Rim & tyre dimensions.

What tyre size do you want? How are you supposed to know and decide?  Bikes used for racing need thin tyres. Hard-core MTB folks want fat tyres. But most of us fall in between.

Added to the issue of width (cross section) is that of diameter. Many people trying to resolve this, and to make the choice easier, have accepted the proposition that 700C rolls better than 26” simply on the basis that 700C is a larger circle and therefore road bumps would seem “smaller” to it. They argue that bumps can be “rolled over” more easily. Look at the marketing associated with the current 29ér fad in MTB. However, research does not absolutely bear this out (see Bicycle Quarterly, Vol.9 No.4 Summer 2011 “Do Larger Wheels Roll Faster?”). The small difference in diameter between the two is less significant than tyre pressure differences.

Narrow tyres may be inflated harder than wider tyres. Harder tyre roll more easily than soft ones.. Also narrower tyres have less air resistance and are normally lighter and therefore require less rotational force to accelerate. But, harder tyres must be kept hard, which is an additional chore if this is your primary transport. They generally puncture more easily and are significantly less comfortable if you ride for 5+ hours in a day. Also, narrower tyres wear out fast and on a tour this may require you to carry spares, offsetting some of the benefits.

Some people imagine that bike touring occurs on rough or sandy tracks requiring fat, rugged tyres. But most cyclists actively avoid soft, sandy surfaces and absolutely avoid mud. Those conditions are simply too exhausting plus they can easily result in harm to the drivetrain. The chain, rear derailleur and cassette, can be worn quite fast if penetrated by sand or mud.  Full chain covers and internal hub gears can cope with some mud and are better for those environments. However tourers know when and where the rainy seasons are, and tend avoid them.

You are unlikely to be seeking out bad roads for the heck of it.  It is more a case of there being occasional bad bits of road which you need to survive without damaging a rim. We want to get “away from it all” but we prefer that this does not involve too much suffering. Damaging a rim on a tour is serious, especially in remote places. Lightweight rims are not recommended for loaded bikes anyway.

Summary so far... for touring, there are cross sections in the 32C-35C-38C zone that are definite candidates. Diameters could be 26" or 700C. And keeping the tyre pressure up (eg above 60psi) will make it easier.

The best quality roads are generally in areas with more people and traffic. But they are busy. We seek out quiter places and these are where roads are sometimes neglected. Here are a few examples in NW Tamil Nadu (India). Whilst sections like this slow you down, they are harmless if your bike is designed to handle them. There is a lot of slowing down and then the need to pick up speed.  Over a longer stretch you’d get sore on high pressure tyres. Added to the higher probability of punctures on high pressure tyres, they don’t seem to be a good idea.  

Meanwhile, there are many great touring routes on which fat tyres would be a unnecessarily slow. On these roads, you could tour on 80psi tyres, move fast and use less energy. So 32C or even 28C for may be ok if you happen to know the route and road surface in detail in advance.  Below on the left is a shot in Iran. Iranian roads are amongst the best in the world. To the right is a shot on the Pampas in Argentina. However these are just good sections and it is not all like that! Carrying spare tyres and swapping them to suit roads is not a good option due to the added weight.

Many roads that are not sealed but still don’t require fat tyres. The shots below are taken within a few k’s of each other in Costa Rica. Fat tyres would be no particular advantage here. Sometimes you need to get off regardless of what tyres you have so as to ensure you get no mud on your chain.

On the diameter issue we like the tyre and rim choices offered in 700C (bead seat diameter of 622). In the 32-35-38 width the range is big. It is far bigger than the 26" range in those widths. Except for extra small riders needing frames where 26 (bead seat 559) in1.4"can be used, 700C"suits our body sizes very well.

Our tyre choice is 700x35C. We use Schwalbe Marathons which are close to puncture proof, very long wearing and can be inflated to 80psi. On good roads, at these pressures, they will roll extremely well. But, subject to loading and speed, the rims are still safe if these tyres are run as low as 45psi.  Hence we base all of our bikes, except for the extra small frame size, around 700x35C tyres. Although Randonneur bikes are made to ride primarily on roads (as distinct from mountain bike trails),  they are expected to get some dirt.

Thanks to sheldonbrown.com for this graphic.

This shot was taken from a 700x35C World Randonneur on a ride with 26x1.75 mountain bikes on mostly rocky trails in mountains in Iran. 700x35 is not the choice if this is all you ride on but if you are on a tour and just want to do a section like this you will be fine. 

Touring in the city.

Our design brief has always been for touring bikes that are ideal for everyday work in towns and cities. It has been interesting to watch the relatively recent growth in cycle commuting in Australian cities with respect to tyre sizes. A lot of carbon fibre road bikes with 23-25C tyres, pumped to 80-90psi are in use. If they pump them up harder they get more punctures but already a puncture a week is common. The riders use backpacks instead of racks and panniers. On some routes these are the most prevalent bikes, especially over distances. We have pondered this and asked ourselves if it suggests we ought to scale our tyres down a size. It seems that narrower tyres are fine and that riders get the speed and efficiency benefits without giving up too much comfort. We suspect that most of these riders also use cars for shopping. But the most telling thing is that the road bikes almost all but vanish from the commuting routes when it is raining. Are they are too dangerous and have to be ridden too slowly to compensate for that danger? Is the greater number of punctures suffered by thin tyres in rain (small bits of wire or glass washed to the sides of the roads) just too annoying? The white markings, wet leaves and steel covers on the roads all become slippery. Hard braking has to be avoided with narrow tyres on wet roads. Holes filled with water are hard to differentiate from simple wet road surfaces. Meanwhile, the 35C Marathon tyres on 500-600gm rims chug along in rain with little or no need to reduce speed.

Some new cyclists imagine that they will commute but in fair weather only. That means they always need another transport option that can be accessed on very short notice. This is not easy to manage even if you do happen to have good public transport near by. If it was fine in the morning but looks like rain in the evening what do you do? Take the bus and leave the bike at work? Our approach is to ride every day and our bikes are designed for that. The exception is really heavy downpours but there are not actually many and they do not typically last for long. Cycle commuters watch the radar and aim to ride between rain storms.

Whilst we recommend 700x35C tyres we agree 32C is ok in several places. This is a basic underlying design decision as the frame and forks have to accommodate 35C tyres. Clearance is needed for mudguards which in turn must match the tyres. Rims have to be suited to 35C (which allows a range so 32C and 38C both fit comfortably). Not wanting to labour the point, we need to emphasis that bikes that start out with rims with an internal width of less than 16.0mm, tyres under 32C and no guards are not likely to accomodate 35C tyres plus guards.

We use a heavier rim on the rear (left drawing). It is 580gms whereas the front is 510gms. The rear wheel has more weight on it. The front has a disc brake so does not need to stay as true and gets no rim wear from the brakes. In an emergency they are interchangable. Both are 36 hole.

What about the claim that tyre size depends on replacement tyre availability?

The claim is sometimes made that regardless of what is appropriate for comfort, efficiency and road conditions, if you can't get spares it was a bad choice. We should all have an issue with this proposition. Most touring bikes use chaingears and long cage, direct mount rear derailleurs that are not available in vast areas of the planet. Even 3/32 chains are not available in many places. So a Rohloff system with a 3/32 chain (which does wear out) has a problem. The gear levers on virtually every modern touring bike are not available in many countries. These things don't stop us. So why should we set out with slow wheels just for a contingency that is avoidable?

We must use very good quality and appropriate parts to begin with. For a long tour, start with new tyres. If they are rated at over 10,000km and you rotate them at 7,000km plus arrange replacements to arrive (somehow) at under 10,000km you ought to be confident.

We should carry spares appropriate for the route. If it is under 10,000km then no spare tyre should be required (assuming you have long-lasting tyres to begin with). But if you are nervous, carry a lightweight folding spare. You can get one at under 250gms as an emergency tyre. And carry a tyre boot (a patch for the inside of the tyre).

We should be ready to have some item sent to us. Just because we are on the ground travelling slowly does not mean things can't be sent to us by air courier. Watch out for customs problems in some places. Use the major couriers and get it from a big on-line store who will get the documentation right.  

Conclusion.

For predominantly on-road touring and commuting, 700x35C is a proven success. For predominantly off-road expeditions you may be better off with something closer to a mountain bike tyre. 700x35C is fine for some off-road work but not good for soft deep dirt or sand. A bike set up for 700x35C can have 32C tyres fitted, is likely to be also ok with 28C, and, depending on mudguard clearances,  may be ok for 38C.

Rim wear.

Of interest are the remarkable lengths gone to by some rim makers to prevent rim wear. Rigida in Europe makes a rim which is blasted with tungsten carbide in a way that fuses it to the rim and presents a very long wearing surface (providing you only use a particular brake block). We have been road testing them and they are good, but very expensive and also heavy. Rigida Andra CSS rims weigh 735gms each. Adding a gram to the outer part of a wheel, due to the need to accelerate it, effectively adds 3 grams to your bike weight. For a very long trip such as 50,000+km, this is an upgrade worth considering.

On our set-up you should find your front rims will last indefinitely and rear rims 15-25,000k depending on the amount of braking (which is much greater in city riding) and your all-up mass. To get the best life out of the rear rim, keep the brake shoes free of grit. Grit that collects in the shoes will abrade the rim. Occasionally you should lightly file or sand the rear brake pads to remove any abrasive material.