There's nothing like nailing a technical section on the Mountain Bike or Sprinting 'til you (almost) puke on the Road Bike, but sometimes it's great to just kick back and enjoy where you are. Here's my some photo's I've taken while out riding and why they are special to me.

 Ashridge woods. about 10am on a November morning. The trails were dry and it was unseasonably warm. The light dappled through the trees in the still air creating visible rays of sunshine.

Wendover woods (just outside the cafe in the woods) 5.45am. It was chaffing cold! The sun had just started to crest the horizon casting an orange haze which gradually faded through Blue to Violet to the Inky Black of night. That the trees framed these colours made it an unmissable shot

This is what freezing fog looks like. It's far heavier than the air around it so lurks in the hollows and makes islands of the hill tops. This was taken atop Ivinghoe Beacon, yet you could have been in the Scottish Highlands.

Same time, same place. The Sun was like a glowing orb and didn't yet have the strength to give that much warmth. I really like the way that taking the shot directly into the Sun makes an outline of the handlebars of the bike.

The Canal Towpath, just outside Tring in the Autumn. I love the glass like reflection in the water and the yellows and browns of the leaves on the path.

Ashridge agricultural college. an impressive building nestled in the heart of Ashridge grounds. I was on the winter road bike and the weather was changeable but the rain had held. It's the clouds that do it for me in this shot.

Just outside Wendover woods whilst testing the Slash 9.0. The Bluebells had just come out and I had to wade into the undergrowth from the side of a lovely undulating piece of Singletrack to get this shot. The tree wasn't fallen, and had grown this way, making a handy bike stand!

I'm sure this year will be full of loads more for me to share with you and I hope I've inspired you to get some arty shots of your own.

Geek corner: Tyres explained.

In this first Geek Corner Blog entry I'd like to talk about those rubber hoops on your bike that get a tough time and are largely mis-understood - although relatively simple things - the tyres. 

I was inspired to write this with the recent spate of 26" vs 29" vs 650B debates in the hope that I can shed some light on what the numbers mean, the positives and negatives of different sizes and widths and how tyres  (in my eyes) could shape the future of MTB set ups. 

Lets's have a look at the writing on the sidewall of your tyres:

FIGURE 1.

26 x 2.40 | 58/60 - 559 

(MTB)

700C x 23  |  23 - 622 

(Road)

What do these numbers mean and do they have any relation to each other? 

We all have grown to know a MTB by it's wheel size, either 26" or 29" - but if you measure a 26" MTB wheel on it's own, the size just doesn't compute - it's much smaller than that measuring around 22 & 3/4"- so the term 26" wheel can be a bit mis-leading

 To get the 'true' 26" you must add the tyre and tube onto the wheel and measure the diameter of the whole wheel / tyre as one assembly. I say 'true' in inverted commas because nowadays it seldom is true. With the advent of increased width knobblies for DH and trail centre use, tyres up to 2.5" are not uncommon.

Take a look at Picture 1. 

I have drawn a square box around the tyre (shaded at the bottom of the wheel) this shows that the height of the tyre is often the same as the width (give or take a few mm's) giving the tyre a box-like cross section.. The wider the tyre, the higher the tyre will become and it is because of this that you will get confusing measurements when placing a tape across a wheel / tyre, with the wider tyres giving you anything up to 27 / 28" numbers. I've found that tyres of a width of 1.75 - 1.9 give the truest 26" reading.

We'll look at how this can affect set up later on in this article, but the figures gleaned here also help us to determine Gear Inches - a topic I'll be covering on a future Geek Corner.

On a Road Bike we get the designation 700C - a metric measurement given in millimetres as opposed to the MTB's imperial inches - which has become the norm for most modern day road going machines, but if we measure the wheel / tyres diameter (as per the MTB) we will rarely get 700mm and can find measurements as low as 680mm on the high end narrow profile tyre race bikes. What gives? Again the tyre width = tyre height comes into play with a narrower width decreasing the overall diameter of the wheel / tyre.

So what does this 'C' thing mean then?

Well it's derived from an old French lettering system which went from A (narrow) through to C (wide) which is a bit of a joke now with 700C tyres available down to an 18mm width these days.

Which brings me onto the second number 700C x 23. and it's very simple, 23 is the width of the tyre in mm's.

Let's take a look at FIGURE 1. again, this time looking at the second set of numbers starting with the MTB of 58 / 60 - 559. 

The first number is the tyres width in Metric (there or thereabouts on a MTB) but don't assume this is the case for the second number. Take a look at PICTURE 1.again

You can see the 'wheel' size listed to the right of the picture and to the left the ETRTO or BSD measurement which is a smaller length dotted line with the numbers 622 and 559 written at the bottom left. 

This measurement is crucial to the fit of your tyre to the rim and is a diameter measurement from wire bead hook to wire bead hook (Bead Seat Diameter or BSD).Without this measurement finding a tyre to fit a given rim would be a logistical nightmare.

This is so important there is a regulatory body called the European Tyre & Rim Technical Organisation (ETRTO) or you may have heard the ISO (International Standards Organisation) tab mentioned also. Interestingly a 29" wheel BSD is 622...

PICTURE 1.

In PICTURE 1. we see at the top of the wheel the ideal tyre widths shown. In reality a rim can support a tyre up to 1 1/2 times its width but this can cause problems.

Let's look at PICTURE 2.

both tyres in the diagram are the same size, but the top tyre is supported by a wider rim, the bottom tyre a narrower rim. 

I wanted to show the importance of correct tyre support here. By using a rim that is as wide as the tyre is high we can prevent the tyre from 'rolling' under lateral (sideways) loading and make it less prone to pinch punctures because of the wider rim profile. Both deal traits for either a MTB or road bike, although the road bike benefits from a tyre profile that similarly matches that of the rim on narrower tyres meaning a much lower weight penalty and good aerodynamics, whereas a MTB equipped with a wide tyre such as the aforementioned 2.40 will require a correspondingly wide rim which will necessitate a heavier or more costly (if materials such as Carbon Fibre are to be used) rim. 

It is worthwhile giving a mention here to the 'wider tyres are faster' debate. This is true, but only at the same pressure. so if we compare a 700c x 37 and a 700c by 23 and we inflate them both to 60 PSI, the 37 will have roll well whereas the 23 will be under-inflated and roll poorly. 

To explain this further we need to look at tyre volume. A rough equation is that if you 'halve the volume - the pressure doubles' and the reverse is true (to a degree) 'double the volume - the pressure halves' 

This can go some way to explaining why your MTB only needs 40 PSI to feel the same as your road bike does at 100 PSI and why a tractor tyre would feel the same at 10 PSI. This also means that as a wider tyre will 'feel' the same as a narrower one at a lower pressure you will have improved traction and comfort giving improved handling and control.

If this is the case then why aren't Pro Tour riders riding wide tyres? Well a wider tyre has more aerodynamic drag than a narrow one does, and as we have seen earlier a narrower tyre will require a higher pressure than a wider one for ride quality, so a compromise must be reached and we sacrifice comfort for aerodynamics (worth it in the long run!) The key to it all is that the wider tyre rolls better than the narrower one AT THE SAME PRESSURE  - the paradox being that higher volume tyres need less pressure - imagine pumping 100 PSI into the tractor tyre!! 

 PICTURE 2.

Why is it that one manufacturers version of a 2.2" width tyre is anothers' 2.4"? well as illustrated above in PICTURE 2. by lines A and B, some manufacturers measure the 'carcass width' (A) and others the tread width (B) giving another confusing set of measurements.

 PICTURE 3.

So how can we affect the set-up of our MTB by using the above information? Well, what PICTURE 3. is attempting to illustrate is that if we use a wider tyre on the front to that on the rear, we will have effectively put a 'bigger' wheel on the front of the bike and a 'smaller' wheel on the back and in these days of offset shock bushes, adjustable head angle frames and headsets and a general 'slacker is better' attitude, we can use the tyres to further slacken the frame angles and drop the Bottom Bracket height (show in PICTURE 3 by the pink lines) giving enhanced handling on steeper technical DH trails.

The advantage to doing this with a high volume wide profile front tyre as opposed to a bigger wheel diameter (such as 650B or 29") on the front (both paired with a standard volume narrow rear tiye) is that you get all of the advantages listed earlier by way of HV (High Volume) = LP (Low Pressure) (better grip, more contact patch, lower rolling resistance) whereas if you simply ran a bigger wheel diameter with the same profile tyre and tube you would only get the geometry change.

The disadvantages as far as I can see are:

Crown clearance - you'd be in danger of buzzing the tyre with the Fork Crown at full bottom out - this can be deadly - you'd have to take the spring medium out and ensure you had he clearance needed

Weight - of the wider tyre and rim combination plus the extra reinforcement needed at the sidewalls to prevent the tyre from folding over which leads us to:

Sidewall flex - a wider tyre if not supported correctly as listed above will suffer greatly from this impairing the advantages

Limited choice of tyres to choose from

Harder to get up to speed than a narrow width 'standard' volume tyre (although it's probably not something you'd notice unduly)

Looks. Trek 69er anyone???

I believe the above listed is something the Manufacturers are missing a trick on - maybe I should get my Patents ready!

There is still much more to explain about these rubber hoops of loveliness, so watch this space for a future Geek Corner where we shall delve into the specifics of grip and what makes a tyre hard or soft and my ideas on future advancements that can be made 'in the field' so to speak.  For now, enjoy Danny MacAskill and Rachel Atherton visiting the Cotinental tyre factory and learning about how their tyres are made.  http://www.youtube.com/watch?v=06QNIjNzqco