Basic ? on tire sizing

[mtdirtbag]

?'s concerning tires on a new 300 RR once received. I'm considering going with the tubliss system once I wear out the OEM tires, most likely will go with the Dunlop AT81 RC at the rear. Not sure on the front, considering the front lasts longer & will make a decision when I replace it (unless I can't get along with the tire that comes on it). The sizing on the rear is throwing me for a loop, I read that a 140/80-18 is the stock tire size and I would think either the 110/90 or 120/80 would be suitable and would think those two sizes are what most go to. The 110/90 is going to have a taller section height than the 120/80 which would be good for rim protection, but not sure how much real world difference it makes. I am vertically challenged so I'm leaning toward the 120/80 - anyone have any insight regarding this please feel free to share.

[Arctra]

Unfortunately the tyre sizing conventions differ between manufacturers. Michelin measure the tyre width from the outside knob across to outside knob, whereas Pirelli measure it from the outside wall to outside wall. So a Michellin 140 is roughly the same width as a Pirelli 120 tyre - I confirmed this when I compared my Pirelli 120/80 tyre with my brothers Michellin 140/80 tyre. I'm not sure about other manufacturers measurements but I believe it is only Michellin that measure it from the outside knobs.

The "/80", "/90", and "/100" part means the height of the tyre in relation to the width. So a Pirelli 120/80 has a height of 80% of 120mm - so it is approximately 96mm high... a 120/100 is 120mm high (100% of 120mm). I'm not sure how that works with the Michellins though I'm afraid, but it is probably the same sort of formula - so a Michellin 140/80 would be 112mm high I'm guessing.

Hope this helps. Another thing you want to consider is the tyre wall strength/flexibility. Due to differences in tyre construction, wall thickness, type of rubber used, the amount the tyre wall flexes differs between tyres. That is why some people find they have to run higher pressures on their tyres to prevent pinch flats (e.g. 14PSI) - because the tyre wall flexes more than other peoples tyres walls who can get away with pressures as low as 8PSI or lower. If you are going to go tubeliss you obviously won't have to worry about pinch flats, but the pressure you run will affect the sort of traction you get and how prone your rims are to being bent because the tyre call flexes too much when hitting rocks and things. I have heard of guys running as low as 4PSI with tubeliss systems which seems insane to me, but then I'm not sure what tyres they are using and how stiff the sidewalls are. I reckon you're going to have to figure it out by trial and error mate.

I was chatting with a suspension tuner recently and he was saying that people don't pay enough attention to their tyre pressures and how they affect the bikes performance. He reckons he can get the suspension good to a point, and then the rider is going to have to experiment with their tyre pressures to extract the best performance out of their bikes. He said people don't seem to realise that the first bit of "suspension" is the flex in the tyre walls. Makes sense to me - that's why the top riders spend so much time testing to get everything right. I asked him why I was having knobs break off the side of my front wheel - he said he could change my valving to prevent that, but given how much I liked the way the suspension was working he would be reluctant to make such a change because it world affect other performance characteristics of my forks, He said given I liked the valving the knobs breaking off was symptomatic of my tyre pressures being too high. After dropping the tyre pressures 3psi the knobs stopped breaking off, I didn't struggle with pinch flats, and my front wheel had better traction in single track. It just confirmed how important testing and playing with tyre pressures for different tyres is.

[mtdirtbag]

Thank you, good points. I was not sure on the sizing in regard to the manufacturers using different points to measure from but did understand the height to width portion. The tire I mentioned is a so-called desert tire and has a stiffer sidewall than the regular Dunlop AT81 and is desirable for lower pressures with the Tubliss system. I've never tried one but they supposedly hold up well in the rock and still provide good traction.

[Enmerdeur]

I am running the AT81 Desert (18-110/90) on my KTM 450 with Tubliss. Works very well. I have it set at 2 PSI. You can run 0 PSI in it if you are doing just slower technical stuff similar to the 739. It is not as stiff as a 739 but is stiffer than most regular tires.

[mtdirtbag]

Ah, real world experience - thank you. How is it holding up both tread wise and sidewall running those kind of pressures?

[Enmerdeur]

Ah, real world experience - thank you. How is it holding up both tread wise and sidewall running those kind of pressures?

I only have about 5 hours on this setup and most of that has been sand so too soon to tell but it is looking good.

[Darkside]

The "/80", "/90", and "/100" part means the height of the tyre in relation to the width. So a Pirelli 120/80 has a height of 80% of 120mm - so it is approximately 96mm high... a 120/100 is 120mm high (100% of 120mm). I'm not sure how that works with the Michellins though I'm afraid, but it is probably the same sort of formula - so a Michellin 140/80 would be 112mm high I'm guessing.

Thanks for posting. I was not aware that the tire height number was a percentage of width. I had always thought it was height in mm.