Butted aluminium tubes - Why?

[DaveP]

I've noticed an aluminium bike frame that is available in two versions, using plain or butted tubes.
The butted version weighs a bit less and costs a bit more - no surprises there. However it is also claimed to offer a more comfortable ride.
I can accept that, but don't understand how this might be. I thought the secret of aluminium fame design was rigidity first and last. If anything I would have expected the plain tubes to do a little better at damping out high frequency vibrations. But what do I know
Can anyone shed a little light?

[pwa]

I think the theory is that the thinner walled middle sections of a butted tube have a little more flex as you go over bumps. I am a bit wary of the idea of aluminium flexing, so I don't trust it to do that without eventually snapping. But others will disagree. I expect the comfort claim to be true, but the effect will be less than you might expect from a change of tyres or saddle.

[Vorpal]

Welding requires thicker tubes and also affects the material properties in the area of of the weld. Both tend to make the frame stiffer, whatever the frame material.

[Mick F]

I had a Vitus Duralinox 979 aluminium frame for a while. Since sold.
Superbly light and the ride was delightful.

[Vantage]

Frame failures tend to happen at the joints where stress is concentrated. The middle section of the tubes can be thinner walled saving weight without losing strength.

[Valbrona]

Marketing spiel, that. I can't imagine an aluminium frame with butted tubes being anymore comfortable than a frame with PG tubes. If there is a difference, it will be in terms of weight.

[Brucey]

butted tubes can be used to make the frame lighter, stronger or stiffer, or indeed some combination of these qualities may arise.

It is even possible to make the frame lighter, stronger, and yet slightly less stiff overall; this is because the strength is limited by the stresses near the tube ends; these can be thickened (so stronger) but the weight can be more than made up by losing weight near the tube centres, where the stresses are lower. Thus the frame can be lighter. The average stress in the frame tubes will likely increase, thus it will (at the same tube diameters) most likely become slightly less stiff.

But having said this if you are looking at a modern aluminium frame it'll have oversized tubes and will most likely be ridiculously stiff with or without butted tubes. If you can tell the difference on a test ride I'd be surprised.

cheers

[al_yrpal]

Easier to weld the thicker sections and the joints are usually where the maximum stresses are too.

Al

[Bonefishblues]

I had a Vitus Duralinox 979 aluminium frame for a while. Since sold.
Superbly light and the ride was delightful.

They had a reputation as being rather flexible IIRC - how did it ride in terms of lateral stiffness?

[Brucey]

I had a Vitus Duralinox 979 aluminium frame for a while. Since sold.
Superbly light and the ride was delightful.

They had a reputation as being rather flexible IIRC - how did it ride in terms of lateral stiffness?

I had a similar one, and honestly, it was like riding a wet noodle when you were honking and giving it some.

I was used to steel frames and on these (even with carefully chosen high quality gear) I could normally make the front mech rub both sides on the big ring. On the vitus the rubbing was far worse and in fact I had to be careful that the chain didn't actually unship. I couldn't surprise anyone with a sneak attack on the Vitus; they would always hear me coming.

So to this day it is a source of mystery to me that the likes of Sean Kelly rode similar frames without having endless problems of various kinds.

The frame was pleasant to ride on when you were just tapping along, and it was about 1/2lb lighter than a good steel frameset. Back then I worried that it was actually slowing me down because it was so flexy, so after a few years it went. A chum still has it and uses it, and I am no longer so sure that it did actually slow me down.

cheers

[NATURAL ANKLING]

Hi,
Which ones had bonded tube to lugs (vitus)
I am thinking that stiff aluminium frames were because cutting down flex was a necessity due to stress fracture

Edited -
Which reminds me to do The Brucey Inpsection on my Diamond Back MTB, riden for a year 3500 miles 20 % off road with all up weight 96 kgs but down hill forks

[Keezx]

The first aluminium frames were made with the same dimensions as steel frames, but thicker walls. (ALAN, Vitus)
They were lighter than the steel ones but very weak because Young's Modulus is 1/3 of steel.
To increase the stiffness but keep the weight low it was necessary to increase diameters and take away material on places where it could> butted.
A non-butted oversized aluminium frame is just as heavy as a steel one.

[Mick F]

I had a Vitus Duralinox 979 aluminium frame for a while. Since sold.
Superbly light and the ride was delightful.

They had a reputation as being rather flexible IIRC - how did it ride in terms of lateral stiffness?

Beautiful ride. Really enjoyed it, but it was a bit small for me.

As for stiffness, yes it was "flexible" but it suited my style of riding very well. It was sheer delight to ride. I think the forks flexed quite a bit, so that might have been a lot to do with the soft ride.

Comparing the lateral stiffness of it to my 531c, there wasn't a great deal in it. I suppose a good test is to stand the bike up holding the saddle and the stem with the nearest pedal at the bottom of the stroke. Push the pedal with a foot, and watch the frame bend.

The Vitus wasn't much different to my 531c, though did bend more easily, but not much.

Sorry, never measured the flex.

[NATURAL ANKLING]

Hi,
http://en.wikipedia.org/wiki/Vitus_(bicycles)
"Vitus is a French bicycle manufacturer best known for its steel cycle frame tubing, and its frames built with aluminium tubes joined to aluminium lugs by bonding - a construction method the company pioneered in the late 1970s.[1"


http://www.rodbikes.com/articles/material-world.html
"These frames resembled conventional frames but had anodized finishes instead of paint and were very light weight (in comparison to the steel frames of that day). The ride was very soft, and worked best for riders who were of lighter weight, and rode with very high cadences. "

"Eventually, these oversized aluminum beasts gained a reputation of being so stiff that they were uncomfortable to ride. This was my experience when I commuted almost 40 miles 3 days per week on one of these frames for nearly 2 years. Soon, riders were running for the Ibuprofen, and scheduling appointments with their bike fitters."[/i]

[i]"DO CARBON FRAMES LAST?
Yes. [Our] frames are guaranteed for five years of racing and training use. When the primary consideration is performance, carbon is the only choice. If you really need your frame to last for fifty years, buy a steel one."

http://www.velocipedesalon.com/forum/f1 ... 27307.html

[foxyrider]

The reason for the extra comfort claimed is actually quite simple and it applies not just to drawn tube aluminium but to steel as well. Your comfort is directly affected by the transference of vibration through the material, vibration travels easiest in straight lines through thick material eg a plain gauge tube. A change of gauge (ie butt) will disrupt the vibration providing a 'damping' effect and multiple butting will enhance the effect.

You might see references to 'continuous butted' tubing, hydroform etc - the wall thickness of these tubes can be made thinner due to a better material matrix and the variable wall thickness that is designed into them puts material where its needed much in the same way that carbon fibre is used. I can almost guarantee that anyone would feel the difference over a reasonable distance ride when compared to a drawn tube frame.

Of course some Al bike frames will use a combination of drawn and cast tubing and just because its cast doesn't neccesarily mean its lighter.

The stuff about weight and welding, whilst part of the story, are certainly not the whole tale.