Up until the mid 70s, the only knobbly tyre size available for bicycles was a 26″ X 1 3/8″ (or 37-590) made by Avon and called ‘Skidway Gripster’. It was discontinued around that time, and then BMX style tyres appeared in the UK, up to two inches wide, and 20″ in diameter.
Experimenting with these showed me that diameter is more important than width. The reasons are that smaller wheels have a shorter contact patch with the ground and potential traction is thus limited, moreover, they can fall into holes more readily, as shown on the illustration above.
The effect may not look too extreme, but the arrow lines show what proportion of your wheel is ‘in the hole’. The larger wheel is only one-third in, whereas the smaller one is half in, requiring more effort to pull it out and continue riding. The drawing is not to scale and so doesn’t exactly represent any particular size of wheel; I’m sure you get the drift.
By the end of the 1970s I had discovered, in Finland, the 26″ X 2 1/8″ (or 54-584, or 650B, or 27 1/2″) size made by Nokia and called ‘Speed Hakkapeliitta’ (with the option of tungsten ice studs) and these were fitted to all the early production Aventuras.
Nowadays there is a wide choice of the larger 29er tyres and the Landseer’s are 60mm wide, made by Schwalbe and have reinforced sidewalls. This feature is critical when using extremely low pressures; without it the tyre simply collapses and your bike becomes difficult to control. Regardless of the experts’ (and manufacturer’s) recommendations, these wide tyres are fitted to the narrowest rims.
Posted by JezT
Those sturdy 29″ downhill tyres and motorcycle inner tubes, run at unusually low pressure, are providing faultless traction. Geoff mentions at this point that we’ve taken the less-muddy route to the one he usually rides.
Internally there are motorcycle-specification innertubes, which cope better with low tyre pressures, around 4 – 8 psi in the front, 8 – 12 psi in the rear, for riding the most challenging terrain.
Posted by JezT
Am I meant to be riding a couple of grand’s worth of shiny technology through all this? Geoff pootles off through the mire, I mutter a couple obscenities and set off after him. Can I hack through without putting a foot down?
Geoff is making my least favourite terrain look a doddle. In fact, it’s quite infuriating to watch this pensioner sail through the gloop while I struggle to keep moving.
Low pressure tyres give disadvantages, and advantages.
The main disadvantage is that rolling resistance is noticeably higher, not ideal if you’re late for an appointment. A secondary issue, particularly with narrow rims, is squirm and wallow; squirm is where your front tyre is gripping the ground more and resulting in a certain amount of oversteer, wallow is where your rear wheel is able to move laterally in relation to the tyre’s contact patch, which can also affect steering somewhat. These effects are quite disconcerting for a short while at the very beginning of a ride but, because they provide such tremendous advantages, are quickly accommodated. Lastly, the fabric of the tyre has to flex much more, inevitably reducing its overall service life.
Posted by ajantom
It [the Landseer] even goes downhill amazingly well. The closest analogy I can give is that it ‘flows’ over stuff. [like water]
Very low tyre pressures – 4-6 psi – in 2.5″ 29er tyres – mean that you have to get used to the tyre squirm and wallow, but it never feels unbalanced, or un-planted.
The advantages are several. The most important is increased rolling-resistance, another way of saying ‘loadsa grip’, mainly because the area of the tyre’s ground-contact-patch is significantly increased with a width of nearly four inches. The Landseer’s narrow rims allow your tyres to flex and mould themselves to the terrain, giving you a sense of fluidity. They’re less likely to pinch the tube because the rim flanges are nearer the centre of the airspace. If my experience is anything to go by, I have had not one single puncture since first using this combination about six years ago; I no longer carry a puncture kit, on any ride.
A further advantage with this set-up is that it provides an extremely efficient high-frequency element of your suspension system, the low-frequency element being your arms and legs, as described previously.
It’s not just size, it’s where you put ’em.
Click here to go to the beginning of the whole story.