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The never ending tyre pressure debate!

August 8, 2016

Mark Morrissey ....The information contained in this blog is provided purely to encorouge motorbike riders to investigate and learn about the most critical thing on your bike.....the TYRES. I am not pushing one or any brand but publishing some information which is public and out there for all to see. You can go toany manufacturer's web site and find the information  page for any tyre you may be considering. What is written below is NOT gospel "you must do this or die" stuff but some basic things to inform and get you thinking.

   

I was a Bridgestone tyre dealer in another phase of my life.

 

Modern tyres are designed to run at higher pressures. And before we start...yes motorbike tyres differ from car tyres but the fundamental principles of tyre design are common to both. Tyres these days are a very different animal to the ones that were on our airheads in say 1971 or even 1996. At the bottom of this blog, I'm going to post a link from car bibles .com and yes...... it is for car tyres but on the pages of this link, the explanation of things relating to tyres and how they actually work and what the terminologies mean are about the best explained I've seen.

 

Some tyres like the Bridgestone BT45 Battleaxes are among the first of the dual tread compound light sidewall construction bike tyres I think. The very latest technology tyres like the Avon Roadriders and the Michelen variation are in another place as far as ride quality and grip are concerned. Certainly if you look at the Michelen it looks more like a track than a road tyre....and there in lies the real story.

 

These days read COMPOUND does the work as much as or maybe even more than tread design. Sidewall design and unsprung weight are a big part of the picture too.

 

I've had endless fun poked at me by people who wave the manual that came with their bike and cite "the BIBLE says....and they built the bike". Happens with everything...look at your phone in 1971 and your phone now...only the naive would think this applies to everything else EXCEPT your tyres.

 

Following on from a massive law suit between Ford and Firestone, we were forbidden as tyre dealers to reccomend pressures to customers. Owners manuals round the world were quietly rewritten with higher reccomended tyre pressures (Remenber, higher pressures bring a harder ride so everything is a compromise). BUT if you get tyres fitted and ask for a suitably higher pressure, you will see the dealer smile to himself.

 

SO....(here we go ) on the side wall of the tyre (any tyre) there is all the information you need to make an informed decision.

(1) there is a serial number and a manufacture date. This is expressed thus: 13-15 or similar and is a universal marking showing the week and the year of manufacture (it is legal to sell a tyre up to 5 years old as a new tyre in Australia) There will also be a max load rating (generally in pounds and KGs, this is per tyre not total.

(2) there are a series of numbers like 90/90/19 V or 3.25 -19. These numbers represent (the first number) 90 (milimeters) or 3.25 (inches) the width of the inflated tyre at THE WIDEST POINT of the cross section of the tyre.

(3) the second number /90 or nothing in the case of imperial measurements is the aspect ratio. In other words the height of the tyre section from the bead to the highest part of the tread on an inflated tyre expressed as a percentage of the width (or first number) so a 90/90 tyre has a height that is 90% of the width. on an imperial measurement, like 3.25 where there is no second number this indicates a 100% aspect ratio. So the height is the same as the width.

(4) the third number, 19 is the size of the hole in the tyre (in other words what size rim it will fit). H and V and possibly other letters (generally the higher in the alphabet the letter the higher the sustained spped rating) following signify the maximum SUSTAINED speed for the tyre to be safe. NOW, HERE IS THE POINT: (please read this carefully and think about it) MAXIMUM SPEED is always calculated at maximum LOAD and at MAXIMUM PRESSURE.

All this information is contained on the sidewall of any passenger tyre regardless of make or model. Have a read and a think about what the manufacturers of the round black things are telling you. Use the link below to begin, then go to the website of the manufacturer you are considering...there are FACT sheets and advertising hyperbole pages. 

I suggest that once you have purchased a tyre, you spend an hour or so on a quiet road experimenting with pressures start low and go up in 2 psi increments noting the change in ride and handling each time. then once you have found THAT sweet spot, vary the pressures front to rear around that sweet pressure (again by 2 psi) with the front lower than the rear and once again, noting the difference in ride and feel...once you have established the best feel....use that...if you are loaded and carrying a pillion increase the pressure by say 4 to 6 psi BUT ONLY UP TO THE MAX PRESSURE LISTED ON THE SIDE WALL OF THE TYRE because this number is  always quoted at MAX LOAD and MAX SPEED for that particular tyre. (have you noticed how often i've said that???)

 

The picture is of a Bridgestone BT45 Battleaxe 90/90/19 on the front of my R75/6. It has done about 13,000 klms and is due to be replaced now. You can see that the tyre is well used  but not scalloped very much at all. 

 

http://www.carbibles.com/tyre_bible_pg4.html

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Copyright 2016 Mark Morrissey, all rights reserved