Wednesday, 19 September 2012

Tyre Fitting: Tread Direction for off road MTB and cyclocross

Which way round should tyres be mounted for off road use and cyclocross racing? I'm fully aware that some manufacturers have rotation arrows on sidewalls and information on websites and in brochures. However, manufacturer's instructions are inconsistent on the subject, as I'll demonstrate below. And then I'll give my views on the answer!


Off road tyres often have a directional tread. Here is a photo of Michelin Mud2.

The knobs form roughly a V shaped chevron or arrow pointing upwards in the photo above. They are marked on their sidewalls with rotation arrows reading "front" and "rear." Michelin's website says:

"Why a particular direction for fitting?
The direction in which a tyre is fitted will enable the user to optimize the performance of the tyres in terms of braking and traction.
In general, the front tyre is more involved in braking while the rear tyre gives maximum traction.
The rolling direction is marked directly on the tyre by an arrow.
The tyres are also marked “Front” and “Rear”."

Is Chuck satisfied with that? No way! In contrast, this is what the Clement tyres website says:

"Q: Which direction should I install the PDX cyclocross tires?
A: We prefer to install the PDX (and most tires) so the arrow-shaped knobs in the center of the tire face forward when viewed from the top. This is true for both front and rear tires. Some riders may install them differently to get different traction characteristics and that’s perfectly OK!"

The arrows on the side of the Vittoria XG Pro (below) are marked "speed" and "traction".

 The Vittoria website says: 

"Is there a mounting direction for my tire?
Mounting direction is shown by a little arrow engraved onto tire sidewall. In the case it is not visible for any reason, do follow the tread pattern design: if it design an arrow, that has to run forward, otherwiese if the tread pattern itself is specular the tire can be mounted either way."

The Vittoria XG Pro have a similar tread pattern to the Challenge Grifos below:

This is what the Challenge website has to say about it:

"The GRIFO is a very special tread pattern which can be used in both directions!
If you point the arrow < < < of the tread forward the tire is faster having low rolling resistance. 
If you turn the tire around with the arrow pointing backwards > > > the tire has much more grip. In this case it is not as fast because there is an increase in rolling resistance. 
It really depends on the type of course you have. So you can actually find your perfect setup according to the course and your style if riding.
The front tire in most cases is kept with the arrow forward, but in a few occasions can also perform better the other way.. 
The rear tire is usually more suitable to switch directions.
Have fun trying !"


So, summing up the above, Challenge and Michelin seem to be saying that this is usually better (viewed from above):

<<<<<< front <<<<<<< ==========BB========>>>>>>>>  rear  >>>>>>>

Whereas Clement and Vittoria seem to be saying that this is usually better (again, viewed from above):

<<<<<< front <<<<<<< ==========BB========<<<<<<<  rear  <<<<<<<

Faced with this, the thorny issue here is about the orientation of the rear tyre tread. I say there's no issue with the front tyre, because every elite MTB or cyclocross bike I've seen has had the front tyre oriented so that looking from above, the chevron/arrow in the tread points forwards and this has always been consistent with the manufacturers recommendations that I've seen.


As to the rear tyre, I've looked at hundreds of photos of elite riders' MTB and Cyclocross bikes on t'net. This includes a number of ex world champions and national champions (eg Stybar, Wyman, Field, Nys). I found the following:

Out of nearly two dozen bikes of elite riders, only one had the rear tyre oriented so that the chevron/arrow points backwards when viewed from the above. The one exception was Tim Johnson's MTB bike (he's more known for CX) on which he had the rear tyre (a Schwalbe Rocket Ron) the other way, with arrow pointing back when viewed from above. The vast majority of bikes had the Vs pointing forwards on both tyres when viewed from above. This included some fitted with Michelin Mud2s, which as I explained  have arrows on them indicating that the rear and front should point in opposite directions. In other words, the riders do not follow the manufacturer's arrows!

I would rate the empirical evidence and practices of world and national CX and MTB champions very highly - presumably they've done enough qualitative testing and timing to form a considered view. 

I run Michelin Mud2 on my CX bike and I've tried them in both configurations. To me the one with the V on each tyre pointing forwards (as viewed from above) works best. Why should this be? Well it could be a placebo effect - that I'm gaining some positive vibes by simply doing what the pros do! 

On the other hand, perhaps there is another reason. Going downhill across a slope, making an off-camber turn, is one of the more risky manoeuvres in cyclocross. The bike can wash out sideways very quickly on a greasy surface. Let's examine that situation a bit more. One is rarely driving the rear wheel hard and may even be braking a bit, front and rear. The diagram below shows the angle of the chevrons as they contact the ground. The green arrows indicate the direction of the slope. In each example A and B, the tyre is rolling from right to left. In other words, not straight downhill, but slightly across the slope. The red oval shows the contact patch schematically. I've put it on the uphill side, because that is the part of the tyre that contacts the ground in this situation. You could imagine the grey chevrons as tyre prints in the mud. Think of the arms of each chevron on the tyre as little skis! In example A, the tyre is mounted with the Vs pointing backwards as the bike is viewed from above. You can see that the uphill arm of the chevron - in the contact patch - is almost parallel with the slope. It is likely to slip easily just like a ski pointing downhill. In example B, the tyre is mounted in the conventional way for a front wheel - that is, Vs pointing forward when looking at the bike from above. In the contact patch, the arms of the uphill edge of the chevron are roughly perpendicular to the slope. The skis are cross-slope in like a skier digging in edges on a side slope. If you're not convinced about the ski analogy, remember that the rider may be braking. There seems to be a clear advantage of B over A, at least going downhill.

Uphill, you can use the same diagram as above but imagine the tyre rolling from left to right. A has tyres mounted with the Vs pointing forward as seen from above. However, uphill, you will be travelling slower and probably not braking. B seems better than A. I guess that's what makes a good tyre tread design - functioning well in both directions. Remember, the diagram above is very simplified - for one thing, it does not show the edge tread, or the diamonds filling out the chevron in the middle as in the Grifo and Vittoria XG Pro, both of which are bound to affect performance. 

So, having tyres fitted like Vittoria and Clement seem to prefer (Vs pointing forwards front and rear when viewed from above) appears more sure-footed for off-camber downhill tracks and cross-slope descents. It seems to be the more popular approach for pro riders. It's also supposed to be the faster orientation according to the tyre manufacturers. However, reversing the rear tyre could be worth a try e.g. where there are significant stretches of slippery off-camber uphill on the course.


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