The old Bonneville needs new tires and inner tubes so I’m wrapping my head around olde fashioned, pre-metric motor-bike tyre sizes.
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| Those would be the Dunlop K70s that came with the Bonnieville back in the day. Fortunately Dunlop is still looking after the bike they designed the tire for. |
The ’71 Triumph Bonneville came with 
It looks like there are modern Dunlop options for vintage SAE (non-metric) wheels at Revco where I’ve gotten my last two sets of tires. The handy chart below shows metric and imperial conversions but after some digging I was able to find SAE/pre-metric modern Dunlop K70s that are the exact fit for the bike. Using recent versions of original equipment that give me the benefit of modern rubber durability and grip while still keeping close to the originally engineering intent in Triumph’s tire choice is fantastic. I’m not trying to recreate riding in 1971, I’m trying to start with that technology and update where prudent for 21st Century use.
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Metric 80/90 90/90 100/90 110/90 120/90 130/90 140/90
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Alphanumeric – NA- MH 90 MJ 90 ML 90 MN90/MP90/MR 90 MT 90 MU 90
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Inch-(Series90) 2.75 3.00/3.25 3.25 3.50 4.00/4.25 5.0 – NA-
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Inch-(Series82) – NA- – NA- 3.60 4.10 4.25/82/4.40 5.10 – NA-
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Size 120/80V16 130/90H16 130/90V16 500S16 MT90H16 3.00 130/90H16 3.00 140/90H16 140/80VB16 140/90H16 150/80V16 160/80H16 160/80H16 200/60VB16 120/80V18 120/90V18 120/90H16 130/80V18 130/70VB18 150/70VB18 140/70V18 170/60VB18 180/55VB18 230/50 X 15 |
Rim 2.75 3.00 3.00 3.00 3.00 3.00 3.50 3.50 3.50 3.50 4.00 4.00 5.50 2.75 2.75 2.75 3.00 3.50 4.00 4.00 4.50 5.50 7.00 to 8.00 |
Overall Width 4.7 5.00 5.2 5.2 5.3 5.4 5.8 5.9 6.0 6.2 6.4 6.8 7.9 4.8 4.8 4.9 5.2 5.4 5.9 6.1 6.9 7.0 9.5 |
Overall Diameter
23.8 25.4 25.0 26.4 25.3 25.6 25.6 24.8 25.8 25.4 26.1 26.1 25.9 25.9 26.4 26.3 26.3 25.4 26.2 25.7 26.0 26.3 25.3 |
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Note: Measurements are based on the given rim widths. A rough rule of thumb: Each additional 1/2-inch of rim width will be approximately 1/4-inch more in each tire width.
The K70s at Revco are just over a hundred bucks a pop and Counteract Balance Beaded inner tubes are actually cheaper than name brand plain old rubber inner tubes. I’ve been using the Counteract beads for years to great effect so they’d be my first choice even if they weren’t cheaper. All in I’m looking at about $320CAN ($250USD) for new rubber for this vintage restoration project, which considering the price of some of the other parts is pretty reasonable.
In order to rebuild the wheels I’ll need to replace the bearings and clean up the brakes before putting new pads and hydraulic brake cylinders back in them. All that and getting the frame sorted out will get me back to a correct rolling chassis ready for the upgraded engine.
Somewhere at the end of all of this will be a road worthy ’71 Triumph Bonneville, but it’s a lot of parts to find and get in and then a lot time in the garage to get there. I’m hoping it’ll be on the road for next year’s riding season. As we thaw out here in Canada I’ll be out on the Tiger and GTR and not spannering so much.
Meanwhile, here are some more motorcycle tire sizing reference charts found on the interwebs:
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