Strut Braces..

[AdamP]

I'd replace the ali bar with some CDS tube with a steel bush of some sort welded onto the each end, or even some cheap rose joints as I did for the rear brace on my Locost:



As its a straight run and therefore completely in tension and compression (hopfully... unless your shell is doing some very strange things) you'd probably find that some thin walled 1" tube would be perfect, much lighter and considerably stiffer.

Aluminium is not a material to make strut braces out of! Constant change between compression and tension leads to short fatigue life.

[andyj]

I've got them now.

Here is a pic, very simple and very functional. I like them but they need to lose weight before they go on the rally car.



The bar will be replaced with tube or box section alloy or stainless. We might even get the plates CNC cut from 5mm alloy if we can get it done on the cheap I.e. Free.

My SC came with the right hand one fitted in the front - wondered where it originated from. I've actually replaced it with a Toyota one as trying to get the car looking original. If anyone wants the right hand one above, for the front, they can have it for say £20?

[stenky]

chunky indeed. a well chosen profile would do better work

[aw11rally]

I'm sure rose joints will be involved. 25mm tube would be more than man enough for the job. We'll see what we can find. Carbon would be nice, none has found it's way onto the car yet, it would be ideal, very strong in tension and compression.

[cogboy]

I'm sure rose joints will be involved. 25mm tube would be more than man enough for the job. We'll see what we can find. Carbon would be nice, none has found it's way onto the car yet, it would be ideal, very strong in tension and compression.

Maybe we could cover the bar with a carbon fibre sleeve, for added lightness? - It would at least look fast.

[PW@Woodsport]

Aluminium is not a material to make strut braces out of! Constant change between compression and tension leads to short fatigue life.

Not sure i agree there, these have lasted ten years or so and many strut braces i have seen are made from aluminium.

[AdamP]

Aluminium is not a material to make strut braces out of! Constant change between compression and tension leads to short fatigue life.

Not sure i agree there, these have lasted ten years or so and many strut braces i have seen are made from aluminium.

Maybe I should have said that it's far from an ideal material. As you have shown, it is possible to make them from aluminium! However if you were to make a functional strut brace from aluminium to the same tensile strength as a steel one you would see signs of fatigue very quickly! You have overcome this problem by increasing the tensile strength and therefore the fatigue life drastically.

The aluminium strut braces you see are either a) considered consumable or b) not functional.

Materials engineering, far from my favourite part of the course but nonetheless an extremely important one.

[andyj]

Well I have a Woodsport one. It's spent ten years on an SC ...cars been driven enthusiastically...been on the track as well. Now it's off the car. I can't bend it or break it! It's not showing any surface deterioration?

Conclusion: not saying it would hold up the Severn Bridge but it seems fit for purpose.

[Jim-SR]

To be fair Adam is right. If you were going to engineer a strut brace to the optimum level of performance, then a key consideration would be that it should be lightweight, whilst still being just strong enough to perform its job. Obviously we aren't talking F1 here, so you would still want it to last a considerable length of time, not get replaced every week lol.

If you were to investigate the loads acting on the strut brace, and then specify hollow aluminium tube that was sufficiently strong to handle them, then it would operate much closer to its limits and over time would fatigue, lose strength, and eventually probably fail. Its only a strut brace and not a significant component, so it would in no way be dangerous, just cost you money to replace.

On the other hand, if you used steel tube instead then it would be stronger, but heavier, and fatigue isn't really an issue.

The reason why the Woodsport braces have never failed (and probably never will) is because they are over-engineered, and the tensile strength of the solid aluminium bar is probably 50 times the required strength!! The drawback being that they are fairly heavy. But ultimately 1-2kg of extra weight on a 1050kg+ car is of little significance. If it was a competition car with room for improvement on the scales then obviously you would do everything you could to reduce that weight. On a road car its better for them to be heavy and strong with no chance of failure for the sake of 1-2kg that you could easily lose just by visiting the toilet prior to getting into the car lol.

The pair of strut braces I posted to aw11rally weighed in at 5.1kg fully boxed up. So its not like they are particularly heavy at all. Giving the arches a clean with a pressure washer would probably dislodge more weight than that in dirt lol. Engineering a bit closer to their limits, you could probably get that weight down nearer 1kg a piece though.

[PW@Woodsport]

All of this over a brace i was making 10 years ago

It was my first attempt when there was pretty much nothing already on the market at all for the Mk1, i didn't have half of the access to tools and equipment that we do these days and i only thought about over engineering the job rather than making something weak.... mission accomplished.

If i were to make one these days it would be much lighter, but i think i'd still use a solid alloy round bar with machined ends to take adjustable rose joints and laser cut flanges for the strut tops.

In my opinion a lot of the hollow tube braces i have seen for sale are doing absolutely nothing in terms of bracing the strut towers and are only sold for show ponies.

[AdamP]

All of this over a brace i was making 10 years ago

It was my first attempt when there was pretty much nothing already on the market at all for the Mk1, i didn't have half of the access to tools and equipment that we do these days and i only thought about over engineering the job rather than making something weak.... mission accomplished.

If i were to make one these days it would be much lighter, but i think i'd still use a solid alloy round bar with machined ends to take adjustable rose joints and laser cut flanges for the strut tops.

In my opinion a lot of the hollow tube braces i have seen for sale are doing absolutely nothing in terms of bracing the strut towers and are only sold for show ponies.

I apologise, I didn't mean to place doubt on the quality of your work, i'm merely coming at it from an engineers point of view.

Any strut brace with a bend in it is going to be doing very little in the way of stiffening things up, whether it's solid or tubular, so you can discount those immediately, but I think you'll find that the vast majority of spaceframes (which a strut brace makes up part of) are made from tubular steel because its properties make it ideal for the application.

I don't really see why you'd want to use solid ali bar though, when CDS is much lighter, much stiffer (less likely to bend under compression), just as strong and much more resilient to fatigue. I know it can look nice all polished up but there is just no engineering reason to use it. For the same reason that you won't find any ali spaceframes, it just doesn't lend itself to use in that application.

[scomr2]

All of this over a brace i was making 10 years ago

It was my first attempt when there was pretty much nothing already on the market at all for the Mk1, i didn't have half of the access to tools and equipment that we do these days and i only thought about over engineering the job rather than making something weak.... mission accomplished.

If i were to make one these days it would be much lighter, but i think i'd still use a solid alloy round bar with machined ends to take adjustable rose joints and laser cut flanges for the strut tops.

In my opinion a lot of the hollow tube braces i have seen for sale are doing absolutely nothing in terms of bracing the strut towers and are only sold for show ponies.

I apologise, I didn't mean to place doubt on the quality of your work, i'm merely coming at it from an engineers point of view.

Any strut brace with a bend in it is going to be doing very little in the way of stiffening things up, whether it's solid or tubular, so you can discount those immediately, but I think you'll find that the vast majority of spaceframes (which a strut brace makes up part of) are made from tubular steel because its properties make it ideal for the application.

I don't really see why you'd want to use solid ali bar though, when CDS is much lighter, much stiffer (less likely to bend under compression), just as strong and much more resilient to fatigue. I know it can look nice all polished up but there is just no engineering reason to use it. For the same reason that you won't find any ali spaceframes, it just doesn't lend itself to use in that application.

You can actually find quite a few aluminium spaceframes - Audi A8, Ford GT40, Lotus Elise...

It's very easy to blame fatigue failure on the wrong material but far more often it's poor detail design.

The stiffness of most strut braces will be dominated by the design of the interface between the tube and the strut tower mounting plate - most look pretty horrible. Ideally the brace tube should be straight and in the same plane as the mounting plate to minimise any bending.

[aw11rally]

You can actually find quite a few aluminium spaceframes - Audi A8, Ford GT40, Lotus Elise...

I'm not sure any of those are alloy "space frames" as such, arn't they epoxy bonded extruded aluminium (semi-)monocoque? The Elise certainly isn't a space frame.

[AdamP]

All of this over a brace i was making 10 years ago

It was my first attempt when there was pretty much nothing already on the market at all for the Mk1, i didn't have half of the access to tools and equipment that we do these days and i only thought about over engineering the job rather than making something weak.... mission accomplished.

If i were to make one these days it would be much lighter, but i think i'd still use a solid alloy round bar with machined ends to take adjustable rose joints and laser cut flanges for the strut tops.

In my opinion a lot of the hollow tube braces i have seen for sale are doing absolutely nothing in terms of bracing the strut towers and are only sold for show ponies.

I apologise, I didn't mean to place doubt on the quality of your work, i'm merely coming at it from an engineers point of view.

Any strut brace with a bend in it is going to be doing very little in the way of stiffening things up, whether it's solid or tubular, so you can discount those immediately, but I think you'll find that the vast majority of spaceframes (which a strut brace makes up part of) are made from tubular steel because its properties make it ideal for the application.

I don't really see why you'd want to use solid ali bar though, when CDS is much lighter, much stiffer (less likely to bend under compression), just as strong and much more resilient to fatigue. I know it can look nice all polished up but there is just no engineering reason to use it. For the same reason that you won't find any ali spaceframes, it just doesn't lend itself to use in that application.

You can actually find quite a few aluminium spaceframes - Audi A8, Ford GT40, Lotus Elise...

It's very easy to blame fatigue failure on the wrong material but far more often it's poor detail design.

The stiffness of most strut braces will be dominated by the design of the interface between the tube and the strut tower mounting plate - most look pretty horrible. Ideally the brace tube should be straight and in the same plane as the mounting plate to minimise any bending.

None of those are spaceframes....

[scomr2]

I apologise, I didn't mean to place doubt on the quality of your work, i'm merely coming at it from an engineers point of view.

Any strut brace with a bend in it is going to be doing very little in the way of stiffening things up, whether it's solid or tubular, so you can discount those immediately, but I think you'll find that the vast majority of spaceframes (which a strut brace makes up part of) are made from tubular steel because its properties make it ideal for the application.

I don't really see why you'd want to use solid ali bar though, when CDS is much lighter, much stiffer (less likely to bend under compression), just as strong and much more resilient to fatigue. I know it can look nice all polished up but there is just no engineering reason to use it. For the same reason that you won't find any ali spaceframes, it just doesn't lend itself to use in that application.

You can actually find quite a few aluminium spaceframes - Audi A8, Ford GT40, Lotus Elise...

It's very easy to blame fatigue failure on the wrong material but far more often it's poor detail design.

The stiffness of most strut braces will be dominated by the design of the interface between the tube and the strut tower mounting plate - most look pretty horrible. Ideally the brace tube should be straight and in the same plane as the mounting plate to minimise any bending.

None of those are spaceframes....

Perhaps not by your definition but others seem to disagree;

Scroll down to audi asf: http://www.autozine.org/technical_schoo ... assis2.htm

Another one: http://www.autoblog.com/2010/03/01/fisk ... -revealed/

Ford GT40: www.aec.org/assets/pdfs/ShwcsFrdGTfinal.pdf

A space frame is a geodesic structure that carries suspension loads with the body panels essentially unstressed - completely different from a monocoque.
Accepted the elise is not a good example - the others are.

[AdamP]

Perhaps not by your definition but others seem to disagree;

Scroll down to audi asf: http://www.autozine.org/technical_schoo ... assis2.htm

Another one: http://www.autoblog.com/2010/03/01/fisk ... -revealed/

Ford GT40: www.aec.org/assets/pdfs/ShwcsFrdGTfinal.pdf

A space frame is a geodesic structure that carries suspension loads with the body panels essentially unstressed - completely different from a monocoque.
Accepted the elise is not a good example - the others are.

Interesting! The definition of a spaceframe I have infront of me is a 'Three-dimensional truss composed of linear elements subject to only tension or compression', which makes referring to any of those three as a space frame dubious, but thats another discussion. They are certainly closer to it than I had realised.

The point is that aluminium WILL fatigue under those conditions, especially at the stress concentrations (in this case the joints). Audi spent many millions of pounds developing the correct alloy and bonding methods to reduce this to an acceptable level while maintaining the weight they were looking for. Woodsport brought it to an acceptable level by increase the tensile strength. It still doesn't make it an ideal material.

[PW@Woodsport]

Oh dear, i think you lads need to get out more

[scomr2]

Perhaps not by your definition but others seem to disagree;

Scroll down to audi asf: http://www.autozine.org/technical_schoo ... assis2.htm

Another one: http://www.autoblog.com/2010/03/01/fisk ... -revealed/

Ford GT40: www.aec.org/assets/pdfs/ShwcsFrdGTfinal.pdf

A space frame is a geodesic structure that carries suspension loads with the body panels essentially unstressed - completely different from a monocoque.
Accepted the elise is not a good example - the others are.

Interesting! The definition of a spaceframe I have infront of me is a 'Three-dimensional truss composed of linear elements subject to only tension or compression', which makes referring to any of those three as a space frame dubious, but thats another discussion. They are certainly closer to it than I had realised.

The point is that aluminium WILL fatigue under those conditions, especially at the stress concentrations (in this case the joints). Audi spent many millions of pounds developing the correct alloy and bonding methods to reduce this to an acceptable level while maintaining the weight they were looking for. Woodsport brought it to an acceptable level by increase the tensile strength. It still doesn't make it an ideal material.

So you are saying that any Aluminium structure exposed to tensile or compressive stresses will fail by fatigue? Better steer clear of Audis, aluminium bikes frames then and definitely no flying!

Metallic materials will all fail in fatigue under an alternating load if the stress level is high enough but below the tensile strength. Aluminium is only different from Steel in that it doesn't have a well defined endurance limit.

[PW@Woodsport]

LOL this is a funny thread, if you want my honest opinion i think my old strut braces are/were crap! But they were a decent enough attempt back then. I made them 10 years ago when i was just starting out producing Mr2 products, i did a short production run of about 20 sets and stopped because it was boring the hell out of me.

They were a cheap and easy solution for a strut brace for a Mk1 when there was nothing on the market. I needed to over-engineer the brace for the very reasons being discussed here, i didn't have the tooling or machine access to make anything fancier, these days i could blow the britches off that brace if i really wanted to produce something very classy.

You engineer types do make me laugh though, you're like battling Pokemon!

[AdamP]

Aluminium is only different from Steel in that it doesn't have a well defined endurance limit.

Exactly. And the fatigue limit at 10^7 cycles is at most half that of steel. If you're aiming for endurance of 10^7 cycles and you have an area at a joint or hole where the stress concentration is around k=2-3 then you're going to have to use aluminium that is 4-6 times the area. Aluminium is around a 3rd the density of steel and so you're looking at a heavier overall structure than if using steel.

Audi have obviously studied at great length all stress concentrations and reduced them to a point where fatigue reaches an acceptable level. At this point it starts to wander from the idea of the 'spaceframe' as loads are transferred as a mixture of bending, torsional and tensile stresses spread along a member rather than as a point load. Aluminium bicycles do suffer badly from fatigue, a friend who cycles competitively has his crack tested each year.