Wednesday, November 26, 2014

Thanksgiving hours and holiday special

HC will be closed for the Thanksgiving holiday weekend, Thursday, November 27 thru Monday, December 1. We will get back to normal hours on Tuesday, December 2. I am more than happy to take orders and answer questions during this time via email during this time. hiawathacyclery at gmail

Don't wait in line for hours like on Thanksgiving and Black Friday for deals. You can get all your deals here at HC. Buy any gift certificate between now and December 31, and I'll add 15% to it. For example, you spend $100 and get a gift certificate for $115, or spend $1000 and get a gift certificate for $1150. You can order this by email or phone, and I can mail it to the intended recipient.

Thursday, November 20, 2014

Are fat wheels heavy?

I think most smart bike people will acknowledge that fat tires have superior traction and flotation on sketchy surfaces, in winter conditions, etc. But the tradeoff is that they're so heavy and slow!

Thirteen years ago, the University of Minnesota decided to grant me a master's degree in geophysics, and I figure it's finally time to apply that credential to some useful purpose. So why not start by measuring something. Here's a 120tpi Dillinger 26x4" on the scale:
The scale reads 1352 g with packaging included. Add in a Q-tubes 26x2.4-2.7 superlight tube and a standard Marge Lite rim, and we have a tire/rim mass of approximately 2.28 kg.

For comparison, consider a more "typical" winter bike wheel. A Schwalbe Marathon Winter 29x2.0 studded tire is listed at 1265g. On a typical 520g rim with a typical 220g tube, this typical winter wheel weighs in at just over 2.00 kg.

But the acceleration of a wheel with an applied torque is not quite as simple as a basic mass comparison. We also have to consider the size of the wheel, since mass farther from the center of rotation is harder to accelerate. The important physical quantity here is moment of inertia. The lower this number, the faster the wheel accelerates under a given torque. Most cyclists prefer a lower value for moment of inertia, though having a higher value increases rolling momentum, which is desirable sometimes. If I assume that the effective radial center of momentum of each wheel is in the center of the tire (halfway between the rim bead seat and the outer tread of the tire), the 29" wheel has an effective radius of approximately 0.340 m. The fat tire 26" wheel has an effective radius of approximately 0.330 m.  The formula for moment of inertia is I = m x r x r. So square the radius and multiply by the mass. For the fat tire 26" wheel, the value of I is 0.24 kgm^2. For the 29" wheel the value is 0.23 kgm^2. These are about double that of a typical high-performance road racing wheel.

I consider these two inertia values to be effectively the same, given the simplifying approximations I made. But the fat tire is widely regarded (by armchair experts) as heavy and slow, while almost nobody thinks to comment on how heavy and slow a typical 700C/29" winter wheel is.

Note that I'm assuming that the mass of a hub and spokes don't contribute to the inertia calculation. Assuming the same hub mass, the 26" wheel has a slight advantage because the spokes are shorter and lighter. But the more important mass is further out on the wheel. That's why I just consider the mass of the rim, tire, and tube. Also, I'm not considering the effect of tire rolling resistance or aerodynamics. My general sense is that a high-quality fat tire inflated to an appropriate pressure has lower rolling resistance than a Schwalbe Marathon Winter, but the skinnier tire has lower aerodynamic drag.

Friday, November 14, 2014

Winter bike stuff

This week, winter set in a little earlier than usual. Given the 10-day forecasts, and the date on the calendar, we probably can't expect much of a break from cold and ice and snow anytime soon.

So I've added a few cold weather items to my Ice Cream Truck:
First, I added 45NRTH Dillinger 26x4.8" studded tires ($250 each or $450/pair). They are costly but worth it, in my opinion (similar 45NRTH tires also available in 26x3.8, 29x2.35", 700x38, 700x30). I missed out on the limited supply of studded fat tires last year, which was unfortunate given the combination of cratered/rutted ice-pack and deeper snow along my commute route for six months of winter. I opted out of bike commuting a lot because it was simply too dangerous on the ice on my fat bike, and too difficult in the snow on my skinnier tire studded bike. So this is the best of both worlds.

Second, the roads get pretty sloppy in places, so I added PDW "Dave's Mud Shovel" fenders to keep myself from getting excessively splattered. I used two front fenders ($18/ea), one on the downtube as intended, and one zip-tied under my Old Man Mountain rack ($135 for the ICT version).

And third, the 45NRTH Cobrafist pogies. These are fairly expensive at $125/pair, but they are a bargain compared to most comparable options, and designed really well. The design allows lots of unencumbered hand movement inside, plus convenient pockets and vents accessible from the inside. The idea of pogies is to allow the rider to ride bare-handed or just wear a thin glove inside the pogies, which means no dexterity loss for shifting and braking, compared to wearing big chopper mitts or lobster claw mitts.
And finally, lights. It gets dark here at 4:30 PM, so having lights that function in cold temps is an essential. I'm running dual NiteRider Lumina 750 lights ($140 each). Unlike alkaline-battery lights, these don't seem to fade in the cold. One is plenty, but having a backup light isn't a horrible idea.