Category Archives: Technical

Tyre Pressure – Real-world & Witch-craft

“It’s nice to see Professor Pivot using ‘real world’ testing for tyre comparisons, and some believable results. A couple of observations though – we tend to quote pressures in pounds per square inch (p.s.i.), but only notice the ‘p’ and ignore the ‘s.i’. For the same indicated pressure, a larger tyre will have more pressure inside and give a harder ride. In the same way, the smaller cross-section Stelvio will have a smaller outside diameter.When I tested 700C against 27″ of the same tyre, the difference was clear.

Finally, what’s all this nonsense about ‘faster up hill’? When climbing hills, the power input is as variable as the descending force is consistent – please don’t mix science with witchcraft!”

Mike Burrows
Norwich

Professor Pivot replies: Narrow section tyres do require higher pressure, and the effect can be quite marked. For example, the cross-sectional diameter of the Brompton 37-349mm tyre is 34mm and the Stelvio 28-349mm tyre is 30mm. If my limited mathematics is correct, the larger tyre has a cross-sectional area 28% larger, so where 80psi might be adequate in the large tyre, it seems reasonable to assume that a little over 100psi would be required in the smaller example.Verification of the theory – and an entertaining parlour game, no doubt – can be found by inflating a bicycle inner tube and a heavy goods vehicle inner tube to the same pressure and sitting on them…

But will the narrow tyre at 100psi react in exactly the same way as the wider example at 80psi? I suspect that if the two tyres strike the same bump under the same load, the cross-sectional area will be temporarily reduced by the same percentage in each case – let us say 20%. Once again, I must apologise if my thesis is running ahead of my mathematics, but I believe a reduction of 20% in the cross-sectional area for the big tyre would result in a deflection of 3.48mm, but for the small tyre the deflection will be only 3.08mm? This suggests that the smaller tyre might transmit smaller ‘choppier’ deflections to the bike – a harsher vibration, in other words.This might increase rolling resistance, but would also tend to reduce energy-wasting bounce when pedalling hard.

A good rule of thumb, whatever the bike, rider, road and tyre combination, is to inflate the tyres (individually, to avoid confusion) until the ride becomes harsh, then set the pressure a little below this point.These personal figures should give a reasonable ride/handling/rolling compromise specific to you and your bike.

Mike also makes an excellent point about keeping ‘seat of the pants’ observations well separated from repeatable and reasonably scientific results such as roll-down tests. But observations – however implausible they might seem at the time – do occasionally lead to a new avenue of enquiry, so they’re worth making, with the proviso that they are treated with the necessary caution.

When testing the Stelvio and the Brompton tyre back-to-back on repeated roll-down tests, I obviously rode back up the hill a number of times. On these return runs, the Stelvio tyres certainly seemed to make smoother progress, allowing the rider to hold on to a higher gear. But with no means of measuring the power input from the rider, I fear this casual observation must remain unproven!

Reamers & Pins

“Recently, whilst replacing my 7-year-old Brompton’s rear mudguard, I noticed play in the rear triangle pivot. Some on-line research suggested that the pin retaining screws cannot readily be unscrewed and would have to be drilled out. A replacement pin is available as part of a kit, but there were ominous references to expensive tooling to deal with the frame bushes. I duly drilled out the screw heads and pushed out the steel pin, which indeed showed some wear at the ends where it runs in the bushes.

To my surprise, the bushes appeared to be only slightly worn and I was able to make up a replacement pin from some over-size silver steel rod.This is now installed and the play has disappeared so I did not need to find a dealer to do the repair.

Assuming the pivot does not seize through lack of lubrication, I doubt if a degree of wear matters much, except when it causes those not used to the Brompton’s rear end bobbing to ask if something is wrong!

George Winspur

Professor Pivot replies: Your observations are entirely correct, George. Brompton does insist that the new bushes be reamed to size using a precise and rather specialist tool. Only a limited number of UK dealers, and distributors in Spain, Germany and the USA are equipped with these tools, so – besides the Brentford factory – these are the only outlets able to undertake a full rebuild.

However, the Brompton functions well with a surprising amount of wear.This can be measured by sitting the bike in the part-folded position and rocking the back of the rear wheel. From my experience, 5mm of play has little or no effect on handling. If modest play has developed, the life of the hinge can be extended by adding a few drops of heavy oil at regular intervals, particularly after riding in the wet.There’s no need to do anything else until play reaches about 10mm, which should take 10,000 miles, or even more. By this time, the bike may feel a bit nervous, particularly on fast reverse curves.

A complete rebuild will probably be needed, but as you also correctly observe, most wear occurs on the pin, so it is often permissible to replace the pin alone. For those with an engineering background, the complete kit (including pin, replacement screws and even a suitable drill should the old screws be seized) costs £11.84 from good Brompton dealers, or the factory.When fitting the pin, make sure the bushes are well coated with grease, but take care to keep grease clear of the threads, both on the screws and the inside of the pin.The screws must be locked in place with a suitable thread sealant – if left dry, they can unwind in use, and ultimately cause the pivot to collapse.

For machines ridden in extreme conditions, the life of the pin can be extended by fitting a grease point, which not only allows grease into the heart of the assembly, but expels grit or water that may have worked its way in.The expert at this treatment is Steve Parry – tel: 01934 516158 or email: spbicycles@aol.com

16-inch performance tyres

“I’ve heard that a new high performance tyre has become available in the Brompton’s size. Is this true? And is it worth buying?”

John Wentworth, Buckinghamshire

Yes, the Schwalbe Stelvio is now available in the Brompton size, but before analysing its performance, we should look briefly at the history of small tyre technology.

With the majority of bicycles designed around tyres of 26 inches in diameter, the technology has always tended to follow the larger sizes, with small tyres – generally destined for children’s bicycles – being bottom of the heap. All this changed in the 1960s, when Dr Moulton introduced his ground-breaking 16-inch machines that performed in a broadly similar way to big-wheelers of their day.This performance was due to a number of factors, including full suspension and relatively high pressure Dunlop and Michelin tyres, developed especially for the machine.

By the 1970s, the mass-market 16-inch Moulton had been replaced by the more specialised 17-inch AM model, and the 16-inch format began to fade away. For the next 20 years, the technology stalled, and when the Brompton arrived in the late 1980s, designer Andrew Ritchie was obliged to choose between two rather mediocre tyres – the Raleigh Record and the Schwalbe, more generally known then by its anglicised name: Swallow. These tyres were heavy with poor rolling characteristics, but they were the best available. Of the two brands, the Raleigh was marginally better.

Fortunately, although the Brompton was only selling in small numbers, recumbent HPVs of various kinds were making a big impact in the USA, creating a completely new market for high quality 16- and 20-inch tyres. Quite what happened next is shrouded in mystery, but it seems Vision – one of the largest recumbent manufacturers – approached Taiwanese tyre manufacturer Cheng Shin with a radical new design.

The tyre they agreed to make was the Primo Comet, and when released in 1996, it transformed the market overnight. Most of the rolling resistance of a tyre is caused by flex in the sidewalls, as the nominally round tyre contacts the flat road and distorts.This effect can be minimised in two ways – by producing a tyre to withstand high inflation pressure, thus reducing the degree of distortion, or by making the sidewalls flexible, reducing the work required when flexing takes place. Clearly, these requirements are somewhat contradictory, but the Primo scored in both departments, withstanding inflation pressure of 90psi or more and rolling better than any small tyre by a fair margin. It was also remarkably light, and tough, provided you didn’t try running a dynamo against the flexible sidewall…

For six years, little changed.The very successful 37-349mm and 37-406mm Primos (13/8″ x 16- and 20-inch respectively) were joined by a narrow racing tyre, but this proved too frail for everyday use and demonstrated no marked benefits over its wider cousins.

…Although some users found the tyre ‘skittish’ in certain conditions, the Brompton tyre was a massive hit…

Brompton was so delighted with the performance benefits of the Primo that the company designed its own tyre based on the same carcass technology and built by Cheng Shin. Launched in early 2000, the Brompton tyre was effectively a Primo with a discreet tread and a dynamo track.The tyre was produced in two versions – conventional, and kevlar-belted for added puncture resistance. Although some users found the tyres ‘skittish’ in slippery conditions, the Brompton tyre was another massive hit, offering long life, reasonable puncture resistance and light weight, together with rolling resistance similar to the Primo. Incidentally, the kevlar version rolls less well, and has been known to fail at high pressures, without necessarily demonstrating greater puncture resistance. At about the same time, Schwalbe updated its 1960s vintage tyre, to produce the Marathon, a very smart kevlar-belted tyre with deep tread and a reflective sidewall.

Arrival of the Stelvio

Last year, the small wheel fraternity got excited all over again with the launch of the Schwalbe Stelvio in small sizes.This narrow racing tyre had been around in 700C form for a year or two, but the first 406mm (20-inch) and 355mm (18-inch, primarily suiting the Birdy) tyres were found to roll better than the opposition, by a fair margin. Schwalbe claims that the improved rolling performance comes from the use of flexible silicon rubber in the sidewalls, and a narrower 28mm tyre tread.Tyre width has a small effect on rolling resistance because narrower tyres have less tread to flex, although in this case, the tyre has been beefed up with a kevlar belt, which probably negates much of the advantage.

Tests of the 406mm tyre, conducted by MIRA on behalf of Inspired Cycle Engineering, demonstrated a significant advantage over the Continental Grand Prix and Primo Comet tyres. However, results from one tyre size do not necessarily stand up for another, and the Primo Comet chosen was 1.75″ in diameter, rather than the sportier 37mm. I decided to gather my own data.

Field Testing

Most rolling resistance tests are carried out in aircraft hangers and other large under- cover spaces, but there’s a lot to be said for trials on ‘real’ surfaces, although field tests must be set up and observed with great care, as wind speed and direction, and air temperature, can have a marked effect on performance. For some years, my tests have been carried out over a 630 metre stretch of ‘B’ category road with what can only be described as a ‘mixed’ road surface. Bicycles are timed over this downhill stretch from a standing start, and – provided weather conditions are similar – it is possible to compare results taken years apart.

In January, wind-free days are rare, although I eventually found the conditions I wanted late one evening.Temperature fluctuated between 3.7 and 6 degrees C, which is on the low side, but the important thing is that all the tyres were tested back to back in near identical conditions. Inner tubes were the standard Raleigh 37-349mm for the Marathon and Brompton tyres, but the smaller Schwalbe tube (claimed to suit any tyre between 32 – 47mm diameter) for the Stelvio. Results are averaged from four or five runs and proved reasonably consistent, although the Marathon took a few minutes to warm up.

The Results

First out was the Schwalbe Marathon.Tyre pressures were set at 85psi rear and 65psi front – pressures that give good results on mixed road surfaces for a cyclist of my build.The Marathon achieved an average roll-down speed of 13.2mph, which is broadly what one might expect from a fairly frumpy kevlar-belted tyre at those temperatures.

Then the new Stelvio tyres were fitted to the bike and inflated to the same pressures. Results were much better, at 13.9mph, which is good for 16-inch, but hardly state of the art. Repeating the runs with inflation pressures of 100psi rear and 80psi front brought a slight improvement to 14mph.

At this point, we know from experience that higher pressures can be counter- productive, as road vibration can actually begin to reduce the rolling speed, an effect that I first observed with the Primo tyre back in Folder 17.This, I should add, is true for the Brompton and others, but fully suspended small-wheeled bikes, such as the Birdy and Moulton, can give very different results.

However, access to the Pantour suspension hub (see page 21), gave a unique opportunity to try the tyres at the maximum pressure of 120psi on a fully suspended Brompton.This produced a more satisfactory roll-down speed of 14.4mph, which is right at the top end of 16-inch performance, particularly in such cold weather.

Remarkable? Well, yes and no.The final test runs (at the lowest temperature, incidentally) were of the standard Brompton tyre, inflated to a comfortable 85psi/65psi. Once again, under the same conditions, the rolling speed was 14.4mph!

Conclusion

One should be cautious interpreting results of this kind.Yes, it’s true that on paper, £20 worth of tyres performed just as well as a £170 suspension/tyre/tube package, but roll-down tests do not give a complete picture.What the figures don’t reveal is that the Stelvio/suspension set up was undoubtably faster uphill, making better use of the power input from the cyclist. It’s probably true too, that the new tyre would perform better on smooth surfaces, so one would expect the Stelvio to shine under racing conditions.

But for day-to-day use, the Brompton tyre still seems to represent the best compromise between comfort, price and performance. Despite descending our test hill at race-tyre speed, it proved extremely comfortable – probably the best of all the combinations tried. Handling was predictable too.The Stelvio was fine at low pressure, but with 120psi front and rear, it proved hard to control, especially on rough tarmac.

Once again I have filled my allotted space, but I hope to return to this matter soon (hopefully in the next issue) with a more detailed review of 16-inch tyres.

The 349mm, 355mm and 406mm Schwalbe Stelvio tyres are available from specialist dealers or by mail order from Westcountry Recumbents mail sales@wrhpv.com Prices start at £13.50 each

Load Carrying

“At present I carry two children to school by car, often picking up shopping on the way home. But the traffic is dreadful and I’m embarrassed to sit fuming in the car when alternatives exist. I saw the ‘Family’ bike at CYCLE 2002 – would this be a suitable bike for my regular school run?”

Hilary Johnson, Blackburn

The Family Bike features a fairly conventional step-thru frame with a number of bolt-on accessories to convert the basic machine into a commuter bike, child-carrier or domestic load-carrier, but it’s hardly a car replacement, and it exhibits a number of disturbing characteristics that make daily use impractical.

Unfortunately, carrying heavy and/or vulnerable loads atop conventional full-size bicycle wheels brings a number of problems.The primary one is stability at low speeds, made much worse in this case by the comparatively flexible step-thru frame. Another serious irritation with carrying a child over the front of the bike, is that the poor creature has to be lifted up, then threaded down through the gear and brake cables into the seat. In practice, most people would find this a terrible chore.

Other weaknesses with the Family include an unreliable stand (a really good stand is essential on a load-carrier) and a crude and rather inadequate derailleur gear system.

…All of these characteristics raise handling issues… the Family Bike is a rather unstable

There’s nothing wrong with the principle though. A cyclist can carry a considerable load (say 30kg) on the flat once they’ve expended a bit of energy accelerating the mass to a modest 10mph or so. At these lowish bicycle speeds, wind resistance is not a serious issue, tyre rolling resistance shouldn’t cause too much hindrance, and mechanical losses can effectively be ignored. So in practical terms, a power output of 100 watts or even less should suffice to keep you rolling, once underway. Stopping involves transforming the kinetic energy into heat, so you need decent brakes, and a load that’s high and/or at the extremities of the machine (see the Oxygen Atala, page 21) may cause handling problems.

On hills, the picture is rather different, because you’re now attempting to lift the mass against the pull of gravity as well as move it forwards, so the power requirement rises very rapidly. For example, if the same rider were to attempt to climb a 10% gradient at 10mph with the same bike and 30kg load, a power output of almost 700 watts would be needed – far in excess of most people’s capabilities.

In practice – assuming the bicycle has suitable gears – most of us climb hills a good deal more slowly.Thus, if our cyclist chooses instead to climb doggedly, expending a steady 100 watts, speed would drop to about 11/2mph, or rather less than walking pace.

Let’s take another look at the Family Bike, and imagine it fully loaded and tackling a hill.With two children and a pile of shopping on board, we have a number of problems: The bike will be moving slowly; the load is positioned high up; it’s mostly carried towards the extremities of the bike; and there’s the extra complication that the child on the front tends to rotate with the front wheel. All of these characteristics raise handling issues. There’s no escaping the fact that the Family Bike is a rather unstable machine – with a heavy load on board, hills would be a nightmare.To make matters worse, the minimum gear ratio of 36″ is much too high to tackle any serious gradient.

The 8-Freight

Let’s put our optimum criteria on a blank sheet of paper and see what we come up with:The best place for the load is in the centre of the bike and low down, so we need a long wheelbase machine, with smallish wheels to keep overall proportions within reasonable limits. Small wheels would also enable the bike to carry panniers at either end for extra capacity, without putting this extra load unreasonably high.

A number of multi-wheel options exist, such as the Cycles Maximus Trishaw, the Brox or the Christiania, but these are large and comparatively heavy machines for ordinary family use and in traffic they get stuck amongst the motorised traffic, putting you back to square one.

For a possible answer we must consult the admirable Mike Burrows. Designing a load-carrying machine for a local welder, Mike produced the 8- Freight – a very long (two-metre wheelbase!) bicycle with 20-inch wheels and an ultra- low well for freight, positioned in the centre of the machine.The 8- Freight couldn’t be more different to the Family Bike – it is not specifically designed as a child carrier, but with a bit of lateral thinking, I feel it fits the criteria rather well.

The 8-Freight answers most of Professor Pivot’s criticisms of the Family Bike. It’s long and low for stability, with a wide, centrally-mounted stand for easy loading and unloading. The long wheelbase reduces manoeuvrability, but it’s still better than a 4-wheeler or a bicycle and trailer

…Safety could hardly be improved upon… the children are positioned centrally in an effective steel cage…

This flexible load carrier is roomy enough to carry two child seats, one behind the other, plus a reasonable load of shopping in panniers, or one child plus extra shopping, or the full supermarket shop, child free.

Safety could hardly be improved upon – the cyclist has excellent all- round visibility and the children are positioned centrally in an effective steel cage.When stationary, the wide stand (again, centrally positioned) provides great stability, enabling the children to hop in and out unaided.The stand is apparently automatic too, descending with the aid of gas-filled struts.

Gearing rather depends on the sort of gradients the bike is likely to see, but brakes are low- maintenance hubs and the 8- Freight is also fitted with Burrow’s trademark mono-blade forks for easy maintenance. In practice, you could probably fix a puncture without removing the load, by swinging the stand down and pulling off the offending wheel.

At a claimed 18kg for the basic machine, weight is remarkably low for this sort of machine, and comfortably lighter than the 21-30kg Family.The sensible weight distribution would make the bike feel lighter too. Obviously a two-metre wheelbase brings problems of manoeuvrability, but unlike a trailer, the 8-Freight is no wider than a conventional bicycle and should be capable of threading through stationary traffic. Remember, we should really be comparing this concept with three- or four-wheeled vehicles, both powered and unpowered – not a conventional bicycle.

Mr Burrows is no power-assist enthusiast, preferring instead to design light, practical machines that can achieve a great deal with human power alone. However, a simple rugged motor such as the Heinzmann would suit the bike well, making this sort of transport practical even in the hilliest areas.

At £400-£700, the Family has the edge price-wise, but even at £975 (excluding any custom extras), the 8-Freight is by far the best option.

Aluminium Rim Wear

If daylight is visible under a straight edge, wear is well underway. This example is close to failure

I always put the correct type of brake blocks on the bike. My steel front wheel is still OK, requires Fibrax brake blocks and the bearings greasing occasionally, though the braking is not as good as the alloy rear.

Ian Fleming
Bath

This is, indeed, a common problem, so you were lucky to get six years use out of the rim! Steel rims will last until they’re consumed by rust, but they’re heavy, and braking effect can be unpredictable, particularly in the wet. Aluminium is lighter and provides a much better friction surface, but the metal wears away rather rapidly, hence the problem. Small wheel rims have less surface area, so wear is greater, and Bromptons seem particularly prone, although statistics being what they are, this may be because Bromptons tend to do a higher all-weather mileage than other folders.

Do alternative brake blocks help? In the absence of proper research, it’s hard to say.Wear is primarily caused by grit caught in the block, rather than the block itself. Larger, softer blocks trap more grit, but apply less pressure to the rim per unit area, and vice versa for the smaller, harder variety. There’s some evidence that the large, soft kind can enhance the machine’s stopping ability without obviously increasing wear, so I’d suggest large blocks if in doubt.The Aztec long block is a good all-round performer, although I would hesitate to claim this was the best, by any means.

Of one thing you can be sure. Ride any distance on a small wheeled bike with alloy rims – particularly on muddy, wet and/or salty roads – and you will eventually experience a rim failure. Rims can fail catastrophically, but this is rare and usually the result of poor maintenance. The chances of failure can be minimised by following a few simple rules:

The first sign of failure. With reduced tyre pressure, a rim like this can be ridden a short distance with great care

1) Inspect the rims regularly, particularly in winter. Failure is caused by many factors, but it’s sensible to start inspecting the rims at 2,000 miles or two years, whichever comes sooner, and checking on a regular basis thereafter. Road grit tends to accumulate around the rear wheel, and it’s here that the inspection should be concentrated. Grinding noises or obvious bright score marks usually mean something nasty trapped in a brake block, and removing it can extend the rim life by many miles. On the other hand, it could mean that the brake block material has worn through, putting the steel backing plate in direct contact with the alloy rim. Never, ever, ride a bike in this condition – the steel can eat through the rim in a matter of minutes.

Failure normally occurs when the rims have worn wafer thin.Wear is easily detected by holding a ruler against the rim, or rubbing your finger tips across it – if the surface feels slightly concave, wear is underway, but if the blocks have worn a pronounced groove, failure may be imminent.This sort of thing can only be judged by experience, so if in doubt, get a second opinion, or simply replace the rim.

2) For obvious reasons, rims tend to fail after the tyres have been inflated. A heavily worn rim can survive for months without any sign of distress, but over-enthusiastic work with a footpump causes a bulge.You can use this to your advantage by deliberately over-inflating the tyres every few weeks and thoroughly inspecting the rims. But don’t try this ten minutes before pedalling to work.

Do nothing and the rim may fail catastrophically...

Generally, a ‘young’ bulge occurs over a couple of centimetres of rim and makes it’s presence felt as a rhythmic thump whenever the brake is applied – mild examples as a gentle tap through the brake lever, but serious bulges may cause the bike to vibrate noticeably. Stop (without a panic brake application, naturally) and take a closer look.

If major damage has occurred, push the bike home. If you attempt to ride, there’s a very real chance that the wheel will either lock solid or disintegrate.

…There is one more option – the brake blocks can usually be realigned closer to the spokes…

3) With mild examples, all is not lost. If the bulge is small, reducing the tyre pressures – 20lbs front and/or 30lbs rear – should get you home. But do ride with caution and stop to investigate any noise or vibration. If there is a growing crack around the damaged section of rim, or you can’t eliminate the bump-thump under braking, there is one more option.The brake blocks can usually be realigned against an unworn section of rim closer to the spokes. If the brakes work smoothly and the bulge in the rim looks stable, it’s permissible to ride on with care.

With the new rim brought up against the old, the spokes can be transferred one at a time

Alloy rims are not expensive, but fitting is time-consuming, so labour costs can push up the price. If you have the confidence, small wheels are much easier to build than the large variety, and with hub gears, there’s usually no complicated rim offset or differential spoke tension to worry about.

The easiest way (provided the spokes and nipples are in good condition), is to put the old wheel in a vice, loosen the spokes, and transfer them one at a time to the new rim.This should be stood alongside the old one, with the tube hole in the same position. Start transferring the spokes nearest to the new rim and work methodically round the rim, fitting the nipples loosely in place as you go. A drop of oil will not go amiss here, as it’s difficult to tension spokes that are tight on their threads, and dry threads will corrode later.When the closest seven (or 9 on a 36-spoke wheel) have been transferred, move straight on to the next group, then cross the wheel to the third and fourth. In an hour, you should be holding a loosely strung wheel.

Small wheels are easy to true as well, but if in doubt, it’s time to call in a professional.Truing involves tightening the spoke nipples very gradually to the proper tension, checking as you go to correct any rim misalignment, either horizontal or vertical.

Experts will run through this process very quickly, but it’s worth taking your time.The wheel can be either mounted in a vice, or refitted to the bike temporarily. It is, however, essential to mount the wheel firmly, in such a way that a fixed point can be brought up against the top or side of the rim as required, to check the alignment.

First hand-tighten the spokes, then give each one a half turn and check that the wheel is starting to come together reasonably straight. As tension begins to build, it’s a good idea to go round the rim, pushing hard against the side of each spoke with your thumbs to ensure the spoke and nipple are properly bedded down.

bicycle-rim-alignment The next stage is to check the vertical and horizontal alignment.To check this, you’ll need a fixed point, such as an old spoke mounted on a heavy block – slide the block forward until the spoke just contacts the high spots on the rim. The aim is to adjust the spoke tension until your fixed point touches the entire rim when the wheel is spun. If the wobble is up and down, gently tighten all the spokes at the high spot and loosen those on the far side of the wheel. If the wobble is side to side, loosen the spokes attached to the appropriate side of the hub, and tighten those on the other side.

With the rim more or less true (don’t spend hours fiddling at this stage), go back to the start and tighten all the spokes by a smaller, but equal amount, recheck, and so on.When the spokes produce a nice satisfying twang when struck, and the rim is as well aligned as you can get it without tearing your hair out, you’re more or less there. If you have time, it’s worth leaving the wheel overnight, squidging all the spokes again with your thumbs, re-check, re-adjust, re-twang, and so on. Perfection is quite unnecessary: provided the brakes work smoothly, there are no vibrations, and the tyre runs a reasonably straight course, the wheel is fit for the road.

After riding for a week or two, it’s worth re-checking the tension. Such attention to detail is sadly all too rare these days, but on a rear wheel in particular, correct tension can greatly enhance the life of the spokes. It’s generally assumed that the spokes break through excessive load – partly true, but the real reason is that a loosely spoked wheel bends and ‘squirms’ as load increases, and this constant fretting movement causes the spokes to fail through metal fatigue.

Brompton Gearing

Having given up hope of getting my Brompton T5 gears fixed, I decided to upgrade to a new six speed version. As I have kept the Schlumpf Mountain Drive from my old Brompton, I have no need of any lower gears. In fact, the transition between Schlumpf top and Brompton bottom is perfect, but I find the Brompton derailleur gears so close together that the difference between them is hardly noticeable. Is it possible to fit a smaller high gear cog, so that I can coast at over 20 mph at a reasonable pedal cadence?

Anthony Lamb, London

Apologies to those who do not own a Brompton, but there is a great deal of interest in this subject and developments are continuing apace. In short, yes, it is possible to fit a smaller sprocket to the Brompton 6- speed, and bigger sprockets with up to 18 teeth can be fitted too.

I am indebted to Steve Parry, engineer of the SP Brompton, for researching this matter. After some experimentation, Steve is now producing a ‘wide ratio’ kit for the 6- speed Brompton consisting of Shimano 12- and 18-tooth sprockets adapted to fit the SRAM hub.The conversion takes less than an hour, but be warned – it does involve cutting, grinding or gently crushing the very rear of one of the frame tubes (invisible, and strength should not be affected) to make room for the 18-tooth sprocket.

The result works surprisingly well.The shift quality is almost on a par with the standard 13/15-tooth combination, but the bike now has a total gear range of 282%.That’s more than a Nexus 7-speed hub, equal to the SRAM 7-speed and almost as wide as the SRAM 3-speed plus Speed Drive (formerly the Mountain Drive Type 2). Leave the standard 50-tooth chainring in place and this extra range will give you a 93-inch top gear which is certainly enough for a 20mph cruise if you have the strength! A better alternative for most people would be to fit the optional 44-tooth chainring, giving a top gear of 82 inches and bottom of 29, as in the chart overleaf.

Before hundreds of readers leap for their cheque books, I should outline a few complications. As the Brompton tensioner is not designed for such a dramatic gear differential, it barely provides sufficient movement to keep the chain tensioned and allow folding in either gear, so a slightly worn chain might cause problems.

Another difficulty, as a glance at the chart will reveal, is a considerable overlap between the gears (although none are identical, as occurs all too frequently with derailleur systems).To avoid disappointment, it’s best to think of this as a four speed system – effectively a three speed with an extra-low bottom gear, or three low ratios plus ‘overdrive’.This leads to another complication: with four ratios where seven would be more usual, the gaps between the gears are quite wide.The final criticism arrived by fax from Holland, courtesy of Simon Korn, Brompton’s inestimable Dutch distributor and another engineer with a long association with the little bikes.

Simon has played with 12-tooth sprockets for years, and claims that sprocket life can be very short – less than a few hundred kilometres in some cases.This seems to be because the derailleur sprockets are designed to be driven by six small pegs, and three must be ground off to fit the SRAM hub. Apparently the pegs ride out of their grooves, bursting the sprocket. All the same, sprocket life depends largely on the rider’s weight and strength, so one fellow’s 200 kilometres, might well be another’s 2,000. An answer might be to braze the 12- and 18-tooth sprockets together, producing a very rugged little block, but that sort of thing can get a bit complicated…

Criticisms aside, the SP conversion does some remarkable things, adding nothing to the weight of a standard 6-speed, and giving a similar range to much costlier, heavier and less efficient options.

My personal preference is for the nippier standard ‘close-ratio’ option, but if I had to choose one bicycle for all occasions, I would probably choose this wide ratio 6-speed Brompton! Whether the sprockets can be made to last will no doubt be revealed. If they don’t, a 13/18-tooth combination would provide a less radical solution. Simon Korn markets all manner of gear options, including Anthony’s set-up: a standard 6-speed plus The 18/12 tooth conversion – space is tight, Mountain Drive, giving 12 evenly spaced gears but it works and a greater range than the Rohloff hub!

Finally, several readers have grumbled that last month’s item lacked fitting instructions and other technical information. Unfortunately, lack of space in a magazine of this kind makes such detail impossible. Both Brompton and the Mountain Drive company can provide written or verbal advice if required (although both may curse me for saying this). But don’t expect Brompton to endorse odd conversions like the above!

A to B Magazine

The alternative transport magazine