Bike Gremlin

Russian (girls) can drink A LOT!

acidfast7

So can the Swedes, but she really impressed me compared to the continental ladies.

People from that party often meet (in London last weekend we we're with a Danish ex-colleague talking about it) and still discuss it. It was in housing for foreign scientists working in Stockholm and the building functioned as a large sundial.

https://www.google.co.uk/search?q=we...tM7aZ&imgdii=_

Jaywalk3r

I'm not so sure.

The smaller wheel must rotate faster for the same tangential speed, so the linear speed of the spokes on the smaller wheel should be faster.

wphamilton

Think of what the tangential speed is at the ground, in the center, and at the top. That's the speed that the spokes (and hub) are moving through the air, at those three points. Do those speeds change when the radius changes?

Jaywalk3r

The spokes do not extend all the way to the ground. The wheel and tire profile must be considered, and as long as either of those are non-zero (true for any spoked bicycle wheel), the spokes of the smaller wheel will have a higher linear speed.

wolfchild

The speed difference and rolling resistence between 700x28 or 700x38 or 26x2.00 is irrelevant for commuting, it's so minor that majority of people wouldn't even notice it. Speed and rolling resistence may be important for racing but not for commuting.

wphamilton

You didn't consider what I suggested

At the ground the tangential speed is zero. At the center of the hub it is the velocity of the bike. At the top, it is twice the bike's velocity. These numbers are the same regardless of the size of the wheel, therefore the spokes will have the same linear speed.

Jaywalk3r

The spokes do not extend the ground, nor to the center of the axle, and those are the only two points along the wheel assembly's radius at which the linear speed will be the same for the spokes of different size wheels. Everywhere else, which is the entire length of the spoke on actual wheels, the linear speeds are different.

Jaywalk3r

My reasoning was incorrect. It's not clear whether, all else equal, the linear speed of the spokes of the smaller wheel would be faster or slower.

The spokes would have the same linear speed at corresponding points along the spoke lengths, assuming the same bicycle speed, if the wheel assemblies were proportionally identical. In other words, the larger wheel would have to also have a proportionally larger hub and a proportionally taller tire. With all else equal, the smaller wheel has a proportionally larger hub and a proportionally larger tire & rim. The two factors result in opposite, but not necessarily equal, effects on the linear speed of the spokes.

trailmix

I agree, it doesn't really matter so long as you arrive at work on time.
In my experience, my fastest commute times have been on 28mm tires. The next fastest were set on 23/25 followed by 32/35. My commute is all uphill then downhill then uphill so that might factor in the equation. YMMV

Sixty Fiver

I will have to find the study but it recall that it was referring to the M series which now runs wheels as large as 22 inches and this is where the engineers found there was a reduction in performance, particularly in the handling department.

Smaller wheels do respond more quickly and will accelerate faster and the lower rolling speed of the larger wheel may have been where the issue arose, it was also noted that despite their testing the market has demanded these oversized wheels.

Maybe these are great when you can top out the car at very high speed but under normal driving they are detrimental.

Your 3 series BMW came with 14 inch wheels and when those rims and tyres are built to a very high standard this eliminates irregularities and better suspension technology can make up for a smaller wheel like it does on my Moulten bicycle.

energyandair

If you mean that a smaller circumference wheel will always be slower with the same power input, my understanding is that this is incorrect and that, if anything, the reverse is true, particularly at high speeds on smooth surfaces.

The reasons I believe that, all else being equal, the small tire is faster are:

• The frontal area of the tire and rim is smaller in the same proportion as the reduction in circumference (or outer diameter) of the tire.
• The frontal area of the spokes is smaller to a greater degree than the reduction in circumference (or outer diameter) of the tire. The primary reason for this is that as the wheel size reduces, spoke length reduces faster because the amount of the diameter occupied by hub, rim, and tire remains the same. There is a secondary smaller effect stemming from a slightly lower velocity at the outer tip of the spoke where the peak velocity is highest and thus matters the most (square law effect). A third potential reduction in drag is that the greater strength of the smaller rim makes it feasible to use fewer spokes.

As far as velocities are concerned, regardless of the wheel size.

• Where the rubber meets the road, its velocity is zero (unless the tire is skidding)
• The axle of the wheel moves forward at the speed of the bike. (for example 20 mph)
• The top of the tire moves forward at twice the speed of the bike (40 mph in this example)

spare_wheel

i sometimes climb 1600 feet on my commute and i can assure you i would notice the extra lb of rotational mass of a 700x37.

acidfast7

Within the context of an average cyclist, this is the "commuter" forum after all, human being riding a bicycle in real world conditions, I would argue that the reduction in aerodynamic drag afforded by a small wheel is negligible compared to the total aerodynamics drag of the bike+rider.

cobrabyte

energyandair

I believe that this is correct. While the smaller wheel is slightly faster due to less aerodynamic drag, for most commuting use the difference is not large enough to be important.

What is more noticeable in some commuting is that when accelerating from a stop the smaller wheel feels significantly faster. (wheel mass and rotational mass are significantly less). If your commute involves crossing multiple busy roads without the benefit of traffic lights, getting across more quickly feels like a big deal as fast moving traffic approaches!.

wphamilton

A fourth potential reduction in drag, which I haven't seen suggested elsewhere, relates to the dynamics of the shape. It is known that if two protuberances are in a given air flow at the right distance, even if both are fixed to the same object, one can in a sense draft the other. The trailing one occupying or near the low pressure region, smoothing out a turbulent wake. With too much separation they're just two pieces hitting the wind. It is conceivable to me that the system of the leading edge, hub and trailing edge might behave in this manner, the smaller separation in the smaller wheel producing a lower drag. Unlikely but conceivable.


Correct, the spoke speeds will be the same measured proportionally from the contact point to the center, where that section of spoke exists in both wheel configurations. So the proportional difference in drag due to spokes (given the same diameter, shape and number of spokes) is simplified for practical purposes to the proportional difference in spoke lengths.

IMO @acidfast7 is correct that none of this matters for time spent in commuting, and it's a little beyond the scope of the initial question. But who's to say that going a tiny bit faster isn't desirable for other reasons than shaving a few seconds off the commute?

alan s 

Can we get the discussion back to something more interesting, like Russian girls and drinking?

grolby

Really? Quick thought experiment for you: given a range between Crr of 0 and Crr of 1, is rolling resistance irrelevant? I should hope you can see that the answer is no, because there's a point long before Crr=1 where the bike is immovable by any human being, and a point long before that where the effort to move the bike is unreasonably high. Obviously, the range between a fast road tire and a slow commuting tire is a lot smaller than 0 to 1, but at what point does Crr become unimportant? This is basically a grain-of-sand question - if accumulation of something is important, how can you say that one atom of that thing doesn't matter?

I'm not just trying to be a smart-ass here. Rolling resistance is not a trivial component of the drag on a bicycle. At moderately high speeds it can be as much as one fifth to one fourth of the total resistance to movement. At slow speeds, it will be a lot more. Most importantly, from the perspective of a commuter, the effect of rolling resistance is constant at all speeds, including very low speeds. For this reason, rolling resistance is arguably a lot MORE relevant to a slow-moving commuter than to a bicycle racer. And as a bike racer myself, the endless invocation of "unless you're racing" gets pretty old. Because of the way bicycle racing works, with drafting and tactics, tire Crr would predict the time of your commute a lot better than it would the outcome of a road race.

The actual reason that most commuters need not concern themselves with rolling resistance is that most commuters are traveling short distances. If you're riding 2 miles each way to get to work and back home, like I do, the difference in total effort and time between a fast tire and a slow tire are miniscule. But as distance expands, so does the cost of higher rolling resistance. I don't blame anyone for thinking about how fast the tires on their commuting bike are. We do want our commutes to take less effort, don't we? We wouldn't turn down the opportunity to get home five minutes earlier for free, would we? Of course there are trade-offs to consider, like tire life, price, cut resistance and so on, but that's really nothing new.

Maybe this seems like splitting hairs, but the insistence that performance doesn't matter for commuting gets under my skin because it's so trivially falsified. OF COURSE you care about speed and rolling resistance! If a bicycle weren't faster and easier than walking, you wouldn't bother with it, would you? How anyone stacks up their priorities is up to them of course, but it's ridiculous to say that performance isn't on the list.

acidfast7

I agree, but I only mentioned speed and not acceleration.

acidfast7

I think it's not even seconds difference, or if it is, it's only when the world's elite are being measured, which is where these techniques/this technology are/is employed.