Originally Posted by
contango
OK, bad use of the word "everyone" on my part.
When I was asking about whether saving 500g on the weight of a tyre would actually make a lot of difference when compared to a 20-odd kg bike and a 100-odd kg rider the almost universal answer I got was that it would make a significant difference.
Well, "they" were wrong! As long as the tyres are otherwise the same (so say a steel and kevlar Extreme of the same width) tyre mass won't alter maximum speed. (However, if the tyre was made heavier by making the rubber thicker - or adding an armoured anti-puncture belt - then it would be slower, because rolling resistance on a smooth road comes from the effort need to keep bending rubber to re-make the contact patch as the tyre rotates, and this increases as the rubber gets thicker.)
Using F=ma was to figure that if m is cut in half then the same F would result in double the a. Ironically it was thinking terms of F=ma that left me surprised at the claim that a loss in m of well under 1% would actually make any appreciable difference (and a difference the other way when I figured that losing the near-bombproof puncture protection meant I wanted to carry a spare tube and a pump).
You'd be correct to be surprised as your %age is correct. However it applies to accelerating the bike, not to maintaining a constant speed! Again, about 90% of the human race find this surprising. But think of a flywheel on very smooth bearings - will it spin for a longer or shorter time if the mass is increased???
A significant part of the riding I do is on hard and fairly smooth surfaces - I often think it's worth pumping the tyres for the sections of road to get me to a trailhead, then softening them for the trail and pumping them again for the ride home.
That's probably correct... as long as you have a good pump!
At the moment the Marathon Plus I had put on when the bike was new. It's got a slick zigzag down the middle with nobbly bits at the side. The idea is that it offers little rolling resistance on tarmac and on softer surfaces as the zigzag sinks in the nobbly bits offer some traction. I've never really been aware of issues at the front, the main issue I've had was spinning the rear wheel. That could be down to a suboptimal tyre or lack of skill on my part - I'd willingly accept either being the cause.
When the Plus goes to meet its maker I'll most likely look at a Dureme or another Extreme. I've been happy with the Extreme on the back and read a few things about a Dureme at the front and Extreme at the back being a good combination. But then I read a few things about how important rotational mass was, so I'll need to fine tune my "believing stuff on the internet" filters
The classic combination BMX racers use is a tread at the front for control and slick at the back for low resistance. As a general rule, any time for you need tread for propulsion, you need it at least as much for control. *However* this is a very general rule - it becomes truer as you ride more aggressively. If you need tread to climb, and then you're coming down slowly, much less so. So an Extreme at the front would let you descend and corner more aggressively, if that's useful.