The psi on the side of the tire is the maximum pressure you can run on that tire without doing damage to it. What BMW recommends is the pressure they want you to put in your tires for your particular bike. So use what BMW says as long as it doesn't go over the limit that is posted on the tire. (It's the same with your car tires)
That number is a load index. basically more load, needs more air. BMW make those tyre pressure recommendations based on the load of the bike and rider.
If you load your bike up heavy, then increase the tyre pressure.
I use the on board tyre pressure read out, and set the tyres to 36 psi front, and 42 psi rear.
My digital gauge tells me that it is actual 3psi higher, but that has worked for me for long enough to leave it alone.
I get on average 14000 km on the rears, and change two rears to one front.
No signs of irregular wear in over 300000 km of riding this way.
Michelin Pilot Road 4 front and back.
The 42 PSI that you see on almost all motorcycle road tires is the maximum safe pressure at 20 degrees centigrade (~ 70 F) at which the tire can be run. It is also the pressure at which the load rating is established.
It is interesting that BMW recommends 42.6 PSI on the rear for a tire that is rated for 40 PSI max.
I have Karoo 3's, the rear side wall max pressure is 38psi, BMW recommends 40psi. If you go to the Metzler website and look up your bike/tire combo in their technical data book for 2019, Metzler recommends 40psi for a tire rated at 38psi.
I contacted Metzeler reguarding the Karoo 3s with 38 psi marked on the rear tire sidewall, vs BMW recommending 42psi and Metzler's Technical Data book saying 42psi as well.
A few emails went back and forth, in the end they had to run my query up the chain of command as they were stumped by the contradiction.
Here is their reply:
Please adhere to vehicle manufacturer specification of 36 front and 42 rear.
We listed 38 psi max due to the tires being designated M+S (Mud and Snow), but since the Karoo 3 is T speed rated tire please adhere to the 118 mph maximum speed rating.
Kindest regards,
Metzeler team
Still don't get it, but my rear is now pumped to 42psi.
brian
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