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As a simple rule of thumb-- as with all "rules" there are many exceptions. The absolute min. number of spokes for an acceptably strong rear wheel dished for 8/9-speeds is (using 700c normal section rims and standard components) is (lower bounds):
|36 spokes||>80 kg|
|32 spokes||60-80 kg.|
|28 spokes||<60 kg.|
Radial spoking is currently quite fashionable and while one can build acceptable (font) radial wheels by using a better flange design and properly stress relieving spokes, one must ask if there is any benefit other than the marginal reduction in mass from slightly shorter spokes? One of the typical arguments for radial wheels are increased lateral stiffness (aprox. 10%) yet the same advocates also tend to promote reduced spoke counts--- which reduces the lateral stiffness. In the front it does not really matter if the wheel is radial or crossed and if one gets the tensions high enough and uniform enough one can build a good wheel (the strength afterall is related to the rim and not the spokes). The rear wheel, however, is less suited to radial spoking (the stresses are, according to the FEM analysis done by von der Osten-Sachsen's group in Aachen and published the Cycling Science Fall '97, 52% larger than for three cross). Even the popular radial and 3 crossed drive side does not appear to be a good combination (40% higher stresses).
While theoretically there is little difference between the load bearing capacity of wheels with different number of spokes-- based upon the preload tension of the sum of spokes-- in praxis the less spoked wheels tend to have lower total tensions given rims of the same basic design: maximum tension a function of mass and diameter. To maintain preload tension, a wheel with less spokes must be built with higher individual spoke tension. With a strong enough (such as some of the heavy deep section) rim there is the problem of getting enough tension in the spokes and with an ultralight box section the problem is overtensioning and pulling out the eyelets. More spokes have the advantage of distributing the tension. Getting enough tension on a wheel with 24 or less spokes can be quite difficult without machines and even with machines thin swagged spokes tend to not fair well under the tightening torque. To reduce stresses one then wants to increase the diameter of spokes (thereby increasing mass). This, generally, means that one should select spokes on the basis of number of holes and weight. As a minimum for the rear wheel:
|For 28,32 spokes||2.0 mm spokes.|
|For 36 spokes||1.8 mm spokes (< 70 kg)|
|2.0 mm spokes (> 70 kg)|
|Campagnolo Shamal 16-spoke||
|HED CX 24-spoke||
|HED disk (lenticular)||
|ZIPP 950 disk (flat sided)||
What you say? A well designed 36 spoke wheel can be lighter than a 32 spoke? But that's not what the industry is telling me. Isn't 32 the standard? 32 is 4 less than 36 and that is a weight saving. If not why 32? The wide-spead use of 32 spoke rear wheels has, I think, had more to do to the introduction and marketing by Mavic of their GP4 rims back in the early 80s than probably anything else. While the rim might indeed appear to hold up to higher tensions and seemed to allow for what appeared to be strong 32 spoked wheels they have been shown to be no better (and in many ways inferior) to previous designs. The hardox does not increase strength, that requires additional mass. The increased strength of the GP4 came from the increased material (wall thickness): 400g compared to the standard sub-360g rims of the day. To improve reliability, in the light of these shortcommings and in line with the trend to more gears and 32 or fewer spokes, the makers have added even more material-- whence increasing weight. While the rim/tire is the significant rotating weight and the reduction in spokes requires more mass, to get the wheels to be lighter the vendors have decreased the weight of the hubs (and quick releases) using titanium, plastic, special flanges, lighter industrial bearings etc..
The current 9-speed drivetrains represent, perhaps, the limit without extending rear axle width. These wheels have been so weakened that, even with 36 holes, the days of sub-300g (Al rims) rear wheels is gone. Typical (nearly all) tubular rims are now around 400g and the current GP4s come in at a heafty 460g! A well built 36-spoke wheel with a 350g tubular rim will be just as reliable as the 32-spoke wheel built with the 400+ gram rim. Since the sole difference between tubular rims with different number of holes is (generally just) the drillings (the hoops are the same) they have, in light of the trend, become just heavier (too heavy). If the choice is for race wheels one can choose the 32 hole but in most circumstances (when one is not bound by a sponsorship contract) one is better off to select a lighter rim with more spokes. The aluminum alloys used in rims, despite contrary consensus by consumers, have not improved--- nearly all improvements, other than aerodynamics, have been in the smoke screen of cosmetics such as welding and colouring. The main improvements to these rims have been in production automation (both in the production of the alloys and in the machines to make the extrusions), packaging, marketing and product differentiation--- things that are of significance for the vendor but offer little advantage for the competitive cyclist. Since production automation, market consolidation and returns-to-scale have allowed the cost to produce good aluminum alloy rims to drop considerably, the kind and quality of rims in the consumer spectrum of the market are substantialy better than a decade ago. At the competitive level-- where a willingness to pay exists and higher margins can be demanded-- to increase profits (as a function of price) some vendors have invested heavily in selling the consumer on the need for welding (while intuitive, its without merit since the rim is held together by the substantial pre-load tension anyway), machining (pre-worn, adding material to the extrusions to than file it off, making for a marginaly more consistant breaking surface on a new but weakened rim with unpredictable and, in parts, too thin walls), adding special coatings to strip it off (improves breaking but why these coatings in the first place?), fancy sounding alloys (while some of these alloys do have certain advantages in special applications, they tend to offer little real advantages for rims), design improvements (selling rims like washing soap, consistently claiming their products are new and improved, even when the only change was a new name) and secret sauses. Forget the voodoo and ad-speak, the strength is a function of pre-load tension. The more the tension per spoke the higher the tendency to pull them through--- more mass and/or more brittle materials are called for. The larger the wheel dish, the greater the tension imbalance between the two sides and the weaker the wheel--- and to increase the overall tension of the weak side means one needs to increase the tension of the strong side and this more needs a larger cross section and more mass.
For the front wheel, by contrast, rims of under 300g and/or fewer spokes are less problematic-- although the aerodynamic advantage of 32 versus 36 is minimal and the potential weight savings, given suitable rims, and the improved reliability of 36 spoke is generally advised. When using the same rims, a 32-spoked front will be much stronger than even the 36-rear, so the combination 32/36 and thinner spokes in the front is generally very reliable.
A 36 spoked wheel can use lighter rims (which given production and lower spoke count trends are increasely harder to get) and other lighter components, esp. thinner (lighter) spokes. Given the weight differences of 1.8mm and 2.0mm spokes.
|DT Swiss Revolution||nirosta||2.0/1.55||266||4.19g/spoke|
|DT Swiss Competition||nirosta||1.8/1.6||266||4.88g/spoke|
|DT Swiss Competition||nirosta||2.0/1.8||266||6.03g/spoke|
|Hi-E||15||alloy||0.235g||requires special socket wrench|
|DT Swiss||15||brass||0.850g||old style disc shaped|
|DT Swiss||14||brass||1.000g||new style trumpet shape|
|DT Swiss||15||brass||1.100g||new style trumpet shape|
|DT Swiss||14||brass||1.140g||old style disc shaped|
|Fiber Flight||14||plastic||0.19g claimed weight|
|Wheel Balancing? Typical bicycle wheels require between 1.5 and 8 gramms additional mass to statically balance. In rolldown tests these wheels consistently function better. As a simple illustration (albeit "cooked"): Take two identical wheels mounted in two identical forks, spin both wheels (synchron) up to 500 RPM and release. The balanced wheels will continue long after the unbalanced wheel has stoped spinning. Continue the experiement by swapping the two wheels around between the two fixtures and then removing the weights from one and attaching them to the other, use different rims, tires etc.|
Weight advantages come from the use of less spokes to under-dimension the wheel, viz. to build weaker wheels. This can work reasonably well for esp. lightweight riders. The increase in the dish of the rear wheel requires higher tensions and more material. Despite the their use of heavy deep section rims and high tension (possible via the special hubs and spokes) the Campa 12-spoke has proven (in the rear) to be unreliable and prone to problems. Wheels such as the Mavic Heliums or the new Campagnolo Climbing Dynamics Series-- Proton and Electron-- combine a box section rim with less spokes and a lighweight special hub (often from their aerodynamic wheelsets) with the explicit aim of optimizing for weight and to a lesser order aerodynamics. These often require thread lockers (such as Loctite or the ACT nipples used by Campa's wheels). Such flimsy wheels can work OK but I would not call them good wheels--- although they still might be acceptable for certain applications. Worse still a spoke failure given their low count can lead to total deformation of the wheel. While these wheels offer some aerodynamic advantages at the cost of decreased reliability (fewer spokes) a wheel with properly paired and designed 32/36 spokes can be not only less expensive, more robust (reliable) but light.
|Fiamme Ergal Rim||280g|
|Tune Mig hub||77g|
|36 DT Revolution spokes||165g|
|36 brass nipples||36g|
|Continental Sprinter 250||280g|
|Total||under 910g (including mastic)|
If you want to reduce weight I would consider looking at tubular tires and rims. They offer more strength for the weight and offer, in my subjective opinion, a better ride.
We are talking about weight and "rotating weight" is important. The current trend towards tuning, titanium and thin walled tubing (TTTT) does not make sense with heavy wheels. How can someone be willing to do anything (and spend a lot of money) to save a few grames in the bicycle frame but accept nearly 300g more rotating weight in the wheels?
The strength of a rim is directly related to its mass, width and cross-section. The taller the section, thicker the sidewalls and the wider the rims, the stronger they are. Since nearly all rim extrudusions are made with 6000 series aluminum alloys, the less the mass the weaker the rim. Forget the ad speak, clincher rims must always be heavier than tubular rims. The shape of a tubular rims is not only closest to the ideal from the view of structural strength but, since tubular tires are glued onto the rim, they don't require a horn.
Granted ultralight "folding" (kevlar core) clincher tires and inner-tubes are commonly available and have allowed one to develop lighter (more narrow) rims-- coming down from well over 500g to near the 400g mark (e.g. the 410g Mavic Reflex). Lighter is better but unfortunately 400g clincher rims are too weak for other than special applications, similar in many ways to sub 300g gram tubular rims. Sure with proper tension one can build a true wheel with these rims but to get towards the 400g mark (like sub 300g tubular rims, e.g. Fiamme Ergals, Mavic GL280s etc.) the rim makers have reduced wall thickness and sometimes chosen harder, more britle, alloys. While softer materials during a clincher rim failure cycle from wear will first bend at the horn, the more britle alloys (can) break with little or no warning--- fortunately the catrostrophic rim breakages occur most often during tire inflation. The min. wall thickness for a clincher rim should be 1 mm or more. Many failures of clincher rims have been documented and the wall thickness of these samples have tended to be between 0.7 mm and 1 mm: See also the Hardcore Bicycle Science archive contributions on "How thin may the braking rim of my wheel get". Lightweight clincher rims should not have walls thinner than 1.5mm (e.g. Mavic MA-2s). Some ultralight clincher rims are made with 1.2 mm walls and offer very little margin for wear. Blue, red or other coloured rims do not provide any additional reliability since the thin eloxial layer, although providing some initial dry wear resistance (which is anyway rather minimal), is usually removed during the very first ride in the rain. Hardox (Grayish Brown, so-called CD by Mavic) rims provide better resistance to breaking wear at the cost of vastly inferior brake performance and increased failures due to cracking. The current limit for reasonable aluminum alloy clincher rims is just under 500g--- and the best of them, still, offer strength in the sub 350g tubular class. Tubular rims with 1.2mm or even 1.0 mm (e.g. Fiamme Ergal) walls provide enough reserve given that they have neither the requirement to resist tire pressure (the tire is held on by tire contraction and mastic) and the structure provides substantialy better resistance to lateral deformation. In this light, a 1.0mm tubular rim is stronger, lighter and will last longer than (even) a 1.5mm clincher rim. Add at least 50g for the horn and one sees that a clincher rims must always be substantialy heavier than a tubular rim.
While a clincher wheel plus tire and inner-tube can these days approach what one often considers the weight of a tubular wheelset, one is comparing a weak ultralight wheel with paper-thin (failure prone) tires with a strong tubular wheel with robust and sturdy tires. A tubular rim weighing 60g less than a clincher rim will still be stronger and despite "common knowledge" the frequency of failures of good tubulars is not higher but lower than equivalent clincher tires (e.g. Continental Competition 240 versus GranPrix, Vittoria CX versus their so-called "Open Tubulars").
The difference in weight of clincher and tubular wheelsets, in this light, is at least 200 grams. That's 200g rotating mass. What's the point of trimming weight with paper-thin (e.g. Columbus Nemo) frames, expensive Titanium or "Coke Cans with Wheels" to then throw away 200 grams where it really counts?
And the ride? Forget the talk about the comfort of different frame materials. A harsh ride comes from the wheels, esp. the tires, and not really from the frame. The feel of the ride of a good pair of tubulars is the standard against which everything else is measured.
The horror stories of tubulars are, again in my opinion, exagerated and unjustified. The main thing that you *must* accept *before* making the switch to tubulars is that you will need to repair them (and not toss them unless you earn more than $200/hour and are totally booked).
|Gluing Tubular Tires|
On poor roads tubulars tires have a distinct reliability advantage. The failure of tubular tires to catch the racing mountain bike scene was due to problems with image and less with the suitability of the concept. While for non-racers the wet, muddy conditions made tire changing impossible this is of little relevance to racing. Since the major sponsors want to sell product they were doomed from the start.
Trashing a $50 tubular is depressing. But I think trashing a $50 clincher is nothing to "light up the day" either. Where clincher tires have a distinct lead over tubulars is in the low cost segment: a cheap clincher will be more reliable, rounder and roll better than a cheap tubular. Cheap tubulars are a waste of money and cost more to operate! In this light, perhaps the poor image of tubulars can be attributed to cheap "training" tires. If you use tubulars you must be prepared, even for touring or training, to select good tires costing at least 50 DM ($30). While more expensive clincher tires are ligher but often more prone to faults (e.g. Continental GranPrix versus SuperSport) the more expensive handmade tubular tires aren't just lighter and ride better but are often more reliable (e.g. Continental Competition 240 versus Sprinter 250). In the current market the cheapest good tubular tire is the (machine made) Continental Sprinter 250, an excellent tire weighing around 260 grams, which retails at prices between 45 DM and 60 DM in Germany (or $35-$50 in the US).
Mounting tubular tires is not the big messy event it is often considered in the popular press. Letting a new tire stretch for a week or two on a clean rim will make mounting easy. While many tires are suited to brutal stretching (e.g. the Continental)-- letting one mount a new tire in minutes-- some, esp. silk and fine cotton, are not. With all tubulars, if you pre-stretch them on a rim for a few weeks they will be easier to mount and be rounder. Tires are like wine and even the synthetics such as the Contis improve with aging, so having a few tires around stretching is a good idea....
Many people tend to confuse the max. pressure with the reccomended pressure (given that generally only the max. is printed on the tire). Although the rolling resistance decreases with pressure, confort more rapidly declines. The table below shows a few values and represent a good starting point for most people:
|Continental||Competition 19 27"||19 mm||225 g.||10.0||12.0|
|Competition 22 27"||22 mm||240 g.||8.0||12.0|
|Competition 25 27"||25 mm||270 g.||8.0||12.0|
|Sprinter 250||22 mm||250 g.||8.0||12.0|
|Giro||22 mm||300 g.||8.0||12.0|
|Do-it-yourself Tubular Repair kit|
When I was a kid I used to repair them on the roadside (my Velox patch kit was my spare)-- I remember once riding with a friend when we rode through a patch of thorns, I think I needed 30 min. to mend all the tires. Sewing a button back onto a shirt is harder and takes just as much time as to sew a tire back together. The hardest part is patching the inner-tube and this is the same for clinchers and tubulars. The time difference between mending a clincher's or a tubular's inner-tube is really only maybe 10-15 minutes (or less). Those using the (silly) 18mm clincher tires will switch the timings around in favor of the tubulars. Greg Lemond in his "Complete Book of Bicycling" claimed that tubulars are difficult to repair and that they take loads of time-- claiming 45 min. to several hours for an experienced mechanic. While he might have been an excellent cyclist and a well known material fetishist (remember how his eyes glowed when he first saw Boardman's Lotus) I think that he perhaps personally found it difficult (I find clinchers a pain but there are many people that don't have problems) or, elected to pass-on "common knowledge"...
|How to Repair Tubular Tires|
a) Unless you have some good boot material (silk and the Polyamind material from Conti Competition tires make reasonably good boots, track tires offer the best!) and its an exceptional tire--- or you really need it, e.g. roadside repair-- its often not worth the effort (except maybe for a needed spare) to boot a severely damaged tire.
And a total failure? Clincher tires have the advantage here since with a spare inner-tube and a tire boot one often can get a clincher back on the road. While one can also boot a tubular and replace an inner-tube its substantially more effort and not really something for the roadside! And if the tire is totally trashed? Either an equipment wagon or enough folded tires (clincher just as tubular) or a long walk (although if you really need to and don't care about the rim one can ride a flat tubular).
While a spare inner-tube and a tire boot (people have been known to use paper money notes or other available materials) might be enough to take on the road while training with clinchers, with tubulars a spare tire is really (despite the practice of my youth) a must. Old, worn, but still intact, tires are the best spares. When the rear tire is low on tread, I tend to rotate the front tire to the rear and the rear to the spare tire collection, yielding in-time a plentifull collection. Sure a spare tire weights more than a boot and inner-tube but it not rotating weight.
For a longer multi-day tour one probably wants, should one use tubulars, to take along 2 spares, repair material and tubular mastic-- compared to 2 inner-tubes, repair material, a folding tire and a boot for clinchers. Some people like tubular mastic tape instead of mastic, I don't. On such a tour one also has clothing, tools and many other things to carry and the additional 300g for the tubulars is a disadvantage but tolerable. While wide heavier weight clincher tires are probably best here, for tubular wheels some of the special Paris-Roubaix Pavé tires (e.g. Vittoria, Clement) offer excellent alternatives (to building a new wheelset for that special tour). For fast Alpine descends one should also select a harder glue and keep an eye on rim heat. Its important to use a good brake technique and avoid continuous use of ones calipers as this can cause significant heat build up. Heat can soften the mastic or, with clincher tires, melt the inner-tube (causing a blow-out).
The absolute #1 problem with tubular tires, beyond incorrect mounting, is heat from braking on the front rim. I'm not sure which is the greater danger, it or clinchers popping?
You say that I'm overstating the clincher popping? It happens just as tubulars can float on "hot" glue. Both are problems, both happen, and luckily both are not frequent--- it would still be much better if they never happen.
So you get tubulars and curse me after your first flat? At this point I think clinchers are kind of like MS-Windows versus Unix. MS-Windows is not the right solution for many but "no one ever got fired over it".
Questions? Post them to http://forums.bsn.com/VeloTech.html (BSn's Velo Technology Discussion Forum).
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