Hoops…

WheelSizeOpt

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.

TyresRims

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.

Advertisements

About gmacleland

Uncool
This entry was posted in Uncategorized. Bookmark the permalink.

5 Responses to Hoops…

  1. Pingback: No sus… | Cleland > L A N D S E E R

  2. 2wheeltrails says:

    I don’t know what you mean by this: “reinforced sidewalls. This feature is critical when using extremely low pressures; without it the tyre simply collapses and your bike becomes difficult to control.”
    Pneumatic tires don’t support the rim directly up from the ground. I run my fatbike with ‘collapsed’ sidewalls at 1-2 psi, and can control it very easily. Can you please explain more clearly what you mean?

    • gmacleland says:

      The tyres I use have more layers in the sidewalls than on the tread area. I have used tyres without this design feature and discovered that it was difficult to control my bike, particularly steering, for the reasons given.

      If pneumatic tyres don’t support the rim directly from the ground up, what does?.

      I can’t really comment on your experience since you don’t indicate the construction of the tyres you use, nor the rim width relative to the tyre width.

      Since you are running at 1 – 2 psi, I would suspect that your sidewalls are ‘reinforced’ in some way, even if this feature is not mentioned in the tyre’s specification.

      Reinforced sidewalls do collapse, but they remain more aligned with the rim than thinner sidewalls, and it is most likely this factor that determines the suitability of any particular tyre to be run at very low pressure, without compromising control.

      • 2wheeltrails says:

        Hi, the upward force on the rim is due to a push by the tire bead. This upward force by the bead occurs at the bottom of the rim. The bead itself is supported from the top of the tire through tension in the sidewall (from the top). The reason this works is because the tire is shaped by air pressure to hold the rim in the center. Of course, air can compress, so the shape of the tire is not round under load, until you reach very high pressure.

        I run 4.6″ tires tubeless on 100 mm-wide rims. The advantage of tubeless is that the tire is more pliable, hence can easier conform to the surface (resulting in more grip) and the inner tube doesn’t rub on the inside of the tire (which lowers rolling resistance).

        I suspect the reason that you love thick sidewalls and thick inner tubes is due to the rebound characteristics. I suspect that you would probably hate tubeless, as it will be very springy: like a basketball bouncing off every rock and root. Downhill tires tend to be made of low (slow) rebound rubber as rolling resistance doesn’t matter as much as control.

        BTW: Love your posts, and thanks for sharing all the design details. Would love to ride your bike. The drivetrain mods would also be excellent for rainy Washington State! Cheers!

  3. gmacleland says:

    Thanks for that.
    We seem to be broadly in agreement, but there are some details where we part company, so to speak.
    I have often wondered about going tubeless, but time is short and what I currently use seems to work well.
    You’ll have to book a holiday in the Scottish Borders!

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s