Bicycling Australia
This article grew out of the question ‘why is a ten-thousand-dollar bike better than a one-thousand-dollar bike, and why does it cost more?’
I decided to try to answer this question in the big sense what goes into designing and manufacturing a bike frame from a clean sheet of paper to a finished product at your local bike shop? Every frame builder from the six-frames-per-year type up to the industry leading corporation has similar issues to deal with. This is part two of a three part series on these bicycle design issues. I realise not everyone will have read the first part (see the September/October issue), so I will start by summarising part one. Once everyone has some idea of what was in part one, I will dive into the meat of part two, bicycle frame geometry.
The big issues from part one were:
- The bike’s intended use (eg, touring, racing, BMX). This is an obvious one in that a road racing bike is very different from a downhill racing bike, even though they are both intended for competition, and they both differ from a touring bike.
- Its intended price (eg, entry level or cost-no-object). This is also quite obvious but from a compromise point of view, the compromises required in a top-of-the-line bicycle are few and those in an entry level bicycle are many.
- What it will be made out of (eg, steel, titanium, carbon). Frame material is quite era specific. Cheap and expensive bikes were virtually all steel 20 years ago and then aluminium was the material of choice for the full spectrum of prices about 10 years ago. Until recently carbon could only be found in big dollar bicycles. Perhaps in a few years even entry level bikes will be displaying full-carbon-goodness.
- How much it will weigh (eg, not important or lightest-on-the-market)?
Building to a design weight—especially when that is around 800 grams for a frame—is probably one of the most challenging assignments in cycling today. Witness the small array of choices at this weight. Even low-end bikes have to be competitively light with their price-point competitors, making weight an issue for virtually all designs.
- What the builder wants it to look like (eg, compact, traditional, or oddball).
In every era there are examples of bikes that just look different.
My final consideration in part one—how long the frame must last before failure (ie, a DH bike is very different from a hybrid)—is a potentially contentious issue. I tend towards the conservative in design (if in doubt make it a bit heavier), and towards liberalism in warranty (a long warranty is a realistic assessment of frame breakage trends in well designed bikes). This is written with the view that anyone reading this article is likely to be in a small minority amongst the global cycling community not only are you reading a bicycle magazine, but you are reading a long article on frame design issues. Most of you will be real bike riders, whether it is Audax events or racing or just riding with mates you are in a minority if you put some real miles on your bike. Remember, if you sit your bike in the shed it is never going to break.In my years in the bike industry, I participated in the design of many bike frames. My involvement has always been in the finer details of frame design—how will the frame fit a human body and how well will it be suited to a particular use. Collectively these are usually referred to as frame geometry, but for today I am going to divide geometry into two sections: bike fit and bike handling.
Bike Fit
Anyone who has shopped for a bike will realise that a 56cm frame from company A is not always the same as a 56cm frame from company B. First of all, A might measure its frames differently than B. Centre-to centre is the most normal method but not the only one. This refers to measuring the frame size from the centre of the bottom bracket shell to the centre of the top tube. When all bikes had horizontal top tubes, this number was much more meaningful than it is for compact frames. Some companies measure centre-to-top, but the top of what? Top of seat collar or top of top tube are both valid choices. At least one company measures from the top of the bottom bracket shell because it is easier to measure to a physical place than to estimate the middle of a big hole. The implication is that exactly the same bike frame that measures 56cm centre-to-centre will be a 53cm frame by the top-to centre method and a 59cm by the centre- to-top method. Beware!
Compact frames can be measured by their physical size. An example is my racing bike, a Scott, which is semi-compact. Listed as a 61cm frame, it is this size from centre-to-top. It is only 57cm centre-to-centre. But it fits the same as my custom-made track bike which is 64cm centre to- top with a reverse sloping top tube (the opposite of a compact frame, the top tube slopes down from the seat to the bars). Compacts can also be designated by shirt sizes—my Scott is also called an XXL frame. Compact frames vary just as much as traditional frames; a Medium from one company can be very different from another’s Medium. I have seen too many people get caught by all this when shopping for a new ride. The bottom line is that you really need to pay attention to the fit of every unique brand, and sometimes each model within a brand, that you are considering for purchase.
If I had to divide the industry into two schools of frame sizing, on the one side would be those companies that use a single seat tube angle for every size in the range. This also includes those who steepen the frame angle for the very small frames but have a constant angle for 52cm or larger. The paradigm behind this is that everything scales up correctly so you don’t have to change the angles (the little frames are an exception to lessen toe overlap with the front wheel). On the opposite side are those companies that slacken the seat angle for each increase in frame size. Such as when the 50cm frame has a 74˚ seat tube while the 60cm has a 72˚ seat tube. Frame-builder and author Lennard Zinn attributes this paradigm to the fact that crank lengths cannot scale up with leg length—there simply are not enough choices on the market. Whichever way a frame is designed, the desire is to have the rider’s relationship between backside and feet (seat and pedals) correct for the job of pedalling.
The third point of contact between bicycle and rider is the handlebars, and their location is controlled primarily by the top tube length. Seat angle and top tube length are interrelated measures. That is, as you change the seat angle you also change the effective top tube length for any specific rider. If your ideal position is well behind the bottom bracket and you try this on a steep angle frame with a big layback post, then you are adding effective top tube to the bike. Obviously the top tube doesn’t change its actual length, but the reach to the bars grows as you slide the seat back on the rails.
The final fit aspect is head tube length. In a traditional frame, with a horizontal top tube, the height of the head tube is set by the frame size. In a compact frame, with a sloping top tube, the head tube can be as long as it needs to be. In a compact the head tubes are roughly the same length as would be found in a comparable horizontal top tube frame. Altering bottom bracket height will have an impact on effective head tube length, but this rarely moves very much in a road bike so it can usually be ignored.
Time trial frames usually throw all these norms out the window in pursuit of aerodynamic efficiency of the rider’s body. Once upon a time, even quite recently, most good pro TT riders had quite slack seat angles. It was the tri influence that introduced five degree steeper geometries to the industry. But look at Dave Zabriskie during his stage win at last year’s Tour de France on his Cervelo—I’d bet that he has his saddle setback right on the 5cm minimum stipulated by the UCI and he is barely sitting on the nose. His effective seat angle is very steep. His whole body has been rotated forward around the bottom bracket, which is not a position everyone likes, or even tolerates. But no one can argue with Zabriskie’s speed. Chris Boardman was one of the first cyclists to come to fame using a forward rotated aero-position when he won the Gold medal at the Barcelona Olympics.
To summarise what I’ve said so far, the feet are anchored by the bottom bracket via crank length. The seat angle and height determine where the cyclist’s backside will be in relation to the feet. Different seat posts and movement on the saddle rails allow a fair range of motion in this relationship. Top tube length works in conjunction with stem length and saddle position to fix the reach to the bars. Finally, head tube length and stem type modify the reach by setting bar height in relation to seat height. Get these three points of contact correct and it does not matter what the frame looks like from a fit point of view (though frame design does affect handling). I hope this demonstrated that designing the fit of a frame is more than just throwing a few numbers together and hoping for the best. Changing any one dimension be it angle or length, has flow-on impacts in other areas. Remember that fit is more important than brand allegiance, frame material or colour scheme in assisting a rider to obtain their best performance. If you cannot find a good fit in one brand or model, try several others until you find the right geometry for you. Custom is always the ultimate way to get a bike to fit you but there are obvious costs associated with this.
Bike Handling
The other components of bike geometry have minimal or no impact on fit, but a large impact on handling. Head angle coupled with fork offset determine trail. The appropriate trail depends on the wheel size—it is different for a 650C bike than it is for a 700C bike, and they are both different from a kid’s 20” wheel. There is a range of trails that most cyclists will deem acceptable but I know I prefer a trail measure that is smack in the middle of the range…neither too twitchy nor too stable. Even though they have a similar wheel size and roughly similar trail measurement, a sprinter’s track bike and a European city bike have incredibly dissimilar handling. The sprinter’s bike is stable when ridden out of the saddle at high speeds while the city bike is stable ridden in the saddle at low speed.
The difference here is mostly due to head angle. The steep angle on the track bike coupled with a low offset fork produces adequate trail and no unwanted lateral movement of the bike when standing. The slack angle on the city bike coupled with a large fork offset makes this bike move around side-to-side when standing, but it takes little effort to keep the bike going where you intend if you remain seated. You might not even notice it if I swapped the trail measurements around between the two bikes, but you cannot miss the results of changing the head angle so radically (perhaps eight degrees).
Cockpit length is an area that a designer can play with fit and handling at the same time. Going towards a shorter stem tends to speed up the steering feel while a longer stem tends to slow down the steering feel. It is no coincidence that so many pro bikes have a really long stem on them. When your office is a bicycle you don’t want a sneeze to dump you on your backside. Using an average length stem in a production bike permits a good fit for the most people, but a custom frame can be built to incorporate whatever handling traits you desire. The Genesis geometry of Gary Fisher mountain frames was a conscious paradigm shift from Gary— a longer top tube and really short stem to move the front wheel forwards and improve handling on steep hills.
On a TT frame I prefer to set them up just like a Fisher Genesis—a short stem and a long top tube (usually the same length as the rider’s regular road bike) pushing the front wheel forwards to get weight off the front wheel and go some way towards restoring handling balance on the bike. Many pros seem to compromise on their bar placement in order to slow the steering down with a long stem, even on their long top tube TT bikes. Chain stay length is just about the only dimension left to discuss. On a typical 2006 racing bike it is difficult to get a thumb between the seat tube and the tyre because the stays are very short (on the order of 40cm). On a typical 1976 racing bike it was possible to mount mudguards and 28mm tyres at the same time (the stays were closer to 42cm in length). My TT bike has 38cm stays and it is not possible to get a piece of paper between a 20mm tyre and the seat tube—a 23mm tyre is right out of possibility. I have never noticed a change in shifting prowess with altered chain stay length, nor does it have a large effect on manoeuvrability (shorter being somewhat more nimble). Short chain stays do feed rear wheel impacts more directly into your saddle.
More subtle than handling, ride quality belongs in this part of the story too. The stiffness of a round and straight tube is the same in all directions. Inexpensive metal tubing is the same thickness from one end to the other, and that thickness has to be able to withstand the heat of assembly. It was 1897 when Reynolds patented its process for butting steel tubes, making them thick at the ends to withstand production heat, but thin in the middle where they were not heated (making the whole bike considerably lighter). Things didn’t move forward all that much for nearly a century. If a metal tube is bent away from straight and round it will support more load in one direction, but less load in other directions. Early attempts to make metal bikes that were both stiff and comfortable were less successful than more recent technology has permitted. Carbon is an ideal medium for fine tuning ride characteristics. Each tube can be laid up to resist lateral flex providing solid response to the pedals while still permitting flex in the vertical direction to minimise road inputs to the rider. It is still a complicated (research and labour intensive, hence expensive) process to produce tubing (metal or carbon) that is differentially shaped internally, externally and in the butting. The result, however, is worth the effort.
As with every other aspect of frame design I have covered in these two instalments, the number of handling compromises required reduces as the investment in the frame’s development increases. For example, a short wheelbase for nimble handling once meant a rough ride. Now ride and wheelbase can be tuned independently with the tubing. In the final section of this article next time I will bring it all together by examining specifics involved in our theoretical $10,000 pro’s bike and our $1000 beginner’s bike.
