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Guide to Bicycle Technology (article index)
In the sections of this second part of the guide, the various functional groups of components will be presented, including extensive coverage of their maintenance and repair. The first part to consider is the frame, the bicycle’s backbone, and the only part generally made by the manufacturer whose name appears on the bike. It forms the basis on which all the other components are mounted. On most bicycles, the frame represents about 40% of the complete bike’s total value. They are sold as complete units comprising the frame and the range of other components selected by the manufacturer for that particular model. However, in the higher price category, it is not uncommon to find what is referred to as custom bikes. In this case, the bare frame is equipped with components selected by the customer in the bike shop. Another step up is the bike based on a real custom frame, which is made to measure for a particular cyclist by a specialist frame builder. When choosing a custom bike or frame, it is also customary to select frame and components from com parable price categories, so the ratio of 40% frame, 60% other parts usually applies here too. However, if you are on a budget, it may be smart to select the highest quality frame you can afford, and save on components of a lower price category. There are two reasons for this. In the first place, the frame can last a lifetime, while the other parts will probably have to be replaced sooner or later, which will be a good time to upgrade them if desired. In the second place, the difference between components may lie more in their finish than in their inherent quality, and it makes no sense to pay extra for a shiny finish when you are on a tight budget. bt_5-1-2.jpg 5.1 and 5.2. Frame builders at work. Above: Manual brazing as done for small series production. Below: automated brazing in a larger factory. Frame Construction Fig. 5.3 shows the different parts that make up the conventional frame. The main frame comprises top tube, seat tube, down tube and head tube. The rear frame, or rear triangle, is built up of seat stays and chain stays. Main frame and rear triangle are joined at the bottom bracket and at the seat lug. Frequently, the frame is sold together with the fork and is then referred to as frameset. In this guide, the fork is covered separately in Section 6, which is devoted to the steering system. 5.3. The frame and its components The main frame tubes are normally joined by means of lugs into which they are brazed, while the bottom bracket is essentially also a big and complicated lug. The rear stays are connected by means of short connecting bridge pieces. At the ends, where seat and chain stays meet, they are attached to flat parts referred to as drop-outs, in which the rear wheel is in stalled. Assuming derailleur gearing is used on the bike, the RH drop-out is usually equipped with an attachment lug for the derailleur. Finally, there are often a number of small parts, referred to as braze-ons, which allow the installation of various components and accessories, and to provide stops or guides for gear and brake cables. 5.4. Dynamic and static loading of frame Frame Design What was said above applies to the conventional diamond shaped frame that is still the most common, if only because it generally offers the best compromise between strength, stability, weight and price. Other frame designs using the same basic construction method have been more or less popular at times, especially for women’s bikes or special-purpose machines. In addition, quite different frame designs, often based on specific manufacturing techniques, have been making some inroads in recent years. Fig. 5.6 shows some of the different designs based on conventional manufacturing techniques. Some of the more revolutionary designs based on differing techniques will be covered separately. 5.5. Wishbone seat stay design To determine which of the shapes shown in Fig. 5.6 is suitable, the purpose must be kept in mind. If the lightest, strongest and most rigid frame possible is de sired, then this is the wrong place to look. If, on the other hand, the most important criterion for a particular rider is to obtain a low straddling point, then this may go at the expense of some other factors. To get the same strength or rigidity, the price of increased weight due to the selection of thicker walled tubing must be accepted. 5.6. Some frame design alternatives Technically seen, the important thing is to form a strong and direct connection between the tie points. A frame structure should resist both the vertical forces applied by the rider’s weight, and the various lateral and torquing forces that are exerted due to asymmetrical loading under movement. Braking, steering and vibrations also cause variable forces that must be absorbed. All these criteria can be easily considered if weight is not the object. But in bicycle design it is, and to minimize the frame weight, a technically well conceived de sign must be worked out. It is relatively simple to com pare the effect of forces on the various frame designs. To do so, you can make models of thin metal or wood rods, pinned together at the joints, freely pivoted. Only the designs that do not collapse are structurally sound, and you will soon find out that this does not apply to the various women’s models shown. Fig. 5.4 shows the distribution of forces on the conventional bicycle frame resulting from vertical, static loading. As you can see, the various tubes are arranged in such a way that the forces are always applied in line with the tube’s linear axis, either in tension or in compression. This design was developed in the late eighteen-eighties and has essentially formed the basis for all bicycle frames since that time. These findings can not only be confirmed in layman’s fashion with the modeling technique mentioned above, but also in engineering terms by means of a relatively easily accessible technique known as finite element analysis. It took a little longer before bicycle designers realized that the asymmetrical and variable forces applied during motion must also be considered. These did not pose such a problem on early bicycle frames, due to the use of very thick-walled tubing or even solid members of significant diameter, which are adequately rigid. In recent times, more attention has been paid to this factor due to the inherent lack of rigidity of very thin- walled tubing. Ideally, the frame should have some vertical flexibility, since this provides shock dampening, while retaining maximum lateral rigidity to prevent in- stability. Of all the designs used to date, the conventional diamond-shaped frame remains the most satisfactory, especially if large diameter, thin-walled tubing is used. 5.7. Split seat stays (short rear triangle) 5.8. Giant’s Cadex. It has a composite frame of carbon fiber reinforced tubing and cast aluminum lugs. 5.9. Raised chain stay design Some different design details have been introduced at times, especially in conjunction with the use of other materials than steel, not all of which offer real advantages. Take for example the so-called wishbone seat stay design. This brings the stays together just above the rear wheel and carries them as one thicker tube to the seat lug, as shown in Fig. 5.5. The result is a rigid rear triangle — but rigid in the wrong direction: the vertical shock absorption is eliminated without providing additional lateral stiffness. Technically less questionable is the very short frame design that can be achieved by splitting the seat tube into two thinner ones, as shown in Fig. 5.7. The recently popular raised chain stay mountain bike frame design shown in Fig. 5.9 loses its triangulated Integrity, just to achieve an amount of chain clearance that is not really needed. Frame Sizing The size of a bicycle is primarily determined by the height of the frame. Fig. 5.10 shows the two methods used to determine this dimension. Although both methods measure along the seat tube, there is some lack of clarity as to the reference points used. French and Italian bicycles, and an increasing number of American mountain bikes, are measured from the center of the bottom bracket to the center of the top tube. The conventional method used in the English speaking world, on the other hand, is to refer to the distance between the center of the bottom bracket and the top of the seat lug. In most cases, the same frame will be quoted approximately 15mm or 1/2” less when measured center-to-center than when measured from the center of the bracket to the top of the seat lug. With the introduction of really different frame designs, such as those made of composites, the conventional frame size becomes a questionable reference, as it al ready is for women’s frame designs. Eventually, one should perhaps give a range of sizes: straddle height of top tube, minimum and maximum distance between saddle and bottom bracket, and minimum and maxi mum handlebar height. Frame Geometry Quite a number of other dimensions and angles, be sides the nominal frame size, are important in the de sign of a bicycle frame, and make one frame more suitable to a particular rider than the other. Fig. 5.10 also shows the most important dimensions that should be considered as they apply to a conventional frame. Just where the rider’s hands, feet and seat must be placed on a bicycle for best control over the machine is a matter of basic ergonomics and has been essentially unchanged since the turn of the century (the recent introduction of handlebars allowing a lower and more stretched out posture only works if speed alone, rather than control over the bike, is considered important). As was explained in Section 4, a shorter tube is basically more rigid than a longer one, which in turn is more flexible, providing better shock absorption. Consequently, a small frame, with short tubes due to mini mal clearances, would be more rigid than one designed to more generous dimensions. This is the reason clearances are minimized to keep tube lengths to a practical minimum. 5.10. Major frame dimensions and angles. 5.11. Jig to hold frame tubes at predetermined angles during assembly. On the other hand, it makes no sense to make the frame so small that seat and handlebars have to be raised and extended by means of an overly long seat post and handlebar stem, since these extensions are themselves less rigid, giving the overall bike not more, but less stability when ridden this way. Another factor that greatly influences overall frame rigidity is the length (and the diameter) of the head tube: frame de signs that maximize head tube length, within the given sizing constraints, achieve optimum rigidity. As for the diameter of the head tube, the recent introduction of oversized headsets with matching head tribe diameters has greatly contributed to overall frame rigidity and steering predictability. 5.12. Comparison of frame geometries Frame Angles The relative angles of the frame tubes are as important as the dimensions. The steering angle, i.e. the angle of the steering tube relative to the horizontal plane, greatly affects both the shock absorption and the steering characteristics of the bike. The latter factor will be covered in Section 6. With respect to the shock absorption properties, the smaller angle gives a softer ride, assuming equal fork blade thickness. The seat tube angle, again measured with respect to the horizontal plane, is usually selected so that the seat arrives in the ergonomically best position relative to the bottom bracket. On racing bikes and aggressively ridden mountain bikes, this angle will be 73-74 degrees. The angle is usually somewhat less on touring and re creational bikes, placing the rider closer to the rear wheel. On small frame sizes, a slightly steeper angle may be selected to achieve the relatively short top tube length that is ergonomically required for most women and many short males. To determine the angle of either tube, refer to Fig. 5.13, using the table contained there to convert the dimension measured into degrees. 5.13. Methods to determine frame angles: commercial angle gauge (operating principle) Although bikes with a small seat tube angle at first seem ergonomically wrong, this only applies when the position is the forward poised one we are familiar with. The same relationship between various fixed points can be achieved with different angles, as it is done most dramatically on the recumbent bicycle design. This can be made to work on relatively normal bikes too: I have ridden bikes with angles down to 60 degrees. In city traffic, this makes it possible for the feet to reach the ground without getting out of the seat, while still maintaining the correct distance between seat an pedals when riding. However, long-term comfort in this position re quires a back rest. The distance between the seat tube and the steerer tube, or rather between the bottom bracket and the front wheel axle, must be such that adequate clearance between the rider’s toes and the front wheel remains. Although this requirement may be waived on racing bikes (since high-speed cycling never requires large steering movements), it is quite critical for mountain bikes, and the use of fenders requires even more clearance. As a guideline, the distance between bottom bracket and front wheel axle is usually about 55cm (22) on a racing bike, about 58cm (23”) on a recreational bike and should be at least 60cm (24”) if fenders will be installed. The distance between the bottom bracket and the rear wheel axle is also minimized to achieve a light and rigid bike. Excessive flex, resulting from long chain stays of thin-wall tubing, can cause tire rubbing, chain scraping and unintentional gear shifting when applying maximum pedaling force. There is no truth in speculations about presumed loss of efficiency due to frame flex, either here or anywhere else — after all, any energy that goes into bending the flexible tube quite far would also have gone into bending the less flexible tube less far, and this same energy is released again as the tube springs back. Racing and aggressively ridden mountain bikes may have chain stays that are about 41 and 43cm (16 and 17”) long, respectively, while they should be at least 2.5cm (1”) longer if fenders are to be installed. 5.14. Helical grooves reinforce this Columbus tubing at the most heavily loaded points. Frame Materials Generally, bicycle frames are made of the materials discussed in Section 4, i.e. tubular steel or aluminum alloys, while titanium or carbon fiber tubes sometimes find application. Frames constructed of molded composites or e.g. cast magnesium are still uncommon, al though their use may increase in the foreseeable future. 5.15. Sectioned frame at bottom bracket, showing just how thin the tubes are on a quality frame built with butted tubes. The customary frame tube diameters in mm are listed as follows, depending whether the frame is built to English (international) or French standards:
On mountain bikes, some of these dimensions provide inadequate strength and rigidity for hard use. Consequently, they are generally built with slightly larger, more recently also with oversize tube diameters. Al though there are significant variations from one make to the other, the following list shows two typical (though not universal) examples of regular and oversize mountain bike tubing sizes:
Because of the material’s much lower modulus of elasticity, aluminum tubes flex too much if built to these standard dimensions. Thus, aluminum frame designers soon found the solution in the use of relatively thin walled oversized tubing. Tube diameters on some aluminum bikes go all the way up to 2” for the down tube — the most heavily torsion-loaded main tube on any bike. Less dramatically oversized tubing dimensions have recently also been introduced on thin-walled steel frames for road bikes, offering increased rigidity with out a weight penalty. Although the difference in weight between a light racing frame and that of a conventional utility bike is quite significant — 1.7kg and 3.9kg, respectively — the difference between frames built of different materials within a certain category is not that dramatic. Thus, the lightest composite frame — that promptly broke when used in the Race Across America — is hardly any lighter than the lightest steel frame, and the same applies to aluminum and titanium. Although lighter frames can be built using these more exotic materials, to make them equally strong, they must also be virtually as heavy. 5.17. Made famous by Greg LeMond in the ‘91 Tour de France, the fibers in this carbon fiber reinforced frame are selectively oriented with respect to the local loading. 5.18. Lugs and bottom bracket locations. Frame Joints Traditionally, bicycle frame tubes are joined by means of brazing the ends together with the aid of lugs. The American utility bike always was an exception, usually being welded, using thicker wall tube with smaller diameters. This explains their quite sensational weight if adequately rigid, or poor stability if made lighter. The various common frame joining methods are depicted in Fig. 5.19. On lugged frames, external lugs are generally used, al though recently, internal lugs have been applied by several manufacturers. On factory-built frames with external lugs, the brazing material is installed in a notch in the tube before the frame is assembled and then heated, causing the brazing material to run around the joint. On hand-built frames, the bronze is added from the outside, a method also used by some of the factories that build frames in small series. Internally lugged frames are almost always factory-built, and the brazing material is installed in a recess in the lug before assembly and heating. Lugs are also used on many aluminum and composite frames. However, in that case the tubes are bonded to the lugs, using an anaerobically-hardening epoxy resin (meaning that the material hardens in a chemical reaction, rather than by drying with air). Some recently introduced frames have aluminum tubes on internal steel lugs, while others combine thin-wall steel tubes with aluminum lugs. Although these developments are still relatively new and not yet time-proven, they seem to be unquestionable from an engineering standpoint. Occasionally, especially if odd-size or odd-shaped tubes must be joined, a lugless brazed construction is used. This technique is usually referred to as fillet brazing, since the brazing metal is not only entered in the narrow gap between the tubes, but is built up around the joint to form a reinforcing fillet. It’s an expensive way of building, due to its labor- and skill- intensiveness. Optically, the result can be very pleasing. Depending on the type of frame tubes and the maximum temperature specified by the manufacturer, both this and other brazing techniques can be done either with bronze or silver brazing rod. Since the latter material flows very thinly at a relatively low temperature, it is harder to work with, but is specified by most manufacturers of ultra high-strength tubing. Whatever the manufacturers specify, nobody uses silver in locations requiring fillets, since it cannot be built up to a bead. 5.19. Frame joining methods If thicker-walled (1.2mm or more at the ends) tubes are used, welding becomes a viable option. Thus, this is a common method for the construction of aluminum frames and most mountain bike frames, whether steel or aluminum. Most aluminum alloys must be stress- relieved at an elevated temperature during a prolonged period subsequent to welding to restore the original strength. As pointed out in Section 4, some alloys of the 7000 series, though slightly weaker than the strongest tempered types of the same series, make life easier for the manufacturer by eliminating the need for stress-relief by means of post weld heat treatment. Essentially all welding operations on bicycle frames are carried out in a protective atmosphere. Although some cheaper thick-walled frames are welded under CO gas, inert gas such as argon is used for all quality frame welding operations, commonly referred to as TIG welding (also see Section 4 for some comments on this welding process). 5.20. Seat lug detail on Vitus frame with carbon fiber reinforced resin tubes bonded to internal aluminum lugs. Note the bulging tubes where they are pressed over the lugs. Frame Details The details of the bicycle frame include the lugs that hold the frame tubes together, the bottom bracket, the drop-outs, the connecting parts, as well as the minor braze-ons used to mount components and accessories. These are the items that will be discussed in the following sections. Lugs and Bottom Bracket 5.21. Haden Polaris lug set. Although made of pressed steel, these are accurate enough for high-quality frames. The cut-outs aid brazing metal penetration. 5.22. Investment cast steel bottom bracket by Reynolds. Fig. 5.21 shows a typical set of lugs. For normal brazed steel frames, there are external lugs in several categories, and the same applies to the bottom bracket shell into which the crankset is mounted. The most important factor affecting bonding quality is the consistency of the gap between the tube arid the lug, which should ideally be about 0.2mm all around. The highest degree of accuracy is generally achieved when these parts are formed by means of the lost wax casting technique, also known as micro-fusion. These parts may be recognized by their relatively sharp and somewhat bulky contours. Since a new mold must be made for each item produced, these parts tend to be quite ex pensive. Cheaper lugs are made of stamped and welded plate. These range from the cheap and often poorly fitting lugs used on mass production frames to quite nicely shaped and more accurately dimensioned ones used on some quality frames. The best lugs are contoured rather delicately and have cut-outs to aid the penetration and inspection of brazing material, which greatly affects the quality of a brazed connection, even though it is generally accepted that a 70% contact is adequate to achieve sufficient bonding in a brazed joint. The seat lug and the bottom bracket shell are some what special. The former is split in the back to allow clamping the seat post in. The two halves are clamped together by means of a bolt that pushes the eye at the back (or sometimes on the front) together. The gap should end in a round 3 - 4mm hole to prevent cracking. On mountain bikes, where the saddle is adjusted often, this point is quite critical, as is the attachment of the eye to the lug. Since a cracked lug would essentially ruin a frame, several manufacturers have replaced the eye by a separate external clamp that goes around the top of the lug. The bottom bracket shell is by far the most complicated lug, connecting down tube, seat tube and chain stays, while providing a mounting for the bottom bracket that must be perfectly aligned perpendicular to the plane formed by the main frame tubes. Usually, the shell is threaded to accept a standard bottom bracket (al though some mountain bike manufacturers install their own special bottom brackets that are held with spring clips). Depending which type will fit, this screw thread may be either of the English, Italian, French or Swiss standard, all of which differ, as is outlined in Table 4. On many racing frames, the bottom bracket shell is cut out in the bottom, which is supposed to allow water to drain off and to save weight. The real benefit escapes me, since the weight savings is negligible and the risk of dirt and water entering there is greater than the benefit of it running off. Other details often included are guides for the gear cables, which are more useful. The LH and RH ends should be perfectly faced (squared) to allow accurate installation of the screwed parts of the bottom bracket bearings. 5.23. Shimano front and rear drop-outs Drop-Outs Drop-outs, or rear fork-ends, are depicted in Fig. 5.24. They are also available in several different versions, the most accurate and strongest of which are either forged or cast with the lost wax method described above. These invariably have some adjusting mechanism to position the rear wheel axle and an eye or lug for the installation of the rear derailleur on the RH side. The optimum attachment point of the rear derailleur relative to the rear axle depends on the make and model in question. This is why most derailleur manufacturers also offer their own drop-outs — with the effect that you get locked into their particular brand of derailleur. 5.24. Rear drop-outs Cheaper frames may have relatively thin drop-outs stamped out of flat plate. These usually have neither an adjustment mechanism nor a derailleur eye, requiring a separate adaptor for the installation of the derailleur. Since these drop-outs are so thin, and since the attachment of the tubes is often too far from the wheel axle, these drop-outs bend too easily. The frames of touring bikes and most mountain bikes have drop-outs with integral eyelets for the installation of stays for fenders and luggage racks. Nowadays, these things are almost always lacking on other bikes, even those meant for recreational riding — purely a fashionable aberration, since they are in nobody’s way and add significant practical benefits for anybody not actually racing. In fact, up to the early seventies, even pure racing bikes usually had eyelets, and the toughest bike racers used them without embarrassment. 5.25. This special Mittendorf’s off-set LH drop-out allows spoking the wheel symmetrically. Vertical drop-outs allow building the rear triangle with smaller wheel clearances, but often lack the adjustment detail, requiring the frame to be built extremely accurately if the wheels are to be aligned properly on the finished bike. To produce an extremely short rear triangle, some manufacturers have used what is known as compact drop-outs. These consist of two parts, connected by means of slotted holes and bolts, allowing adjustment and replacement. The drop-outs on welded aluminum frames had better be extremely thick, to the point of being ugly. Another solution would be a two-part design, with the major part, containing the wheel slot and the threaded holes for derailleur attachment, made of e.g. stainless steel. Bonded frames can be built with steel or aluminum tubes bonded to more or less normal steel drop-outs. 5.26. Some typical braze-ons: Attachment bosses, or eyelets, Front derailleur lug, Cable tunnels 5.27. Racing frame built with Reynolds 531 tubing and investment cast lugs by the same manufacturer. Other Frame Parts Most of what was said about the lugs and drop-outs also applies mutatis mutandis to the various other frame parts. Thus, the connecting pieces between the rear stays are best when accurately made and shaped to match the respective frame tubes to which they connect. The following is a list of the various braze-ons used on many bikes, briefly summarizing the points that are important in their design or installation:
5.28. Some finish detailing is still done by hand, even on relatively simple frames like this. Frame Finish The subject of finishing processes as covered in Section 4 applies largely to the frame. Virtually all steel frames and most welded aluminum frames are painted or lacquered. Chrome plating can be used to advantage on particularly scratch-sensitive locations on steel frames, such as the RH chain stays, the seat lug and the drop-outs. What is not painted or otherwise coated on an aluminum frame should preferably be anodized, although a really good natural polish can look very attractive and remains that way unless the bike gets into a salty atmosphere. Titanium is also anodized or polished, while carbon fiber reinforced epoxy is generally pigmented throughout. 5.29. Two-part rear drop-out of cantilever design on Cannondale welded aluminum frame. Another excellent frame finish is nickel plating. Popular around the turn of the century, this process was long forgotten, until some mountain bike frame builders reintroduced it recently. It is highly scratch resistant, and although it is less shiny then chrome plating, it is more durable than either chrome or paint. Maintenance and Repairs Although the frame is the bicycle’s major component, it does not rate very high amongst the parts requiring maintenance. If anything does happen, it is often so catastrophic that there is little hope of correcting it: you may have to buy a whole new frame or — more typically — a whole new bike. What little there is to do on the frame will be covered below. Upon frontal impact, the front end of the bike will be pushed in and this may result in damage to the down tube as shown in Fig. 5.30. This results in a severely weakened frame and usually also affects the steering. Check for this kind of damage alter a fall or collision. 5.30. Typical downtube damage due to frontal collision. On a really good frame, it is often worthwhile to get a damaged tube replaced if you can find a frame builder who will take on this kind of job. After the repair, the frame will have to be wholly or partly repainted, but it may still be cheaper than buying a new frame. Even if there is no damage of the type described above, you should thoroughly inspect the front fork and the frame for correct alignment after any serious accident, which will be described in the next section. Frame Inspection Before the actual inspection, the rear wheel and any interfering accessories must be removed. The professional way is by using a special frame alignment gauge, but it can be done quite satisfactorily with 3m (10 ft) twine, a ruler with mm marking and a metal straight edge Procedure 1. Place the twine around the head tube and along both sides towards the drop-outs, pulling it taut per Fig. 5.31. 5.31 and 5.32. Left: frame alignment inspection. Right: drop-out alignment check. 2. Measure the distance between the twine and the seat tube on both sides to one mm accuracy. 3. Compare the two measured values. If they differ by more than 2mm, there is lateral misalignment of the frame. 4. Check whether the drop-outs are still straight. To do this, place the metal straightedge with the thin side against the outside of each drop-out in turn and check whether the distance between the straightedge and the seat tube is the same for both (see Fig. 5.32). 5. If the two values differ by more than 3mm, at least one of the drop-outs is bent. Frame Alignment This is the kind of job best left to a bicycle mechanic. Just the same, it is quite possible to do it yourself if you go about it carefully. It will also give you a much better feel for how bicycle tubes deform and what kind of forces are needed to do so. Generally, it suffices to move the two halves of the rear triangle relative to the center plane of the main frame until a frame check confirms that the deviations are within the tolerances specified above. Procedure: 1. On the basis of the frame inspection, establish which half must be moved how far in which direction. 2. Place the frame horizontally on a firm flat surface as shown in Fig. 5.33. Let an assistant stand on the main frame, holding the head tube with one leg, the seat tube with the other to assure these points stay flat on the working surface during the bending operation. 5.33. Crude cold-setting method to straighten rear frame triangle. 3. Using firm but even force, push or pull one side of the rear triangle over its entire length into the de sired direction. 4. Check the result by repeating the frame inspection as outlined above, repeating the bending operation until the desired results are achieved. 5. Repeat the drop-out check from point 4 and 5 of the Frame Inspection description, and if necessary, perform the following procedure. Drop-Out Alignment First check to make sure the slot for the wheel is still straight and there are no cracks in the drop-outs that will eventually cause them to break. In case of serious damage, a frame builder could replace a drop-out on a high-quality frame at a fraction of the cost you would pay for a new frame. 5.34. Crude drop-out straightening method If the drop-outs are only bent, without serious damage, you can probably do a repair yourself. This requires a special tool when the RH dropout is bent in such a way that the derailleur eye is affected. In other cases, it can be done with nothing but a heavy metal working vice mounted on a sturdy work bench. Procedure: 1. Establish which drop-out has to be bent, and deter mine:
2. For the LH drop-out, clamp it in the vice as shown in Fig. 5.34, so that the bent location is just above the top of the vice. 3. Firmly grab both stays on that side just above the drop-out and steadily push the whole rear triangle into the appropriate direction. 4. Repeat the drop-out alignment check from the Frame Inspection checking procedure and repeat until the alignment is within the tolerances given there. 5. If necessary, get the RH drop-out straightened by a bike mechanic using special alignment tools. Note: If cracks develop, the drop-out must be replaced, which can be done by a frame builder (most bike shops have an appropriate contact). 5.35. Professional tools for drop-out inspection and alignment. |
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