Mailbag #5: tyre pressures

From this mail onwards, I'm switching the font convention. Since my answers are always waay longer than the questions, I'm going to take the regular font for my text and that italics font for the queries.

Long time. Got a question for your mailbag : )

This one is about tyre pressure. Your post on unidirectional tyres got me thinking. This one is in two parts…

Q1 While the recommended front tyre pressure stays the same, the rear tyre has 2 'settings', for single and pillion riding. More often than not, I find myself doing a mix of riding. Whether to drop people off or to pick them up. What's the solution for this? Do you go with one setting and ignore the other or do you average the air pressure? Currently, I use the pillion setting when I know that I'm going to have a 'mixed' ride.

Q2 Ok, I'm sure you've seen the blaze tyres. maybe its just me, but when I look at them, their 'ground contact patch' seems circular rather than the 'flat' of a pulsar etc. I've not had a problem with them in the dry, but add some water on the road and I feel the rear acting up. I first felt that while riding in the wet sand at Gorai (the waves were coming in... no, i was not recreating some movie scene, I got caught in the high tide) and then again on regular road which had water splattered on it. do I have to be extra careful with this near circular cross-section tyres? any other recommendations?

Nyctophobia

A1 I actually tend to ignore manufacturer recommendations (I'll call it MRP, manufacturer recommended pressure) almost entirely, and when I do use them, I look at them as a guideline. I am told this isn't the best way to do this, but I'm comfortable with it. One component in the MRP is the superior puncture resistance of a more inflated tyre. Another, is the best compromise between a larger contact patch and higher fuel economy. But the crucial one, is the rider's weight (or in the case of pillion+rider) their combined weight. As is with many things motorcycling, the MRP, therefore, is the best compromise for the rider/combination they profiled for that machine. Which isn't me in many ways.

For instance, I ride much harder than the average rider, so my tyre loads must be different. I use a lot more lean angle, a lot more brake force (I brake later, and harder, rather than softly and earlier). For this, I need more grip, and not more economy, I find that keeping the front tyre to about 24 PSI works for most bikes. On days when I knew I was going to go hell for leather, I'd take my RD350's front tyre down to as little as 18. And my mechanic, an ex-racer, prefered 14! Similarly, I tend to let the rear tyre pressure also be far lower than MRP. It varies with the front, but the difference is usually between 7-10 PSI more than the front tyre. Now, when I have a passenger, I will always alter the pressure. Since I don't ride anywhere close to as hard when I am two-up, I find it convenient to let the pressure be the MRP.

Since tyres are one of the most crucial parts of the ride, I tend not to look at changing/refilling tyre pressure as a chore. You already know I am an OCD case, so I carry my own digital tyre pressure gauge with me...

But to answer you question, if I had to pick between the solo and pillion for a mixed ride, I would also pick the pillion setting.

A2 It's just you. The tyre profile, to reduce it to a very simplistic level, has but one effect. The more rounded it is, the easier the bike will lean. Race bikes, for instance, have tyre profiles that look almost like cones so that they can go from the vertical to knee down in no time at all. So, the Blaze's more rounded tyre, versus Pulsar's more flat tyre? It's in your head as far as acting up in the wet is concerned.

However, when you are riding in the wet, tyre pressures are an interesting debate.

One school of thought says, over-inflated tyres work best. The extra tyre pressure reduces the contact patch, increasing traction (the theory is that the same weight over a smaller patch produces more force pushing downward and therefore more grip) and reducing the chances of hydroplaning (extension of the same logic). The other school of thought says, underinflate. Lower tyre pressure makes for a softer tyre (sidewalls become more absorbent to shocks and help the transition of power from the bike to the road smoother). The wider contact patch offers more grip (force per square inch goes down, but a larger contact patch means more braking or accelerating force can be applied). Any extra heat is taken care, because the tyre is water cooled and while hydroplaning will occur earlier, you only need to be really cautious through standing water. And of course, there is the third school that believes both theories are total bull and only the recommended pressures are the best path. I don't know if I am right or wrong (which is why this is a debate), I subscribe to the underinflate school of thought... But maybe this season, I'll try the recommended pressure school and see.

Here are some links on this subject that I found interesting

• SV650.net FAQs (look about one third down the page or search for Avon)
• Survival Skills UK on Wet Tyre Pressures
• MCN - Track Days
  
• MCN - Advice