 30 x 15 with a 1mm wall thickness

16 x 16mm RHS with 1mm wall thickness

3/4" round 16 gauge
19mm round 2.0mm wall thickness

35 x 20 RHS with a 1.5mm wall thickness

There isn't much clearance between bonnet and engine so the thinner sections would be nicer but will they be up to the job. There's no point if not.

MikeRJ - 18/11/09 at 04:11 PM

I think you need to define "best" in this case. Stiffest, lightest etc.

Nick Skidmore - 18/11/09 at 04:13 PM

I have some 32 x 22 x 1.5 eliptical oval ERW.

Ł5 per metre plus post no minimum.

U2U or 07903 474191

smart51 - 18/11/09 at 04:14 PM

quote:
Originally posted by MikeRJ
I think you need to define "best" in this case. Stiffest, lightest etc.

Yes, that's right. Have you ever not been entirely sure?

I believe they're under compression more often than tension. There's not a lot of torsion applied to them and as they're bolted on with a single bolt at each end, only so much bending force as can be resisted by a tightened up bolt.

MikeCapon - 18/11/09 at 04:18 PM

The shape of the material does not really affect it's performance in the case of a strut brace. All a strut brace can do is to prevent (or limit) the two strut towers approaching one another or moving away from one another as the chassis tries to flex. Providing the tube is straight and adequately fixed at each end the only criteria you are interested in are the compression and tensile strengths. As these are expressed in force/surface area, for equal materials the surface area become the sole variable.

As for "not up to the job" how will you ever know? The tube is not going to break. It will just stretch/compress with the chassis movements. Any strut brace is going to be better than no strut brace. How stiff do you want it to be??

I've run a 22X1.0 tube on a gravel rally car that was abused enormously. It never showed any signs of distress although at the same time it probably didn't contribute a huge amount to the chassis stiffness.

ETA If you are going to look at different material specs then you'll find that drawn tube is much better in terms of tensile strength as the drawing process helps to align the grain structure along the length of the tube.

[Edited on 18/11/09 by MikeCapon]

dinosaurjuice - 18/11/09 at 04:42 PM

quote:
Originally posted by MikeCapon
The shape of the material does not really affect it's performance in the case of a strut brace. All a strut brace can do is to prevent (or limit) the two strut towers approaching one another or moving away from one another as the chassis tries to flex. Providing the tube is straight and adequately fixed at each end the only criteria you are interested in are the compression and tensile strengths. As these are expressed in force/surface area, for equal materials the surface area become the sole variable.

[Edited on 18/11/09 by MikeCapon]

anything 'slender' will buckle under compressive load. This introduces the length (length squared actually) and the I value of the component. only tensile loadings are as simple as force and area, unfortunately.

answering your questions Smart51, i think what Nick Skidmore is offering is best. (best being cheap and stiff)

l0rd - 18/11/09 at 04:52 PM

I always had the idea that strut braces only work if you race the car on twisty circuits.

Someone here will prove me wrong though.

Also, in case someone crashes on one side of your front side of the car, the brace will actually damage both sides of the car.

smart51 - 18/11/09 at 06:12 PM

quote:
Originally posted by dinosaurjuice
answering your questions Smart51, i think what Nick Skidmore is offering is best. (best being cheap and stiff)

Sadly, His 22mm profile is just too thick. It was designed with Ralt's 16.7mm profle in mind. 19mm may be OK but I suspect 22mm is too much.

quote:
Originally posted by l0rd
I always had the idea that strut braces only work if you race the car on twisty circuits.

There's enough flex in the cappuccino that it would be of benefit on the road. For moderate driving it is great. Driven with enthusiasm and it starts to become a little ragged. Of course it may not do much but the fun is with the tinkering.

jacko - 18/11/09 at 07:19 PM

You need some thing like this

r34engine /2

flak monkey - 18/11/09 at 07:48 PM

As I havent turned the bushes for it yet, if you want to change the size of the tube let me know (20mm bar in stock which would be ok for 22mm oval with a 1mm or bigger wall)

I will be making them tomorrow and everything in the post on friday afternoon.

As others have said the best tube is that with the highest second moment of area as it will be most resistant to bucking under compressive loads and also most resistant to bending.

David

[Edited on 18/11/09 by flak monkey]

smart51 - 18/11/09 at 08:56 PM

quote:
Originally posted by flak monkey
As others have said the best tube is that with the highest second moment of area as it will be most resistant to bucking under compressive loads and also most resistant to bending.

Ooo that's interesting. If wikipedia is to be believed, the thin wall flat sided oval is actually quite stiff in second moment of area, stronger than 3/4" round even with a thicker wall. stronger even than the larger ralt oval and that has a 2mm wall thickness. Wall thickness is not so important, you've got to get the metal away from the axis.

Some worked examples. 19mm RHS is 71% stiffer than 19mm round tube in the same wall thickness.
19mm x 1.5 wall round tube is 2% stiffer than the 30 x 15 flat sided oval in 1mm wall thickenss.
36x16.7mm oval is only 50% the stiffness of the 1" round tube it was rolled from and only 58% of the stiffness of RHS of the same major dimensions.

Its amazing what you learn when you put some numbers on it.

flak monkey - 18/11/09 at 09:06 PM

It depends which direction you bend it in with oval.

If across the narrow dimension it will be less stiff than across the bigger dimension.

You are quite right about the position of the metal. A 0.5mm wall thickness tube is nearly as stiff as a solid bar of the same diameter but its no where near as strong or as resistant to buckling.

David