Tyre sizes for GS650G (M) "Baby Katana"?

[2BRacing]

I have just finished the revival of my brother's 1981 GS650 M "Baby Katana" after storage for about 25 years. The bike has only done about 10 000 km! I have posted about it in other threads.

My experience with tyres is only related to my GS850G where I run 100/90-19 front and 130/90-17 rear BT45's on the OEM 1.85 and 2.50 rims. The 650 M also has the OEM 1.85 front and 2.50 rear rims, and the factory service manual's specifications for tyre sizes (still in the "old" units) is 3.25-19 front and 4.25-17 rear. However, the tyre size specifications for my 850 in the "old" units is 3.50-19 front and 4.50-17 rear, even though the rim widths are the same for the two bikes.

I would like to hear from owners of 650's which tyre sizes they are using, as I need to know if I should use 100/90-19 and 130/90-17, or go down one size in the width of the tyres as implied by the difference in OEM tyre specifications for the 650 vs 850?

Thanks!

[tom203]

I'm using the 100/90-19 front and the 130/90 rear .My "L" uses the 16 inch rim, but there is good clearance to clear swingarm tube, so 17 inch should be no problem

[Steve]

I don't have a 650M, but I would guess that the smaller tyres were because it was a lighter bike.

Something else to consider: if you take the 3.50 spec for the front tyre and multiply by 25.4 (metric conversion), you end up with 88.9. According to that, we should be running 90/90-19 tyres on our 850s. Many of us have found that handling is not sacrificed by going to the 100/90, which removes most of the speedometer error.

Along the same line, the 4.50 spec for the rear works out to 114.3mm, so we should probably be running 120s on the rear, but we don't.

For the 650, the 3.25 front works out to 82.8mm, so a 90 front tyre would be called for. The 4.25 on the rear works out to 107.9, so a 110 or 120 might be better.

Bottom line: you could probably run the same 100 front and 130 rear, but handling might be a bit crisper with 90 front and 120 rear.

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[Gorminrider]

The 650 M also has the OEM 1.85 front and 2.50 rear rims, and the factory service manual's specifications for tyre sizes (still in the "old" units) is 3.25-19 front and 4.25-17 rear

This is the same as my normal 650G and I have 100/90 19" Front 120/90 17" rear. They are fine. I think these bikes are nice handling and it'd be a shame to run fat tires on them...which seem to be harder to lean. Stay as close to stock as you can within the parameters of the tires you want and can afford is what I would say...I wanted a "set" so I ended up with the sizes above at a very reasonable cost but I looked first for the originals in all respects.

[2BRacing]

Thanks for the replies guys. Unlike many, I am not a fan of "the wider the better" tyres, and prefer decent handling instead.

I would have preferred to go with 90 front and 120 rear, but when I enquired about those sizes for the Bridgestone Battlax BT45 tyres I want to fit, I was told that although those sizes are made, the local distributor does not bring them in to South Africa.

So I will have no choice but to go with 100 front and 130 rear!

[Steve]

I do not think you will be disappointed.

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