minimizing rotational mass

[thomason2wheels]

First let me apologize in advance if this has been addressed already in another thread, but it seems to me more you reduce the available horsepower, like buying a ninja 250 instead of a 600, the moreimportant saving weight is and especially in the wheel tire combo. There, an additional pound is like 10 lbs added to the chassis. Has anyone researched what the lightest available tires are for the ninja 250?

[Michael Moore]

FWIW, the 250 Production folks who race EX250s report that changing from a 5XX (OEM) chain and sprocket to a 410/415/428 size will show a bump of about 1-1.5hp on an inertial dyno. Obviously, that doesn't actually create more horsepower. Rather, it is the lower rotating mass that lets the dyno accelerate quicker making it look like more power.

IIRC the 2.50x18 Bridgestones used by many of the CB160 Honda racers are about half the weight of a set of 80/90x18 Dunlop or Avon race tires. That is significant when you figure the mass is at a 9" radius and the bike doesn't have a lot of power.

When talking about EX250 tires you need to specify if you want 16" (Gen 1 and 2) or the 17"s that came as OEM about 2008 onwards. The Gen 2 EX I had came to me with an Avon Venom on the back and that thing was very heavy compared to the Dunlop (K391?) I replaced it with.

The spec sheets you'll find at most of the manufacturers seldom show the tire weights. You need to take a scale to the local shop and weigh them yourself. In the early 70s even tires that were the same size/brand/model could vary a pound or so. I suspect they've gotten more consistent since then.

Some of the modern sporting street tires are very light. But that's in the sizes for 600/750/1000s, there's not as much attention paid to the small bikes any more.

cheers,
Michael

[beatr911]

Originally Posted by Michael Moore

...The Gen 2 EX I had came to me with an Avon Venom on the back and that thing was very heavy compared to the Dunlop (K391?) I replaced it with.

Michael, did you notice a difference in how the bike reacted (with respect to rotational mass) with the lighter tire? Was the size the same?

[thomason2wheels]

Hi Michael, my ninja is an 09 with 17 inch wheels....and i think u must be right about tire weight, it just isnt posted on the tire manufacturers websites. Btw thanks for the detailed reply.

[Michael Moore]

I never rode the EX250 with the Avons. They were worn out, so I replaced them first thing. But even with the tread practically gone in the middle on the rear the Avon was still several pounds heavier than the same size Dunlop.

In

Index of /graphics

there are scans of a series of articles from Cycle Guide (files named "ltwtweapon") in which a sub-200 pound 250cc Yamaha MXer was built up. The writer weighed every component that went into the bike, which quickly made it very clear that you can't build something really light without comparing the weights on all the bits. Tires, tubes, handlebars, levers, sprockets, etc etc etc all varied a lot for pieces from different manufacturers.

Once you get started weighing things it is easy to find you've just got to have a nice lab-grade scale to let you chase tenths of a gram.

cheers,
Michael

[sendler]

Is your Ninja not fast enough for you? I have found either of my 250's is easily capable of keeping up with traffic or making a pass at 50 mph. Reducing the tire mass will only help fuel economy if you are in stop and go traffic often and might make the bike feel less stable on the road at speed. It will even tend to reduce your best mpg on the highway if you are good at Pulse n Glide. I would recommend that you place tire wear and wet traction at a high priority and forget the weight unless you are racing. Bridgestone BT023. Or grab the perfectly good new take offs the Ninja and CBR250R throw away from IRC.