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**Features

Lightweight bikes

In the mountain bike industry the word “lightweight” is considered by some to be a swear word. Visions of buckled wheels and snapped handlebars spring to mind. Lightweight equates to the “single season only” hardtails raced by the world’s elite. An elite that also happen to be pretty damn lightweight themselves and carry a truck full of mechanics to repair whatever they break. Surely a sub-20lb XC race bike or a pair of 125g handlebars just can’t hack it in the neglect and abuse of real mountain biking?
Unfortunately in order to even enter into the world of cycling you have to enter into the realm of ultra-lightweight structures. The bicycle wheel is perhaps the most efficient structure ever constructed by man, in comparison family cars are light years behind in terms of strength to weight ratio. Believe me, even a 750kg F1 car capable of over 200mph is nothing compared to a 12kg vehicle capable of 40mph whilst carrying a payload of 100kg.
If mountain bikes were designed with safety margins first and weight targets second, you would probably be unable to lift them! However beefy or gusseted your steed, however DH orientated or whether it can survive a landing from 10 feet, it is still an ultra lightweight structure. And this is why so many problems arise.


How it all went wrong

Back in the early nineties MTB design went lightweight crazy. Suddenly no bike could weigh more than 24lbs, and bars were down to 120g. Within a few months frames were cracked, chins were gushing blood from snapped handlebars and punctures were rife. By the mid nineties alloy spoke nipples, latex tubes, featherweight components and bolt replacement kits were all assigned to history.
But wait a moment, I still to this day use aluminium bolts, latex inner tubes, alloy spoke nipples and 140g handlebars, and I weigh 100kg and regularly do multi-day rides. I have had no problems what-so-ever with these (what would be considered today) “dodgy” components. The trouble came because the MTB industry in its endless search for profit wanted to introduce all these “trick” weight saving products at minimal cost to themselves. So we got nasty lightweight inner tubes that split, soft alloy nipples that rounded off, and handlebars that had the same wall thickness as those £60 ones, but were made with cheaper alloy and poor quality control for only £6. The final nail in the lightweight coffin was extortionately priced CNCed cranks and rear mechs that snapped before you got them half way up the nearest farm track. Cheap and nasty is bad enough, but expensive and nasty is the ultimate insult. I use DT alloy spoke nipples, Continental latex tubes, and ITM handlebars, and I’ll say no more about that.


Beefy is better?

If weight equalled strength then life would be simple. A heavier bike would be stronger – and many mountain bikers still seem to believe this. More weight equals more metal equals more strength, right? Wrong! If this were true then why does the £100 two tonne ATB that the kid next door owns simply fall apart at the merest hint of offroad? Why does my 24lb bike shrug off everything I throw at it, unlike my first 30lb+ bike on which I bust almost every component before it celebrated its first birthday? Because expensive bikes are made from stronger, more expensive materials perhaps. So does light and expensive work, but heavy and cheap doesn’t? Well, things are even more complex than that.
Many companies enter mountain bike design without the knowledge required to design a tea pot, never mind a critical component such as a frame or handlebar. The lure of quick profits seems to blind them of the fact that a snapped component can equal a scarred face for life, or worse.
A handlebar is simply a slightly bent piece of tubing with a bulge in the centre. Designing a handlebar would be child’s play if it weren’t for the fact that they are supposed to weigh around 150g. Suddenly, designing a handlebar becomes an incredibly complex task.
But who is to say that any bar less than 150g is unsafe, or that carbon bars snap, or titanium bars are best? One company’s 95g carbon bar might (and is) stronger than another company’s 350g braced DH riser bar. The riser bar has to be made from a weaker alloy to allow the double bend to be formed, the bend itself also weakens the bar structurally, so a brace has to be added. The brace is welded in place which further weakens the bar at exactly the same place as the bend weakens it – oooops! Catch my drift?


Less is more

There is no substitute for knowledge when building bike bits, and unbelievably low weight can be achieved with perfect safety – but only by companies who know what they are doing. These are also not necessarily the flashiest or most expensive components around, nor the ones which shun far east manufacturing. Even tripling the weight of a component, and adding untold extra gussets will never compensate for an initially poor design. Bizarre as it may seem, adding weight to a component can make it weaker, and removing it can make it stronger. Also a flexible component can often be stronger than a stiff component of the same weight. The secret is not the amount of metal you use, but in placing the metal (or composite) in exactly the right places.
A classic example of this is double butting. Double butted frame tubes and spokes are not only lighter but stronger than plain gauge ones. This is because by making the central sections thinner, the highest stress levels are moved there, away from the complex (and usually impossible to calculate) stress concentrations at the welds / threads. Also, as the whole structure becomes more elastic and flexible, stress levels are further reduced at the ends. How often do you see a frame tube crack NOT at a weld, or a spoke snap in the middle?


Smoothly does it

The significance of the old saying “form follows function” is often underestimated. Many mountain bikers believe that the flowing lines and smooth curves of racing bike components are Italian aesthetics over-riding engineering common sense. After-all engineering is about big ugly welds, CNC machined cut-outs, girders and rivets. Actually the reverse is true. Those Italian road components are actually stronger because they have fluid flowing lines that reduce stress concentrations. Even the highly polished finish serves a structural purpose, by eliminating the tiny surface scratches that can initiate fatigue cracks. With the “terminator” look being all the rage at the moment, you may like to know that maximum strength is achieved by using the minimum amount of welding, some recent frames seem to be comprised almost entirely of weld alone! Another example is that Easton recommends that its tubesets are NOT used with external gussets, and as far as those expensive CNC machined ‘girder’ stems are concerned – bin them before they kill you!
The strongest shape for tubing is round and straight. Curved tubes are only produced to provide clearances and/or deliberately make the frame more flexible (e.g. Ritchey plexus or Cannondale seat stays). Non-round tubes are only produced to maximise the weldable joint area or to simply look good.
Even our best UK manufacturers can be guilty of severely weakening their products by machining their name into a stressed area. If you can CNC your company logo 3mm deep on a product, then you can probably completely remove 5mm of metal from that surface, making a much lighter product, and laser engrave your name on it!


Aerospace titanium fibre matrix composite
(as used on the space shuttle)

Yet another fallacy of the (mainly USA) bike scene is that exotic materials can save the day. If you want a lighter bottom bracket, then simply make it from titanium, as everyone knows that titanium is a wonder metal which is lighter, stronger, more fatigue resist and sexier than boring old steel. Well, many mountain bikers now have scar tissue on their calves where broken titanium bottom bracket axles dug into their flesh. Titanium actually has almost exactly the same strength to weight ratio as a high quality steel. Therefore, if you make a bottom bracket axle from titanium it will be lighter than a steel one, but only because it is weaker and less stiff than a steel one. Oooops again!
Campagnolo learned this lesson back in the mid eighties: that titanium is simply another choice of material, not a wonder metal. The MTB world ignored their painful lessons almost with contempt, and they decided to make these mistakes all over again. Wouldn’t it be much easier to simply redesign a steel axle with a spline so that you can safely drill a bigger hole in the centre – doh! Similarly, steel sprockets mounted on an aluminium spider have done far more to reduce cassette weight than titanium cogs ever did.
The only materials with a huge strength-to-weight advantage are composites, but these are many times more difficult to work with than metals, which actually reinforces the fact that expensive materials are no substitute for good design. So, when buying a component produced from the black stuff, careful choice of manufacturer is even more critical than with a metal component.


How light is too light?

If a designer knows what he/she is doing, then almost any weight is possible. The solution does not lie in unnecessary extra welding, badly executed gussets and braces, nor CNC machining from billet. Fatigue and strength tests published in the German press, show that it is nearly always the lightest components that are the strongest, and vice versa. Why? Because manufacturer X knows exactly what he is doing and manufacturer Y couldn’t engineer his way out of a paper bag!
In summary, bikes can and will get much lighter than they are today, and probably without any loss in strength. You must believe that a safe sub 100g handlebar is possible, or a 300g crank – just because most companies can’t produce them doesn’t mean that no-one can. Don’t be scared to use ultra-lightweight components even on the most brutal of rides – provided it is from a manufacturer that you trust. Easton carbon riser bars weigh only 150g but I’ve never heard of any snapping. The weight of road bike forks has dropped from around 700g to almost 300g in recent years without any increase in breakages.
As final proof I’d like to let to you know of a German company, Tune, who are undoubtedly the best lightweight component manufacturer in the world. They produce 185g rear hubs which are strong enough to be used in the Transalp, and 125g bars which are used in world class downhilling. So exactly how light is too light?
Kevin Hodgson
January 2001

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