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aluminium tube frame, has anyone done this ?
mark chandler - 3/7/05 at 09:05 PM

Hi, new to the forum and have started planning my first locost, 1000 budget for an bike engined effort so weight is key.

As I can weld aluminium I was wondering if anyone has built a frame using ali, then tigged the sheets to this ? (I hate rivits, hurt the wrist and rattle loose after a while.....).

Also being that round tube is torsionally stronger than square is it okay to knock up a frame from this.

I plan to track day the product of my efforts, however second question is based around Camaraderie, do you arrange social track days ?

Regards Mark

phelpsa - 3/7/05 at 09:12 PM

I'm not sure this has been attempted, but im sure it could work with a lot of developement. Im not sure it could be done too well with a 1000 budget though.


kb58 - 3/7/05 at 09:23 PM

Do a search here on that... it's been covered several times. There are two *very* opposing camps on it, one saying it should be fine, the other saying it'll work-harden and crack. I believe the latter.

mark chandler - 3/7/05 at 09:31 PM

I can stretch the budget if required but a 1000 goal keeps thing interesting. I have priced up equivilent ali, not much in it when you add the costs of rivits. I plan to build a reverse gear using a Landrover centre diff so save a lump of weight and money here, to my mind the most expensive bit beyond the engine (R1/blade's go for around 600) will be the GAZ shocks. As I have a decent AC Tig welder and an understanding wife (at the moment) the big cost to me will be time.

nb/ I noticed other posts about ali, however I will be welding all the panels/ floor etc to the frame so will effectively build a monocoque

cymtriks - 3/7/05 at 09:38 PM

You could do this but don't be confused by the aluminium "spaceframes" the certain exotics are now based on. Basically they are much closer to ladder frames or multitube chassis!

A spaceframe is a triangulated structure that derives its structural worth from that triangulation. If you change the tubes in a spaceframe you will end up with a change in chassis weight, strength and stiffness.

For aluminium a tube would be a third of the weight but only a third as stiff or as strong as a steel tube. To get back the stiffness you could triple the wal thickness which restores the stiffness and strength but gives you the same weight as a steel chassis. No scope for improvement at all!

If you really want to make a stiff and light weight structure then you could try my mods as given in "kitcaranalysis" on the site but with the tubes in 18 gauge steel and the floor in 1mm steel. This will still be about 50% stiffer than the book chassis but should save 30 to 40 pounds.

Personally I think that the extra difficulties in welding 18 gauge as opposed to the standard and road proven 16 gauge might outweigh any advantages, not so much because of the build, but because if a job's harder to do it's also harder to do right so you're taking a bigger risk that the welds might be duff. Possibly brazing would be a better way to build an 18 gauge chassis than welding but it sounds as if you know more about welding than I do anyway!

Triton - 3/7/05 at 10:01 PM

Go for 18swg erw or better still use T45(big quids tho!) and tig that rather than using cymtriks mentioned you need to increase wall thickness if using ali so no point as weight goes up.
I seem to remember Martin Keenan(MK eng) made an ali Locost a few years back.


mark chandler - 3/7/05 at 10:11 PM

I cannot find your notes 'kitcaranalysis" have you a url ? Lots of people have talked about Ali but it seems hardly anyone has tried I,ll shelve that idea as I was worried about it work hardening and cracking around the suspension pickup points, was just looking for a bit of feedback.

The great thing about TIG welding is the precise control you get, its also very enjoyable (maybe I,m just sad) so a steel frame coming on with steel sides and floor as light as I can make it !

mangogrooveworkshop - 3/7/05 at 10:36 PM

Make us lot ali tanks and sell em then you can buy subk2002 mk for cash.......

mark chandler - 3/7/05 at 10:44 PM

I do not mind welding someones tank for a drink, as long as the metal is folded so all I have to do is stick it togethe.

Probally more money in a budget reverse gear for a bike engined car I think I can sort out something with virtually no backlash for 100 would that be more interesting (good for 200 BHP).

Anyway I thought the joy was the building, anyone with cash can go fast... its the going fast without money that the key.

Volvorsport - 3/7/05 at 11:00 PM

i dont think you can do it for 1000 - if its only trackdays , maybe , but youd have to trailer it, or go thro sva to drive , and it gets a bit more complicated , what you going to do for bodywork etc ?

bimbleuk - 4/7/05 at 04:59 AM

I know of one Seven style kit car made from all aluminium and it weighs in at less than 400 kg!

However its track only and worse its been left standing in a garage since it was built

Dave Ashurst - 4/7/05 at 07:25 AM

No I'm no expert in design with aluminium, but:

Unless you REALLY know what you are doing with your fatigue design calculations and practical fatigue detailing I suggest it would be safer NOT to use aluminium for a car chassis.

Of course it's possible to MAKE an aluminium chassis. You might be good at TIG, enjoy it and (rightfully) take great pride in it. Depending on design the aluminium frame you make might be strong enough based on a 3d spaceframe static analysis and it might be strong enough based on detailed dynamic analysis.

The problem with Aluminium is FATIGUE:
- aluminium has a low cutoff stress threshold
- aluminium has a severely limited fatigue life in use under dynamic loading, compared to steel.

i.e. If you design for strength alone, your aluminium car chassis WILL break in use due to fatigue; and potentially quite soon.

So if you don't design for fatigue then it will break. The questions are :

Where will it break?
When will it break?
How will you spot the signs? and
Will you spot them early enough?

(in my opinion.)


mark chandler - 4/7/05 at 08:20 AM

Just picked up a fireblade engine for 400 on Ebay last night so commited.

Steel for the frame - 150, ali - 50
Shocks - 300 (GAZ)
Donor sierra - 50
Prop adaptor 75 (ebay)
Reverse gear modified LT230 transfer box.

I currently race a landrover so have a trailer and loads of useful bits like trackrod ends, lights indicators etc may even be able to flog the engine and box off the sierra and recoup a bit.

Okay a nice set of wheels and an screen will be on top, but I am hopeful of getting a driveable car for my 1K.

David Jenkins - 4/7/05 at 08:44 AM

The other problem to add to Dave's list is that Ali doesn't just work-harden and develop stress cracks - it also age-hardens all by itself. Anneal a piece of ali, leave it for a year or so, then try bending it.

I have some pieces of old ali sheet in my garage that have to be annealled before I can attempt a 90-degree bend, otherwise they just crack straight along the bend.

Some grades of ali might be better than others (there are some aircraft grades that are alloyed with stuff like copper) but ordinary commercial grades have to be treated with suspicion.

Anyway, in most cases where weight is an issue, the highest gains are usually found by reducing the mass of the soft, squishy thing holding the steering wheel!
...certainly in my case...


britishtrident - 4/7/05 at 09:51 AM

Generally a steel spaceframe is the lightest method of construction, when I was racing in 1000cc specials sallons/GT I watched the weights tumble, it was reckoned the best Maguire spaceframe Imps tipped the scales at little more than 600 pounds ready to race, this may have been paddock psyco hype but I know they were a lot lighter than the Davrian monocoque cars.

awinter - 4/7/05 at 12:17 PM

Just out of interest, they have been building mountain bike out of Ali. These have the benifits of being lighter and alot stiffer than a simalar steel bike. If the long term durability of ali was a problem why would a manufacturer choose to build an aluminium fame bike? Think of the loading imposed on the front of the frame at the head set and bottom bracket where the chainset runs through the frame.
The frames do need to use oversize tube to retain strenght but are still lighter and stiffer than a steel frame. Is the increase in tube size the key to getting durability and the fact that the aluminium age hardens over time then a benifit?
I don't know much about this but would be interested in how this progresses as this would have been something I would be interested in if I had more time.

andkilde - 4/7/05 at 03:52 PM

Originally posted by awinter
If the long term durability of ali was a problem why would a manufacturer choose to build an aluminium fame bike?

Ali bicycles do suffer from fatgue cracking and failure Allan, particularly at the head tube join, and bicycles don't have to deal with the vibration of an engine or the sort of mileage that can be racked up in a car.

You have hit the nail on the head though, in order to get the strength they need they go far bigger on the tube diameter.

There are all sorts of aluminium alloys, all with differing hardness and fatigue characteristics. If you poke around a bit on the net you'll be able to find out about "age hardening", "artificial aging" "tempering" and a number of other things done to Ali to improve it's strength and toughness.

The one thing steel has over Ali is that steel has vastly better fatigue resistance in its non-heat-treated form. So, from a Locoster's standpoint, it's far more forgiving to the things we do to it, welding, forming etc.

If you have the engineering skills, access to proper heat treating facilities and the desire, I'm sure an acceptable Ali frame could be built.

Cheers, Ted

britishtrident - 4/7/05 at 05:44 PM

One important difference is that a bike frame is of a size that can be heat treated as a whole unit after welding.

rocket - 4/7/05 at 09:24 PM

Alot of bikes use magnesium rich alloys. thats the key.

I'm building an 18g frame which will have ali underfloor. so it should be light.

All in all I would guess something like 20 kg. it's easy to weld ( I'm a fabricator) unless you have a really cheap mig that has to run full up just to arc up, or you are using stick and are a novice.

But just to throw a spanner in the works look at delta box frame on super bikes?

These guys have been using 5% mag alloy for years no probs.

Lotus elise? even has alloy brake discs!!

the only argument against an alloy frame is impact.

When they tested the elise they found ali weld is strong until it cracks then it goes v. week and falls apart. When impact testing they found glueing and riveting was better in a crash, this was because the joint moved and absorbed impact instead of cracking.

The best frame would be titamium but is too expensive.

Best idea is to save weight some where else!

But then again. . . by the time you have put wilwood calipers alloy hubs, magnesium bell housing light wieght racing wheels blah blah blah thats over 1000.

So build a titanium frame then fit the normal stuff on!!

Or if you have abudget of around 8000 do both!!



cymtriks - 4/7/05 at 10:00 PM

Mark, try
kitcaranalysis is in there!

Volvorsport - 4/7/05 at 10:54 PM

an ali frame will be better in an accident on a strength for strength basis .

RallyHarry - 5/7/05 at 05:17 AM

How expensive is titanium ?, or is it the fabrication that's expensive ?
The good part will be that small dents will flex back to original shape !

And not least, is the main properties right ?


RallyHarry - 5/7/05 at 05:29 AM

To answer my own question:
Link to mountainbike frame materials

In short:
"Look at the chart again. You'll see that identical steel vs titanium frames would be about equal in strength, but that the titanium frame would be about half the weight and half the stiffness.

Such a frame would likely have a whippy feel due to the reduced stiffness, especially in loaded touring applications. To compensate, builders of titanium frames use somewhat larger diameter tubes to bring the stiffness more into line with what riders like. This tends to increase the weight a bit, but by making the walls of the larger tubes a bit thinner, they can compensate to some extent, and come up with a frame that is still lighter than a normal steel frame. "


rocket - 5/7/05 at 09:14 AM

It approx 3 times more expensive than 316 st/st ( if purchased in quantity)



rocket - 5/7/05 at 09:20 AM

Originally posted by Volvorsport
an ali frame will be better in an accident on a strength for strength basis .

not if it's welded together.

gazza285 - 5/7/05 at 09:22 AM

Originally posted by RallyHarry
How expensive is titanium ?,

"How to build a sports car for 25,000"

rocket - 5/7/05 at 09:25 AM

JAD - 5/7/05 at 03:07 PM

Porsche used aluminum tubing for their 917 types; however, due to cracking of the welds (or heat affected zone) they had to pressurize the tubing with gas. All tubing intersections has drilled holes so that a common path was created for the pressurized gas. A pressure gage was welded into the frame and when the pressure dropped, back to the factory with the frame to repair the cracks.

silex - 5/7/05 at 03:59 PM

Well you could always opt for little machined corner joints that the tubes would slide into - bonded in place of course so as to eliminate weld stress.

It would look like a Knex kit, but foe big boys

crbrlfrost - 5/7/05 at 04:32 PM

Aluminium can have a descent fatigue life, if the alloy is carefully choosen and the loads limited to the lower slope of the fatigue curve, kind of like a 1/x graph. Anyway, the whole point of using a lighter and lower IE materials is so it can be better utilized by increasing distance between loaded surfaces, increasing the moment arm in the materials favor. So it wouldn't really make sense to weld together small AL tubes when the materials could be put to better use with larger thinwall sections, hence the use of big AL extrusions by the manufacturers. Titanium has the same issues aluminium does in regards to weight/stiffness, but it has better high temperature properties and is a pain to fabricate, not to mention price (molten Ti eats through investment tooling and casting equipment, drives up manufacturing costs, oh, and has to be done under vacuum). Anyway, to make a short story long, I'd have to say either go steel spaceframe, aluminium monocoque, or s happy mixture of the two. Cheers!

mark chandler - 5/7/05 at 11:07 PM


Thanks for the many posts, my mind is now made up, I will follow the recommendations as per kitcaranalysis.doc in 18swg steel.

First task is build a table and carefully mark out as per the book and chassis corrections.... its all getting confusing but the setting out should clarify things. I,ve sourced my cortina hubs now, just got to collect the IRS and worry about reverse and engine position.

Cheers Mark

rocket - 6/7/05 at 08:07 AM

Popular miss conception about titanium being welded in a vacuum!!

It doesn't !



NS Dev - 6/7/05 at 12:13 PM

Rocket, you missed the point a bit I think!

He was saying it has to be CAST under vacuum, which is entirely true unless you want a casting with the properties of a sponge.

It has to be welded in a pure inert gas atmosphere though (the setups I have seen use old baby incubators filled with argon) which is still hardly convenient!!

RallyHarry - 7/7/05 at 06:06 AM

It'll really be a small car then if it'll fit in a baby incubator, small car would mean small cost though.


rocket - 7/7/05 at 08:14 AM

smart51 - 7/7/05 at 11:25 AM

Titanium is a fantastic engineering material but diamond is better. A diamond cored carbon fibre laminate with titanium fixing brackets would make the best chassis with magnesium-polypropylene body panels.

For a locost that is built in your own home, midl steel, aluminium sheet and GRP are your best friends.

rocket - 8/7/05 at 08:59 AM

diamond would be too heavy!!

NS Dev - 8/7/05 at 09:19 AM

what's wrong with good ol' wood?

I thought my oak framed special with contiboard cladding looked quite smart!

smart51 - 8/7/05 at 09:23 AM

diamond is light - it's made of carbon. It's lighter than aluminium, just.
I was thinking of using depleted uranium with a riven granite dash but that would be quite heavy. Those of you using pinto engines might want to consider it as it will hardly affect the car's overall weight

smart51 - 8/7/05 at 09:26 AM

A wooden car. Fantastic. You could have a Diamuid Gavin inspired one made out of used railway sleepers! A bit of teak oil and it would look beautiful. It might even float.

NS Dev - 8/7/05 at 11:10 AM

I think I've seen the future!!!

Wouldn't even need too much cuprinol if you used railway sleepers. Telegraph poles aren't bad either!!

Thought about using cardboard cladding for the bodywork too.

MikeR - 8/7/05 at 11:11 AM

So thats how the Gibbs Aquida works ...... now i know where all the railway sleepers have gone in Nuneaton !!!!!!!

geoffreyh - 28/12/05 at 11:14 PM

If you are looking for someone who did it already try the following link



rocket - 29/12/05 at 05:51 PM

It's not worth it all that trouble and expense for a 620kg car!!!!!!

My chassis is st/st and I won't be going past 550kg with a sierra base and zetec engine!!!

Probly be cracked in a year or so ( like mine)



geoffreyh - 29/12/05 at 05:56 PM

Maybe Aluminium tubing is not the best solution but by plating the chassis you get a very stiff chassis. A stiff chassis can eventually cope with more engine forces.

I think the building idea is not so bad.