Why all the excitement about Grand Bois Tyres? Posted on 17 Jul 19:37 , 0 comments
Until Grand Bois started making their range of tyres, there were 2 classes of tyre available: lightweight narrow racing tyres and sturdy, but heavy, wider tyres for commuters, cycle tourists and leisure riders.
Racers typically use a 19 - 25mm wide tyre which gives a smaller "envelope" of air touching the road. This means that the tyre has to be run at very high pressure (110-125 psi) to avoid the rim "pinching" the inner tube over bumps and causing punctures. When I was a racer back in the 1960's we were advised to inflate our tyres "board" hard.
Now these hard narrow tyres work very well on perfectly smooth surfaces, but their lack of compliance actually makes them slower on normal roads. (remember a solid tyre with almost no compliance is very much slower than a pneumatic). A slightly wider tyre can make a large difference to the volume of air in the envelope as volume is proportional to the square of the diameter. For example a 25mm racing tyre has 18% more volume than a 23mm tyre. Interestingly racers have gone from 23mm or even 19mm tyres a few years ago up to 25mm wide tyres now in recognition of this and run on slightly lower pressures giving more compliance on normal road surfaces.
Envelope size and compliance are not the only factors in favour of a wider tyre. There is the matter of frictional losses. The friction losses in a tyre are mainly due to the flexing of the tyre wall when the tyre is compressed against the road. The surprising thing about this is that wider tyres have lower frictional losses! This is because the amount of friction is proportional to length of the "footprint" of the tyre in contact with the road. A longer length of tyre wall being flexed will create more friction. At the same tyre pressure all tyres have the same area footprint. If a bike plus rider equals 200 pounds and the tyre pressures are at 100psi, the each tyre will have a contact patch of just 1 square inch. On a 25mm wide tyre this equates to an approximately 1" length of tyre wall being flexed, but a 47mm tyre will have more like 1/2 inch being compressed as it has a shorter, but wider footprint. So why don't racing cyclists use fat tyres?
At racing speeds, aerodynamics come into it: the aerodynamic drag of a 2 inch wide tyre is twice that of a 1"wide tyre, but aerodynamics only come into play at higher speeds. The aerodynamic drag is proportional to the square of the speed. A racer doing 25 mph will experience 4 times the drag of a cycle tourist doing 12 mph. To the tourist, the aerodynamic drag of the tyre is relatively negligible, but to a racer it's important. So for racers it will always be a compromise and at the moment 25mm seems to be the compromise. Also if you have mudguards fitted then the drag of the mudguards is what matters and the size of the tyres is irrelevant. Many people think that racers don't fit mudguards because of the weight, and we have people making very expensive carbon fibre mudguards. It's a waste of money, for a racer mudguards act as parachutes!
So why aren't all of us non-racers fitting wider tyres? Back in the 50s -80s the standard clincher was 1 1/4" for racers and 1 3/8" for utility bikes. This equates to 32mm for racers and 35mm for utility bikes. Most classic steel racers at least up until the 80s were designed to take 32mm tyres and you would be well advised to fit a pair of Grand Bois Cypres. I did to my 1960 Viking Hosteller and it made a huge difference.
One thing is we all like to emulate the racers, thinking that if we have the same kit we will be as fast as them! Also most manufacturers only make wider tyres in heavier construction, as their main market for these sizes are for commuters/utility riders who typically make short journeys are looking for a more robust tyre.
Well what about weight then? Does weight make much difference? A Grand Bois Cerf Blue in 26mm weighs 260 grammes and that is a pretty light tyre. A Schwalbe Marathon in the same size is twice the weight at 520 grammes. The weight difference is less than the contents of your water bottle! Of course the tyre is on the outside of your tyre so the inertia comes into play when accelerating. Again this is important for racers but not for the rest of us. So the weight is not a huge penalty. So why do these heavy utility tyres feel "dead" and slow compared with a lightweight racing tyre?
The answer is compliance. As stated above, the friction in a tyre is due to the flexing of the walls. Racing tyres have thin flexible walls made of a finer fabric with maybe 320 threads per inch or more, compared with a "standard" tyre of 75 threads per inch or less.
Grand Bois tyres are built like racing tyres: light and pliable, but in wider sizes. This makes them unique. You can have a tyre that is comfortable but fast even over rough surfaces. No other tyre can provide this. Some Grand Bois users understand the science behind them, but most just ride them because they know that they are just the best tyres money can buy.
Choosing Your Tyres
Grand Bois Tyres are available in 700C and 650B. Choose the widest your frame can take. There are hardly any decent road tyres available in 650B. Hetre and Lierre are the ones to choose.