Trials Tires

Trials Tires

When Cindy purchased her 5.7 it still had the original Michelin X-Lite tires.  The X-Lite is the lightest and most expensive of all trials tires.  They seem to be highly regarded.  Although not excessively worn, the rear did not grip well.  The rubber seemed quite stiff when pressing on the tread to judge inflation pressure.  I used my Intercomp tire durometer gauge to take the following readings and create the table below.

The results are telling.  Newer trials tires measure 45-ish.  Older trials tires are 50-ish.  A tire that measures 60 does not work right!  I'm guessing the Texas sun baked all the flex right out of that tire.

I'll mention an aside here that may or may not be relevant.  Early in my trials “career,” I overheard the best rider in the club saying how much he hated the X-Lite.  At the time, my OSSA had come with those tires and was still wearing them.  They were black and round and seemed to work as tires should.  I wondered what would cause such a strong dislike (and what worked better) but was too green to ask.

Anyway, I replaced the 5.7's Michelin X-Lite with a Dunlop D803.  Doing so incurred a pretty big weight penalty.  The old, worn, X-Lite scaled-in at 12 pounds 5, ounces.  The new D803 weighed 13 pounds, 13 ounces.

Adding weight to a trials bike is usually not desirable, but in this case, it seems to have had a positive side effect.  Since a torque-mode controller has a propensity to “spin up” the rear wheel with any loss of traction, the increased rotational inertia of the heavier tire moderates that somewhat.  It seems to have made a noticeable improvement.  My suggestion to those of you who own an ePure Race is, don't be afraid to try the Dunlop.

Trials Tire Pressure

I'll mention this here because if you are new to trials, the very low inflation pressures may come as a surprise.   A rider just starting out with our trials club asked me to have a go on his bike and report what the heck was wrong with it.  It was basically unrideable - not hooking up anywhere.  I asked him how much air was in the tires.  Same as his sportbike, 30 psi.  Well, 4 psi in the rear is fairly standard.  The front would be slightly higher, maybe 5 psi. 

The pressure gauge can make a big difference too.  The nice digital one I have for normal use is inaccurate at those low pressures.  I bought a special Moroso analog gauge used for drag racing slicks.  It tops out at 15 psi.  Once you get accustomed to those pressures, you can get a pretty good idea of the pressure just by pressing on the tread and seeing how easily it deforms.  

When I started in trials I did not realize just how important the tires are.  Although having an excellent front tire may be nice, it's really much more important to have an excellent rear tire.  It's not uncommon to “flip” the rear tire halfway through its useful life to again have sharp blocks on the leading edge of the tread.   

How Tire Pressure Affects Contact Patch

Consider a tire with no sidewall stiffness, which is a reasonable assumption for a rear trials tire.

Let’s assume the rear tire is supporting a combined bike and rider weight of 150 pounds.

To make the following calculation easy, inflate the tire to a pressure of 15 psi (pounds per square inch).

This internal pressure exerts an outward force of 15 pounds on each square inch of the tire’s surface.

Now, imagine lowering the wheel to contact the ground.  As this happens, a flat spot forms on the tire where it touches the ground.  This flat spot increases in size as the bike is further lowered.

To support a weight of 150 pounds, the contact patch needs to cover 10 square inches (150 / 15 = 10).

If the pressure is decreased or the weight is increased, the size of the contact patch grows.

There is an optimal-size contact patch for any given terrain.  Low-speed riding generally benefits from a decrease in tire pressure.  Higher-speed riding requires increased tire pressure.

Rolling Resistance

At low speeds (say, less than 30 mph) the aerodynamic resistance of the vehicle is usually much smaller than its rolling resistance.  Thus, when attempting to optimize the range of an off-road EV, tire pressure is worth considering.  Although not specific to trials tires, the following quote from Wong's Theory of Ground Vehicles is applicable.

“Inflation pressure affects the flexibility of the tire.  Depending on the deformability of the ground, the inflation pressure affects the rolling resistance in different manners.  On hard surfaces, the rolling resistance decreases slightly with the increase of inflation pressure.   On deformable surfaces, such as sand, high inflation pressure results in increased ground penetration work and therefore higher rolling resistance.  Conversely lower inflation pressure, while decreasing ground penetration, increases the deflection of the tire and hence hysteresis losses.  Therefore, an optimum inflation pressure exists for a particular surface condition.”

The coefficient of rolling resistance for sand can be 2 - 3 times that of medium-hard soil.  Even with very low inflation pressures, concrete offers much lower rolling resistance than soil or sand.  You probably already have a feeling for this just from pushing a motorcycle around on various surfaces.

Rear Tubeless Tire Mounting

Here is a tip.  Prior to mounting the rear, inflate an 18" tube inside the tire to stretch and expand its sidewall.  This is best done at least a day beforehand, preferably out in the sun.