6/12/2010

06-12-10 - Some Bike Notes

I found this interesting article on Optimizing Tire Pressure Drop (PDF) . Now I certainly do agree that too many people in the quest for "performance" (both on cars and bikes) think that stiff ride = fast. With that in mind they over-inflate their bike tires, and use too narrow bike tires, and on cars they use over-size rims on tires without enough rubber. Anyway I'm not sure I believe the article, because if I follow it, it suggests really crazy pressures :

I'm about 200 pounds.  Assuming the 60/40 weight distribution is right, that's 120 on the rear and 80 pounds
on the front.

My lightspeed has 23 mm tires front and rear, so the pressures should be roughly 80 psi front and 120 psi rear.
(it's roughly 1:1 for 23 mm which is very unclear from the stupid way they draw the graph)

My city bike has 25 mm tire rear and 28 mm tire front.  The pressures should be :
rear : 110 psi, front : 60 psi

I've been running something like 115 rear, 100 front on my lightspeed and 110 rear, 80 front on my city bike, so according to this article I need to drop the front pressure even a lot more.

One problem I have with this analysis is that it assumes your weight distribution is static. In reality the 60/40 is only when I'm seated. When I am braking downhill my weight transfers forward a lot and with these low pressure the front tire will mush.

I've been having a lot of trouble on my blue bike because of the wheels I bought from Pricepoint. They're good components (Shimano hubs on Mavic Open Pro rims) but I think the spokes are shitty and the lacing job was done wrong somehow, because they keep coming loose; I thighten and true them, and then a week later they're loose again. Contrary to the popular press, I have found that the "pre-built" wheelsets (like Mavic Cosmos or whatever) are excellent quality and hold up great, but the "hand built" wheels that are supposed to be so superior are only as good as the build-up, which if you buy form some cheapo online place is probably not very good.

Anyway, because of my wheel trouble I swapped out my rear wheel for another that I have sitting around with an 8 speed cassette on it (my bike is normally a 10 speed). I switched the downtube lever to friction shifting (instead of index) and off you go. Friction shifting on these many-speed modern bikes is sort of interesting; the cogs are so close together and the shift is so smooth that it almost feels like a completely analog shift. You just move the lever and it silently slips into a very slightly different gear. You can just dial the lever to the gear ratio you want like an analog slider. I wonder when we will have continuous transmissions for bicycles; you could imagine just having a single cog with a ratcheting mechanism to get bigger or smaller, or perhaps more realistically a cone gear with a belt drive like some of the early CVT's on cars (a guide holds the belt to one part of the cone which sets the gear).

Anyway, friction shifting for 8+ gears is not awesome. On flat ground with no load it's fine, but if your bike has any flex at all, it will cause you to change gears when you stand up and dig up a hill, the gears are just too close together to avoid hop. I don't think you can friction shift past 6, maybe 7 gears.

IMO 8 gears was probably the perfect amount. I know some purists will say 5 was enough. Eh, not really. 5 is plenty if you are on flat terrain, sure, but for varied terrain you do want some very small gears, and also some big ones for flats (though I agree with Dave Moulton that the very big gears most bikes come with these days are very pointless; sure it's fun to go 30 mph on a downhill, but you could go 27 mph and it would be almost as good). At 8 you can have enough range and also fine enough steps in the "money zone" where you spend most of your time. Beyond 8, there's little gain from the additional gears, and you start having more problems because the cogs are so very close together, it's more finnicky about having the index adjustment just right, if things aren't right it's easier to get hops into the wrong gear, and of course the chain has to be thinner and weaker.

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old rants