Elements of Speed
 

I have a theoretical question.

Suppose two cyclists of equal talent and fitness are riding together on paved roads. Rider A is riding a decent, $1500 road bike of 2005 vintage. Rider B is riding an old mountain bike, 1990 vintage, with knobby tires and platform pedals. Rider B is obviously going to be exerting significantly more effort to maintain the same speed.

The question is: what elements of the clunker bike, if changed individually, would have the most impact on increasing the speed of the clunker bike?

This isn't meant to be a practical question -- clearly, it would be better to scrap the old bike and buy new. It is more to examine what elements make a modern road bike faster, on the road, than a clunker bike.

My first-pass rating of changes most increasing of speed (ranked from greatest to least impact on speed), are as follows:

1. Change tires to narrower, high pressure slicks.
2. Change to clipless pedals and shoes.
3. Change to dropped bars, for reduced aerodynamic drag.
4. Change to 700 diameter wheels.
5. Change to lighter frameset.

Would be interested in hearing what everybody else has to say.

number 3-4-1-2-5 It's not so much the weight of the frameset it is the geomatry that allows you to get in a tucked position

You can not change out 26" wheels for 700s. It just won't work unless you put in tons of effort.

After slicks number 2 has to be change the gearing. My mountian bike is 90s and once slicks are on it unless going uphill or into a nasty headwind any decent rider will quickly run out of gears at the high end.

Clipless pedals are worthwhile in any case, moreso for a nonveteran rider who wants to go fast than anyone else. Come to think of it clipless could come before gearing depending on rider skill and power.

The smaller the wheel the 'faster' it is, because of less rotating mass and less aerodynamic drag.

A while back there was a thread '700c and Unsprung Saddles a Lie?' where the fact that a 20" Moulton holds the upright speed record of 51.29mph was brought up often. Apparently the 700c becomes more effective only after 33mph because of gyroscopic effect.

I'm sure the rest of the folder forum will be along any time now...:)

http://members.localnet.com/~milliken/liner/

Nothing in this post has any basis in proper physics.

I am reporting only what's on the Moulton website, including the gyroscopic stuff.

Relative speed aside, doesn't a smaller wheel have lower air drag? Doesn't it also have a smaller rotating mass?

I would go in this order of importance:

3. Change to dropped bars, for reduced aerodynamic drag.
1. Change tires to narrower, high pressure slicks.
2. Change to clipless pedals and shoes.
4. Change to 700 diameter wheels.
5. Change to lighter frameset.

I think #1 & 2 are close to even as far as benefit goes, but it also depends upon the speed. The faster you go, the less difference rolling-resistance makes and more aerodynamics come into play. At low speeds, rolling-resistance is a higher-percentage of the drag and the knobbies will have A LOT more resistance than slicks. At higher-speeds, the difference in rolling-resistance compared to total-drag won't be as big a deal, but bigger tyres will certainly cause more air-drag.

So it depends upon the speed. I've raced a collegiate-Bs crit on my commuter MTB once because I didn't have my road-bike built-up in time. With drop-bars and slicks it was pretty much just as fast as my road-bike, and I definitely had better cornering with the Tioga City-Slickers MTB tyres that looks like it came off a rice-rocket motorcycle. Don't forget close-ratio straight-block gearing to let you find just the right gear at your cadence sweet-spot. :)

I don't know if this has any relevance but even in the electric bicycle forum, there is a concensus that smaller wheels are more efficient than comparable bigger wheels. Of course, there is a question of different mechanics, and electric bikes are significantly heavier, but the topic of less rotating mass flares up from time to time.

The Moulton record was set with an aero box built around the bike and rider.

A smaller wheel has less drag, but if you go purely by frontal area, then a 20" bike looks about the same as a road bike.

Smaller rotating mass has no effect on speed, only with acceleration, and a neglible effect at that. The rotating mass thing is brought up every once so often, but it has absolutely no basis in physics whatsoever. It is just repeated so often it is universally believed. And it has no negative (or positive) effect on climbing. Weight is the sole issue.

Being a former auto technician and road racer I know that there are some benefits to SMALLER WHEELS.

Smaller diameter wheels allow for quicker acceleration but can loose efficiency in the top end. A 700c wheel is better for higher speed consistancy. This is why FWD drag race cars have small 14" or 15" wheels, they just want to get the faster acceleration.

The answers probably depend on what speed the riders expect, too. For instance, aero doesn't matter at all if both bikes are ridden on a rail trail at 12 mph. At virtually any speed, tires would be the first choice. Big fat knobbies have more rolling resistance AND more aero drag. For 17-20 mph speeds, I'd rate them as:

1. narrow slicks
3. drop bars
4. aero section wheels (yeah, they have to be 26" or maybe 650c depending on the frame)
2. clipless pedals
5. lighter frame

Frame weight wouldn't matter at all, except for startups and hills. (Geometry is another issue.) Moulton's claims about small wheels are pure hype. Consider the claims against the fact that he markets a line of folding bikes which use small wheels, and that to sell his bikes he must overcome the general public perception of small wheels belonging only on kids' bikes.

this school of thought was why 650 wheels were big in triathlon for a while. However it's not confirmed in wind tunnel testing. While the smaller wheel presents a smaller profile to the wind, it has to rotate more times at a higher speed, for any given bike speed. Thus increasing the drag from the spokes, or the "egg beater" effect. The studies I've seen indicate its about a wash.

Didn't someone just break the hour record using wheels that had extra weight added?

Weight (or more correctly mass) has nothing to do with speed, only acceleration, at least on the flat.

I went to skinny slicks and aero bars when I had to set up an MTB for a double Century. The other improvements would have gained trivial amounts compared to the drag caused by the rear rack and pannier I carried for gear.

Thanks for the feedback and discussion.

This question originally arose when, in fact, I was riding my own 90s-era MTB (Fisher) along the bike path in Southern California. I don't get to the trails all that often, and so decided to switch out my knobbies -- which just by the noise alone they generated, were clearly scrubbing off alot of speed -- to commuter slicks. Whereas before I was averaging about 13-15 mph on knobbies, I was now averaging about 16-18 mph, with less effort. That is why I, rather unscientifically, ranked tire change first in the list, due to my empirical experience. Good point that a few have made regarding varying effectiveness of tires, drop bars, etc. depending on speed.

It seems many here have ranked a clipless pedal system as fairly low on the list. I'm old-school, and used to use clips on an old road bike, but have never used clipless systems. Nevertheless, I can recall that toeclips and shoes used to help quite a bit, especially in that they allow you to "pull" a bit with the non-driving leg.

Clipless pedals help in spreading the pedalling load to other muscles so they last longer, important since cycling is mostly an endurance activity. They may also help in getting more speed in sprinting since more muscles are recruited. It is not absolutely clear that they would make a significant difference to riders who do not try to get every last little bit of energy into forward speed. Your choice of taking the knobblies off was the most important one.

I used flat pedals as a kid, toe clips as a young adult (into my mid-thirties), and have used clipless pedals for nearly ten years now. As you undoubtedly know from your own experience, toe clips are much better than flat pedals (with no retention) as far as keeping your foot planted and adding some efficiency because of your foot being retained on the pedal somewhat. If you go clipless, you'll find that the increase in efficiency, compared to toe clips, is even greater than the difference between flat pedals and toe clips.........You occasionally hear the analogy that a cyclist's legs are like his/her "pistons." If you know anything about the internal combustion engine, you also know that there are connecting rods between the pistons and the crankshaft. I like to think of the biomechanical interface between the rider's body and the bike's drivetrain in this way, with the foot-shoe-pedal-crankarm being the equivalent of a connecting rod, and the bottom bracket spindle being the equivalent of the crankshaft. Clipless pedal systems play a key role, because they allow mechanical attachment of the shoe to the pedal, much better than simply laying the shoe on the pedal, or enclosing it in a flexible cage such as a toe clip assembly. Obviously, this is not a perfect analogy, but for me it points out the importance of clipless systems in regard to efficiency...........I also suspect that when you initially listed the change to 700c wheels, you were thinking more in terms of adding gear inches (which 700c would do, compared to 26ers). The problem, as others have pointed out already, is that on a standard mtb, that wouldn't be a simple, easy change to make. Adding gear inches through larger chainrings would be the best bet there.

700c wheels are necessarily heavier than 26" wheels because of the presence of additional metal.

The heavier wheel provides a beneficial flywheel effect that is helpful to smooth out pedal strokes and provide some coasting power during gear changes.

This also allows for plentiful lightweight tires. Lighter tires are pointless. More flexible tires are usually lighter, and the flexibility IS the point. Road surface impacts push backwards on your bike. Flexability in the tire overcomes this.

The lighter the combination of bike and rider, the less flywheel effect is needed.
Shoe attachment, biopace chainrings, lower gears, spinning, and quicker shifting all make for less reliance on the flywheel effect.
The heavier bike and rider absolutely require more flywheel effect in order to go at the same speed as the lighter bike and rider. Mavic makes some especially heavy road rims for this purpose.


For your 26" bike, the technology will not work unless you convert it into a road bike--at the cost of the loss of a mountain bike/crusier/hardtail.


A technology from the world of motocross solves the problem for the heavier 26" bike/rider. They're called rollers. They're enormous and can hit the 20's speed range and hold it--IF your bike fits you right and derailer drag is minimized through careful angle/pitch alignment.
Some of the faster options are Maxxis Holy Rollers, Schwinn Typhoon Cord, Schwable Big Apple (now available in a lighter version with a different name and same tread--for your alloy hardtail).
These add the benefits of the 700c diameter onto your gearing.
These add a huge amount of flywheel effect so that shoe attachment is not especially required.
Rollers that work all have a "rolling bricks" or checkerboard pattern.
Rollers that work are made of rather dry rubber that requires a small amount of tread.
It is the dry, non-sticky rubber that gives the greatest efficiency.
The very modest tread is engineered so that it sits on top of the tire casing, causing great flexability, and it is inherently sheilded so that it does not produce areo drag.


More physics:

The reason that we used to throw away bald tires is that they got too much traction on dry surfaces and slid like crazy in the wet. This was fixed for road bikes with the 1980's invention of sticky rubber. Obviously, you don't want much of that to touch the road, and a modest tread is still helpful in keeping more of it off of the road. When sticky, silica rubber tires are engorged to mountain bike size, too much traction/friction occurs. For the mountain bike, classic dry rubber prevents excess friction, but it is unsafe except at large sizes. That is available.

You may not expect criterion bike speeds from this approach. Criterion bikes sacrafice safety, comfort, reliability and strength in return for a bit higher speed. This will not happen by putting rollers onto a mountain bike/hardtail.

You may expect to keep pace with classic road bikes.

You may expect to gain features instead of losing them.


There's several caveats:

Most mountain bikes are slightly undergeared. You will need to purchase a cassette with an 11 tooth gear or perhaps a slightly larger chainring for the front. Most crusiers of the alloy sort that could keep a high pace come with a slouchy riding position (use identical pattern handlebars in a far smaller size) and they are severely undergeared. Most hardtails have enough gears already, so you're set for the next caveat. . .

Most all 26" bikes are too small for the rider. Expect a lot of messing around with the handlebars and seat before you get some speed. The Over The Bars expression that is so common it was shortened to OTB is because most of these have the handlebars too low, thus putting your body's power into the front wheel instead of the back wheel. Ideal handlebar heights range from a minimum of level with the seat to a maximum of 3 inches above. Anywhere here is fine as long as you don't slouch and that your back is flat.

Rollers are hypersensitive to the pressure of the front tire. Get a speedometer and experiment. Expect 40 to 45 pounds in the front tire and 60 to 65 pounds in the back tire. All tires this size accept this pressure range despite the label that might say otherwise. Larger tires are less flexible, and you must manually cause it to flex a very tiny amount by regulating the air pressure. Efficiency can vary by air pressure as much as 30%. For some reason, it doesn't matter as much for the back tire.
Rider position and weight affect roller load handling greatly and may require yet more messing around with the handlebars and seat.

This is a proven technology in that a motorcycle outfitted with rollers goes far faster than the exact same machine outfitted with silica slicks. I have personally tested it with a bicycle, and it is effective at speed.

And then you realize the more flexible the tire is the slower it is.