81 GS750E rear tire size?

[Guest]

my bike needs new tires and originally it had 4.00x18 rear. I want to put a metric size on but there doesn't appear to be a direct conversion from 4.00 rear. it has a 130-90 on now and while it fits I think it is too wide for the wheel. I'm thinking a 120-90x18 would be a better choice but I don't want to screw up the handling. Any thoughts on either a 110 or 120 rear? Upsides? Downsides?

[Nessism]

I'd run the 120. In theory it's a little wide for the 2.15" wide wheel but that's okay. Kawasaki ran 120's from stock on a bunch of KZ model bikes and they also used a 2.15" wide wheel.

[jdvorchak]

thanks Ed. I know within the past year or so I've bought a few sets of tires for models like the KZ750 and KZ550 and I used 110's and 120's on them. This 130 just looks too wide for that rim. I want to avoid excessive sidewall flexing the wide tire would give me.

[Steve]

Besides the flexing, using too wide a tire on a narrower rim will pull the tire into a tighter radius, which actually puts LESS rubber on the road.

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[jdvorchak]

I agree Steve. The contact patch has got to be smaller which is never a good thing. Not to mention a waste of money as the wider tires are more expensive. I know a lot of folks think the wider tire is better looking and they may be thinking it will add more traction. That only works with a wider rim. The engineers had plenty of sizes to choose and if they wanted a 5.00 (130) they would have put one on with an appropriate sized wheel. Most of the bikes of the era had 100 on the front and 110 or 120 on the rear as I remember.

[John Park]

Besides the flexing, using too wide a tire on a narrower rim will pull the tire into a tighter radius, which actually puts LESS rubber on the road.

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Actually, the area of rubber on the road is a function of inflation pressure. A gentler radius tire will produce a wider and shorter contact area; a pinched up tire like this example would have a longer and narrower area. This generates more flex and heat and wear.

Whether a long and narrow contact produces a different amount of traction from a wide and short one is academic. Wide tires will flex a lot less, have more heat radiating area and generate less heat to start with, and that allows the use of stickier compounds, heavier bikes and higher hp and speed.

Personally, I prefer the feel of skinny tires, but then I'm a bicycle guy. Modern chassis designs may make wide tires function acceptably, but I still don't care for how they handle. Plus, the offset loads leaned over are an invitation to the high side hookup - amongst other tradeoffs.

Nobody seems to question how a 'modern' bike will have a 110 - 120 wide front tire but still manages to have balanced traction front to rear when fitted with a 180 on the back. I mean, there's so much more rubber on the road…..