Does any one have any experience or knows of any expert reports regarding the long term viability of the tube frame chassis supporting the Locost.

I have read something that amounted to a rather pessimistic account pointing to the inherent weakness of the aluminium rivet to mild steel assembly which the design calls for. But I know of little specifics.

Any views on the subject?

Philippe.

that would probably be down to dissimilar metal corrosion. It doesnt seem to be as big a problem as the pessimists would make out. Anyway, you can always get a new frame if it rusted over 5 years. My frame is about 4 years old and is not bad at all.

Common sense can go a long way toward avoiding the reaction between ally and steel. Butyl rubber or inner tube can be used as a bond breaker and if you dip the rivet in paint or varnish it helps seal it when you pop it shut. Obviously Land Rover never deployed these tactics in the past or the present.
Alex

I have heard of a different kind of failure caused by rivets fretting out.

Flex of the chassis gradually works the heads loose until the panel starts to peel off.

I've heard of rivets with extra large heads to prevent panels from peeling off.

The only way to stop this is to either track down these rivets or make the chassis a lot stiffer. That's how Caterham get away with their panels being attached by rivets.

Caroll Smith advocated dipping the rivets in epoxy right before popping them to fill in gaps and help prevent fretting. Most gap fillers could be used in between the sheet and the frame itself, such as a liquid nails, silicon, etc; although a bonding agent such as hysol or others would perhaps contribute added strength. Being that the panels themselves aren't terribly load bearing in the book configuration, I wouldn't worry about them structurally and once the rivets started to fret, they could be drilled and replaced with the next bigger. Gives the chassis one extra life. And if worried about internal corrosion, I know the aircraft supplied sell a anti-rust solution that they put into the structure prior to sealing it. Just a couple of thoughts. Cheers

For dissimilar metal corrosion to take place you need an electrolyte, In this case usually water). If you seal a rivetted joint thouroughly you will exclude the water and galvanic action will not be a problem.
Kill two birds with one stone and bond the joint with a structural adhesive and you should not have a problem with fretting.
The suggestion to dip rivets into a sealing agent prior to setting is very good advice

Mick

Spot on Mick.

Isn't DMC also between the Al rivet and the steel mandrel inside it?

Personally I've never been keen on pop rivets. They are after all only a tube, not like a solid rivet.

Paul G

Isn't DMC also between the Al rivet and the steel mandrel inside it?

It's a possibility, but thats one of the reasons to dip rivets before setting. If you really want to be sure seal the heads after setting.

Mick

quote:

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Originally posted by Mix
If you really want to be sure seal the heads after setting.

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id just use a flexible adhesive to hold the panels on, and if you dont trust it, use a single rivet in each corner.

quote:

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Originally posted by Mix
For dissimilar metal corrosion to take place you need an electrolyte, In this case usually water). If you seal a rivetted joint thouroughly you will exclude the water and galvanic action will not be a problem.
Kill two birds with one stone and bond the joint with a structural adhesive and you should not have a problem with fretting.
The suggestion to dip rivets into a sealing agent prior to setting is very good advice

Mick

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quote:

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Originally posted by 907
Spot on Mick.

Isn't DMC also between the Al rivet and the steel mandrel inside it?

Personally I've never been keen on pop rivets. They are after all only a tube, not like a solid rivet.

Paul G

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OK as I understand all previously stated comments I have a question or two.
1 Are we better to use alu or steel pop rivets?
2 What about using stainless steel fixings either rivets/mono/magnabolts or drill and tap and use machine screws.

I have a trailer made from 50 mm box with an 18g galv floor pop riveted on with 5 mm ali sealed end rivets.
When unloading sand it's dead easy to cut the top off a rivet with the shovel.
Replaced loads of them over the years.

As for the car, if I was to rivet Al to steel I think I'd go for stainless rivets.

As I have a s/s chassis with a welded in floor I only have the Al side panels to fix,
and for them I'm using M4 dome head Allen screws.

Paul G

quote:

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Originally posted by omega 24 v6
OK as I understand all previously stated comments I have a question or two.
1 Are we better to use alu or steel pop rivets?
2 What about using stainless steel fixings either rivets/mono/magnabolts or drill and tap and use machine screws.

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I had wondered about seperating the ali sheet from the steel chassis with a double side sticky tape or a foam type draught excluder strip 1 inch wide.

quote:

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Originally posted by omega 24 v6
OK as I understand all previously stated comments I have a question or two.
1 Are we better to use alu or steel pop rivets?
2 What about using stainless steel fixings either rivets/mono/magnabolts or drill and tap and use machine screws.

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Not a thing to worry over much much about just use decent stainless closed head rivets, -- on Sevens for cosmetic the paneling tends to get renewed every few years.

I would caution anybody thinking of using normal silcone adhesives the general purpose grades are acidic.
3M Double sided adhesive tape has been reported used by some builders.


I might add that a Lotus S2 Seven chassis flexies an awful lot more than even a book Locost, Chapman on a cost/weight cutting exercise decreed that a lot of the diagonal bracing should be omitted from the already sparse chassis used on the the first series.

The best way to prevent corrosion in an aluminium / mild steel joint is to use 'wet assembly'.
The two components are brought together with a layer of 'jointing compound' in between and rivetted whilst still 'wet'.
Using this method in conjunction with dipping the rivets into the 'jointing compound' will minimise the possibility of galvanic action within the joint.
As BT says, some of the cheaper silicone sealants are acid, (acetic), based and are hence not ideal.
I will be using aluminium pop rivets in conjunction with structural adhesive to secure my bodywork, IMO stainless is OTT in this application.

Mick (an aircraft engineer )

I've had a couple of rivets give up and it's not a problem, I just put another in, the hole's already there. It's nothing structural and they only go one at a time so it's not like the panels are all going to fall off at once.

[Edited on 8/1/06 by Peteff]

Rivets it is then and structural adhesive.
Although with the stuff we use at work I don't fancy ever having to renew a panel as it sticks like sith to a blanket and is mega strong and grippy.

[Edited on 8/1/06 by omega 24 v6]

I used a PU adhesive (similar to Sikaflex) between the chassis and the ali. When I finally put the sheet in place the PU oozed out of the rivet hole - I turned the end of the rivet in that before setting it in place.

The PU got everywhere, but it cleans off with white spirit if you get to it before it hardens.

David

I have used ally closed end rivets with PU as well, not too much bother and as David says, the PU cleans off ok with solvent while still "wet".

Re fretting etc, I have a problem on my grasser where the solid mounted engine sets up frequencies in the chassis that repeatedly pops the heads of 4 and 5mm rivets clean off! Usually have to replace 4 or 5 each race meeting!!!! (vibration doesn't help vision at some revs either, my eyes itch!)

quote:

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Originally posted by NS Dev
I have used ally closed end rivets with PU as well, not too much bother and as David says, the PU cleans off ok with solvent while still "wet".

Re fretting etc, I have a problem on my grasser where the solid mounted engine sets up frequencies in the chassis that repeatedly pops the heads of 4 and 5mm rivets clean off! Usually have to replace 4 or 5 each race meeting!!!! (vibration doesn't help vision at some revs either, my eyes itch!)

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