Tire Pressure vs Carry Weight

[Sdaniels]

What about my pressures?  35F/36R?

[lunkhead]

34.5 psi w/198 lb rider (mid way between the extremes) for front and 36 psi for rear is what Yamaha recommends. Since the rate of psi to lbs isn't linear, you can't use the "for every 49 lbs ADDED, add 1 psi". As weight is REMOVED, you need to decrease pressure more and more for a given weight. The 49 lbs I wrote was a rough rule of thumb for that part of the curve. For a weight bias that close to 50/50%, I'd run FR 34.5, RR 35 psi.

[Sdaniels]

Gotcha.  What about my tire question?  You suggested a 140 instead of the stock 150.  Would I go with a taller profile?  Avon has their cobra chrome in 140/90-16.

[lunkhead]

A 140/90 is probably what you want if you're already scraping pegs and to keep the speedo close to correct. I went with a /90 on the Zero to get past the 85 mph limiter. With a 110/80-18 on the rear, it'll go 0-60 mph in under 4 seconds but cuts power at less than 85 mph.

I see the Cobra is belted and reinforced (heavy) and meant to carry a heavier load (LI 908 lbs) than the stock 150 (LI 761 lbs). I suggest shopping around but there aren't many choices for bias tires in that size.

[lunkhead]

For some reason the OEM Commander II has a lower load rating than the same store-bought tire of the same size so it seems the higher load rating is normal for any replacement. The Commander III might be a better match for the stock front than the Chrome and it's said to weigh less than the Commander II.

[Sdaniels]

I have a Koso so any speedo discrepancy can be adjusted.  I've been wanting to try the Cobra chrome for the rear because they make a stock sized radial....I know, I know, you've mentioned radials on our narrow rims before  but I still want to give it a go.  It's 71V & should be lighter too.

[lunkhead]

There's no 140/90-16 radial. Radials are usually low profile with short sidewalls and fit to wide rims to keep them from flexing sideways. If you use the bias, it'll feel more stable because of the stiffer construction. The SCR uses a 140 radial but on a 17" rim to get back the lost 1/2" ride height. The Avon Storm 3D X-M 150 radial is the only radial I've found made for a 3.5" rim. The Chrome is probably a better tire and although a 3.5" rim isn't recommended, Avon considers it acceptable. The Pirelli Route 66 140/90 might work well if you go with bias.

[lunkhead]

A bias ply connects the contact patch to a large portion of the rim through it's plies so there's really good feedback to the rider. Radial plies only connect to a very small section of rim on the sides which allows the contact patch to easily move sideways relative to the rim and the rest of the tire. Keeping the edge of the tread close to the rim is the only way to stop it without adding weight.

Some people run car tires on the rear and get away with it so no matter what you decide, it couldn't be any worse. Just be careful!

[Sdaniels]

Heard about dark side years ago  yeah, I'm typically a 'don't knock it til you try it' kind of guy but even I won't stick a car tire on my bike.  I had a VTX 1300 & lots of guys on one of the boards did that.  I went a different direction & stuck a 230/60 on mine after one guy tried it first.  Stock size is 170/80.  That rim was 4" wide, once the tire was mounted it pinched the width down to about 205.  It was a radial too.  Never had any issues with it, beads never unseated.

[lunkhead]

A 150 radial is really meant for wider rims on a lighter bike that can lean farther. The best radial for the rear of the Bolt would be at least a 160 on a 5.5" rim. That will spread out the tread to give a wider contact patch and use more of tread's edge. If you squeeze a 150 radial on a 3.5" rim you'll get the opposite effect and the tire will wear faster because of the higher pressure to contact area. A 140/90 bias will spread out more on a 3.5" rim than the stock 150/80 so the profile and contact patch area/pressure ratio stay roughly the same but with a smaller, lighter tire. A 150 radial would need a lower tire pressure to get the same contact patch pressure and run too hot. Running the right tire pressure to keep the rubber from flexing too much would give a smaller, higher pressure contact area which will cause faster wear.

[lunkhead]

I have a whole front and rear end off an FZ07. I know an R1 sprocket will fit the countershaft so a chain and longer shocks can be used. I think the rear has the stock 180/55 tire on it which would be good for carrying a passenger. The front would be shorter with the 17" rim but I think the longer rear shocks would more than make up for it.