Matching tubes to tires ... or not

[jharvey]

Here’s an odd question - possibly of little or no consequence, but just to be safe - and then a fairly normal question.

I picked up my bike this morning after the installation of a new pair of tires and tubes for my 1980 GS400 (Battlax BT46, 100/90-18 front, 110/90-18 rear (“Tubeless - Use Tube on Tube Type Rim” models)). After putting in 100 or so km towards ‘scuffing in’, I went home and noticed in the work order that while the entries for the tires differentiated between sizes (the front at 100, the rear at 110) the entry for the tubes in just ‘2 tubes - 100/90-18’.

I got the bike just before they closed for a half-day, and they won’t be back till probably mid-next week, so I can’t ask till then if this was just a case of listing the 2 tubes by one size even though they were different on the work order, but I figured someone here might already know the answer to this question assuming the work order is accurate. Would putting a 100 tube in a 110 rim be a disaster waiting to happen, or would it just expand into the extra space? (Or would a 100 tube in a 110 rim be so obviously a wrong fit that an installer couldn’t miss it?)




And since I’m starting out with a fresh set, what are the opinions about tire pressure here - full pressure that the tire’s rated for, or some formulaic number below that?




Thanks folks.

[Nessism]

I think you are fine. Tubes expand to fill the available space. A 100 tube would easily expand enough for a 110 tire.

[Rob S.]

Full pressure is for a full load. Are you carrying a passenger and luggage?

[Grimly]

Tubes generally cover three or so sizes of tyre widths, so nothing to be concerned about.
The max pressure is the maximum pressure permitted under full load - you get nothing near that, so start with the handbook or tyre maker's recommendations for your bike and typical loading and speeds.
Probably 32F and 36R, but might differ a bit from that.

[jharvey]

Thanks for the input everyone. I also went online to a Metzler tube site and saw that some of their tubes were rated for multiple sizes - none that matched my particular combination, but enough to establish in my mind that the practice exists.




Tire pressures are even more interesting. (For reference, 400lb bike and 185lb rider - no extras. Bridgestone Spitfires for as long as I can remember, switched for the BT46s this time around as the Spitfires were no longer available in my sizes. Both Spitfires and BT45s have a 41lb maximum pressure.)

The comments above pretty much mirror my understanding over the years, though it seems every different mechanic in that time has had a different idea of 'what's best.' For the first years that I had the bike (original owner) I went by the manual, which was something like 24/28 city, 28/32 high speed. Then at some point, one of the shops (and there have been a few shops over the last 42 years - some went out of business while others ... lost my trust) would set them at 32/36 when the bike would be in for maintenance (and it may have been my imagination or confirmation bias, but it seemed that tires would last longer before the sidewalls would start cracking at the higher pressure). The place I've gone to for the last half-dozen years or so (no complaints here), would have them set at 34/34 when I would pick it up, until this last time, when they were 34/40.

This also had me wondering if the 40 in the rear was to help 'stretching' the 100 tube into a 110 rim, or if there had been some change in tire technologies since my last set that would explain that higher rear pressure, but I guess it was just that particular mechanic's 'default.'

Thanks again

[Brendan W]

Pressure is a balancing act between not too high to reduce your contact patch and not so low as to cause excessive sidewall flexing and heating.
A rule of thumb is to check before and after a ride and if the hot pressure is 10% above the cold you are in the sweet spot. More than that and you are under inflated and vice versa.
Tyres available today are very different from when our manuals were written and generally need higher pressures.
For example the 850 manual specifies 25/32 psi for normal two up riding on the original 3.50/4.50 tyres. I ran them at 30 front and 36 rear on 100 and 130 Continentals.
Talk to the manufacturer of your tyre for advice.

[rphillips]

Things have changed, yrs. past most tires had the recommended pressure, checked when cold, to run. Today many just have Max. pressure stamped on them, that tells me nothing about what should be a normal operating. pressure. I liked the old way best.

[alke46]

Totally agree with Grimly. 32/36. Use this setup before going out on a ride long enough to heat the tires up. Then check them and you should be at 35/39 or maybe 35/40. That's the 10 percent rule. Should have you real close.