Bicycle Technology: Bikes for the Short and the Tall





Guide to Bicycle Technology (article index)

The sections of this last part of the guide are devoted to the perhaps 1% of all bicycles on the road that differ from the run of the mill. Here we shall cover the special bicycles, machines that are technically different for one or more reasons. In the present section, we shall cover machines for riders whose physical size makes off-the- peg bikes uncomfortable. In the remaining sections, we shall first look at the tandem, and then at the various other uncommon bicycle designs, followed in the last section by a tongue-in-cheek evaluation of the bike of the future.

15.1. The biggest and smallest bikes that can be considered ‘normal’: A youth’s bike with 24-inch wheels and a 45cm frame and a 28-inch frame on 700mm wheels.

The most common reason for choosing a bike that differs from the norm is that no standard design comfortably fits the particular rider. Long ignored by the industry and large segments of the bike trade, some riders just can’t find standard bikes to fit them. Often, the answer to the need for such a machine has simply been either to modify an existing bike somewhat, or at best to build a version that is either scaled up or down a little. Many women have problems finding a bike to fit. That is not only due to their overall size, but also to the fact that most bikes are still made with the typical male proportions in mind. Fig. 15.2 shows in what respects even an average female physique differs from that of an average male of the same size.

15.2. This illustration, based on one taken from Georgena Terry’s catalog, illustrates the typical differences between the physiques of the average man and the average women — even with the same overall body height. Clearly, their bicycles should be adapted to these differences. Compared to a male of the same overall height, the typical woman has: narrower shoulders, shorter trunk wider pelvis shorter arms smaller hands, longer legs, smaller feet.

Tall riders typically not only need seat and handlebars that are higher, they also need a frame that achieves these positions without requiring excessively long (and therefore flexible) extensions of seat post and handle bar stem. The length of the top tube should probably be longer than on an average bike. In addition to the frame, crank length (and consequently bottom bracket height) and quite a number of other items should also be adapted to the rider’s proportions. Achieving all this while maintaining proper steering and handling is no mean feat.

For short riders — be they small adults or children — the problems are at least as severe. Only too often, the height of the frame is all that is reduced, and that just isn’t enough: they need a shorter top tube while maintaining adequate steering tube length, shorter cranks (and consequently a lower bottom bracket) and quite a few other modifications.

Even the best intended attempts to accommodate short or tall riders often go amiss. To give an example, merely scaling down a regular bike to achieve a design with smaller wheels will likely result in insufficient trail for proper steering stability. In recent years, some manufacturers have taken these problems a little more seriously, with the result that more choices are avail able today — at least among high priced machines. Even when ordering a custom-built frame, it is good to consider the points that will be explained’ in this section, because not all frame builders have a thorough enough grasp of the ergonomic needs of small and big riders.

15.3. One way of making it big: This Gazelle fitness cycling frame has a long head tube for maximum stability and a slightly lowered top tube. Suitable for riders who are both tall and relatively heavy.

Often quite minor points may matter a lot. Take the brake cables for example: it is common to run them over the top of the top tube. But that adds about 6mm (¼”) to the effective top tube height when trying to straddle the frame. That does not seem like much, but a few minor points like that soon add up to a difference of 2.5cm — a whole inch, which is a lot when you are trying to minimize frame size for a short rider.

The Frame

Obviously the frame should be small for short riders. Yet that usually can’t be achieved by either shortening only the seat tube or by scaling all dimensions down. The first solution does not achieve the required shorter top tube length, nor does it address the need for an adequate head tube length — the most important criterion for overall frame rigidity and steering predict ability. The second solution can’t be carried out within the constraints of the required minimum clearances around parts such as the wheels.

One solution for a small bicycle is the design pioneered by frame builder Bill Boston, later successfully implemented and perfected by Georgena Terry in her production bicycles for women. This design uses a smaller front wheel, providing adequate head tube length even in the smallest frame sizes, as shown in Fig 15.10. Other methods that can be successfully used include using a steeper seat angle and a shallower head tube angle (providing the other steering dimensions are adjusted to suit).

For most tall riders, another problem should be ad dressed as well: not only must the frame be bigger overall, it also needs to be stronger. There are two reasons for this: in the first place, the longer tubes of the bigger frame (assuming top tube and down tube are also longer) tend to flex more under the same load; in the second place, the taller rider is likely to be heavier and stronger.

All this has to be achieved using standard wheel sizes, since larger ones are just not available. In addition, the distribution of weight and the correct steering geometry must be maintained. No mean feat, and numerous frame builders have had to disappoint their tall customers, some of whom got nearly unridable machines as a result of the vagaries of oversize frame design.

15.4. The effective frame size can be de creased by means of a sloping top tube. height reduction due to sloping top tube; minimum required head tube length; horizontal top tube

15.5. One good way to make a woman’s frame with a low top tube without sacrificing rigidity. This is achieved with additional bracing tubes between the rear axle and the point where the top tube meets the seat tube.

15.6 and 15.7. Two examples of suitable bracing of a large frame, using horizontal and diagonal bracing tubes for torsional rigidity.

15.8. Lowered effective frame height due to use of shallower head tube angle.

The longer frame length that may be required for a tall rider has to be compensated by means of an additional stiffening member. Fig. 15.6 and 15.7 show common solutions, which seems to be required when frames exceed a size of 65 cm (26). On the other hand, this adds more strength than stiffness, while both problems may be adequately addressed by running the seat stays in such a way that they cross the seat tube and tie into the top tube at a point forward of the seat lug, as shown in Fig. 15.9.

15.9. Frame with seat stays crossing the seat tube for increased rigidity.

Other methods to achieve increased frame strength and rigidity include the use of thick-wall and oversize tubing. Obviously, bikes for large riders should not be made with the lightest tubes, and there may be good reasons to choose mountain bike tubing with its greater diameters and wall thicknesses for such frames if they will be subjected to hard use.

Whether big or small, different size frames often involve different angles between the various tubes. Thus, the use of precision cast steel lugs is often not possible for such frames. Stamped plate lugs, though not as accurately dimensioned and often not as pretty, at least allow some bending without doing damage. If the difference becomes more than 3 degrees, a lugless construction should be used instead, so as not to stress the lugs excessively.

The Steering System

To steer a bike with a big frame properly, it has to be long enough so the rider is not too close to the steering axis. On small bikes, sufficient trail must be provided, even if the front wheel is smaller than usual, or if the head tube angle is different. This means that the fork rake should not be the same on bikes with different wheel sizes or with different head tube angles. A small front wheel requires less rake, as does a steeper head tube angle, to achieve the same trail. The effect of wheel size on steering stability was discussed in section 6.

Racing type handlebars appear to be readily available to fit any male gorilla, but to find one that matches a small rider with relatively narrow body build may be next to impossible. The correct posture keeps the arms parallel if the bars are held at the drops. There are some children’s size bars that may do the trick, but very few of them seem to be available in the quality that is justified in the price category where one lands when getting a custom built bike. Mountain bike handlebars can at least be cut short, assuming they are not bent too far from the center. Even the handlebar diameter should be reduced for riders with small hands.

The Seat

Very small or big riders often differ in that part of the anatomy that contacts the seat. Although there are now quite a number of reasonably comfortable women’s seats available, people who are both heavy and broadly built may have difficulty finding something to match. Although it looks weird on an otherwise fine racing bike, the kind of leather saddle with spiral springs in the (very wide) back seems to be the only comfortable solution for many women and quite a few males.

For big riders, the seatpost may have to be rather long if the frame is not perfectly matched to the leg length. In recent years, very long seatposts have become avail able — but only in diameters to match the typical in side diameters of mountain bike seat tubes, which are slightly smaller than those on most racing and touring bikes. It is possible to install a shim around the seat- post in the seat lug, but this had better be quite long: at least 10cm (4”) are required for adequately secure clamping.

15.10. Lowered top tube on account of small front wheel.

The Wheels

Normal wheel sizes for adult bikes are nominally 26, 27 and 28 inch (again nominally: 650, 675 and 700mm). In reality, these usually do not differ very much from each other, due to the fact that the nominally smaller wheels are frequently equipped with wider tires, the larger ones with very narrow tires. As we’ve seen in Section 8, wheels called 700mm (28”) actually have smaller rims than those referred to as 675mm (27”).

15.11. Small riders probably also need smaller handlebar grips, such as Gary Fisher’s design (top).

For small riders, there are some 26” wheels available with narrow 1 1/4” tires. This size is still readily available in France, Italy and to a lesser extent in Britain, while the are virtually impossible to find in the US. The next step down is the 24” wheel, available both with relatively wide tires, as used on so-called dirt bikes and some mountain bike rear wheels, and with moderate tire widths.

Even more serious than the tire situation is that of the rims. In the US, it is often very hard to obtain strong aluminum rims in any size other than those that fit 27” and 700mm ten-speed or 26” mountain bike tires. When my children were so small that their bikes required 22, 24 and 26” tires, I stocked up on those rims and tires on a trip to Italy, but I can’t seriously suggest you go there to buy rims and tires on a regular basis. Most of these smaller rims come with fewer than the standard number of 36 spoke holes: anything from 24 to 32. That means you also need matching hubs, which are hard to find, although they are now more readily available — at a price — thanks to the recent craze for light arid supposedly aerodynamic equipment.

Tall cyclist can’t count on finding bigger wheels, but their wheels still need special attention. Heavy riders require particularly strong wheels, calling for relatively heavy rims and more than the usual number of spokes. This also affects the need for special hubs, which are available (usually intended for tandem use) but again hard to find.

The spoking pattern should be selected to match the number of spokes. On wheels with 36 spokes, 3- and 4-cross patterns are optimally strong, while fewer crossings are required for (smaller) wheels with fewer spokes, more crossings for wheels with more spokes.

15.12. Terry’s advertising is as original as her special bikes.

15.13. Design basis for Terry’s small- wheel woman’s bike design.

Drivetrain and Gearing

Even small cyclists generally want to keep up with others as much as possible. Consequently, the drive- train had better be adjusted to their special needs, especially if smaller wheels are installed. The biggest problem occurs on small bikes, especially those for children. Manufacturers often overlook the fact that the smaller wheel already provides a lower gear. This effect is illustrated in Fig. 15.14. Instead of equipping the bike with the right gearing to compensate for this, they tend to install scaled-down versions of the chainring, with fewer teeth, aggravating the problem even more.

True, a tiny bike with small wheels looks weird if it has a big chainring, but it is technically required to obtain useful gearing. Another solution is to choose a free- wheel block with smaller sprockets. Sprockets with 12 teeth are readily available these days, and even smaller ones (all the way down to 9 teeth) can be found. In that case, it will be quite essential to make sure the chain wraps as far as possible around the smallest sprockets, by selecting the chain length and the derailleur orientation appropriately.

Cranks must match the rider’s leg length as well as possible. Expensive cranksets are available in a variety of crank lengths ranging from 165mm to 180 or even from 160 to 185mm. This may be satisfactory for most big and small adults, but it still does not satisfy the needs of children, who may require even shorter ones. It also leaves those on a budget without consolation, because the more modest quality found on affordable bikes is just not available in enough different sizes to satisfy all. Really small children’s cranks are usually of very inferior quality.

Pedals are available in a limited number of sizes, but most high quality pedals in only one size. Actually, this is often not a problem on racing bikes, since most modern pedals for use on the drop handlebar derailleur bike are of the clipless design, and are not so much influenced by the size of the foot as was the case for conventional pedals with toeclips. The problem is really most severe on touring and utility bikes for large riders, where few pedals will prove wide enough. One solution may be to install mountain bike pedals, which tend to be rather wide. Toeclips for conventional pedals also are available in a number of different sizes, al though they rarely match children’s or small women’s feet. If it suits your kind of riding, the use of clipless pedals will solve most problems — but only if you don’t plan on getting off the bike to walk.

15.12. Comparison of gearing effect with large and small wheels. high gear (or long development); Large-wheel bike; low gear or short development.

The Brakes

Nowadays, brakes are not really a problem even on bikes of quite extreme sizes. Brake levers have been introduced that are readily adjustable with respect to the hand’s reach. Other models, such as those intended for BMX bikes, can often be used successfully on mountain bikes for small riders. Fig. 15.15 shows the critical dimension for reach of the lever.

Another recently introduced improvement is the brake with return springs both in the lever and in the brake calipers. This feature makes brake operation very light and accurate, even for riders with relatively weak hands.

Finally, in this respect, the modern hydraulic brakes should be mentioned. Their operation is really an experience to behold, requiring absolutely minimal force. Unfortunately, there is not a suitable range of brake levers for these brakes available to date: either a small or an adjustable version is sorely needed.

15.15. Brake lever reach definition.

Accessories

Whereas the lock and lighting are no different on a small or big bike than on any other machine, some other accessories may pose particular problems, especially for small bikes. Finding a good small frame pump, light fenders or a sturdy luggage rack to fit a small bike, can be very hard, since many of such items are not available off-the-shell.

If you are willing and able to pay the price, the problem may be less severe. Anyone who is prepared to buy himself, his children or his spouse a custom-built bike should also investigate the possibility of matching accessories, which many frame builders will gladly design and produce. It does leave those on a budget out on a limb, but being different always costs a little more, it seems.

15.16. The wrong way to make a small frame: The continuous twin tubes don’t provide adequate rigidity. Interesting detail the ergonomic ‘guidonuet’ bar-end brake levers.

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