A bicycle is a pretty simple and elegant device. The basic bicycle has been around for around 100 years. Most ideas for bicycle parts have withstood the test of time. Nonetheless, there are always new ideas. Some of these are good, some are bad, and some are ugly (like the Campagnoly Delta brakes). The best overall source of advice is a gentleman by the name of Jobst Brandt, a mechanical engineer. His thoughts can be found on rec.bicycles.tech . There are a few places where his advice and mine differ, but not many.
There are some people who are obsessed with the weight of a bicycle and/or the parts on a bicycle. Unless you are a professional (one who makes money riding a bicycle), or have really minimal bodyfat (like 5% for men, and 12% for women), I think it is silly to worry about a few ounces or even a pound of weight on a bicycle. After all, a full water bottle weighs over a pound. You can likely stand to lose a pound. Many bicycle designs to shave weight do so at the expense of safety, reliably, or cost. I have seen really silly attempts to save weight, including a frame with a series of 1 inch diameter holes drilled in it. That being said, there is no reason to have a frame like the original Schwinn Varsity which likely weighed 10 pounds.
The frame is the heart of the bicycle. There are many designs, but the most common (and overall best) is the so called 'diamond' frame. It is rigid and light. For offroad bicycles with a rear suspension, there are many modifications to this basic design. These modifications allow suspension travel which is good, at the expense of weight, rigidity, or durability.
There are many different frame materials. The most common ones are steel, aluminum, titanium, and carbon fiber. All of these materials can make a good bike frame, as well as a bad bike frame. The quality depends on the material, the design, the quality of the construction. Most frames are made out of tubes. These tubes need to be joined to each other at odd angles in order to form a frame. This requires reasonably high precision and good techniques. One interesting feature about most things constructed of tubes is the stress is highest near the joints. One clever thing to do is to make the tube thicker near the joints and thinner near the middle. This is known as 'butting'. Quality tubes are 'double butted' which means they are thin in the middle, and thicker at both ends.
Steel is the most proven material, and can make a light, durable, reasonably inexpensive frame. There are two common families of steel used to make frames. One is chromium-molybdenum (Columbus) and one is manganese-molybdenum (Reynolds 531). These steel tubes generally come from the factory already heat treated. Therefore, when joining them it is a good idea to minimize the temperature. Welding is generally a bad idea, as it involves melting and fusing of the steel, destroying the heat treatment. Brazing is a lower temperature method of joining tubes. Silver soldering is brazing using a silver brazing material, and is done at a lower temperature than conventional brazing. Although it is possible to build the frame, and then heat treat the finished assembly, I have never heard of it being done for steel frames (it is common for aluminum frames).
Aluminum is available in a wide variety of alloys for making bike frames. Aluminum is lower in density than steel. Early Aluminum frames such as the Alan and Viscount, has similar diameter tubes as steel frames. They weren't much lighter, and they were quite flexible. (This is because the stiffness of a tube is proportional to its radius to the 4th power. That means that a 2 inch diameter tube is 16 times stiffer than a 1 inch diameter tube.) Since aluminum is less stiff than steel, an aluminum tube with the diameter of a steel tube will be less stiff. (The modulus of elasticity for steel is 190-210, aluminum is 69, titanium is 105-120, carbon fiber reinforced plastic 125-150.) So the obvious solution, is to make the aluminum tubes bigger in diameter. This will produce a frame that is as stiff or stiffer than a steel frame, and almost always lighter. The downside is many parts used to be made to fit the standard steel frame. This isn't an issue any more, due to the popularity of 'oversized' tube aluminum frames. Cannondale uses 6061 aluminum, and heat treats the entire frame after welding. This can produce slightly warped or bent tubes. However, all the mounting points are perfectly aligned, so it is a minor cosmetic issue. Well known brands are Klein (who popularized oversized aluminum frames) and Cannondale (who made aluminum frames popular.)
Titanium frames are available in two common alloys 3AL-2.5V and 6AL-4V. 6AL-4V is stronger, though most tubes are formed from flat sheets and have seams. This is heavy and weakens the tube. It is also more expensive than 3AL-2.5V. Many titanium frames don't have butted tubes. The first titanium frame was the Teledyne Titan. I remember seeing a road and track bike in 1974. The problem with the frame was the welding was not done in an inert atmosphere, and titanium when hot is quite reactive. The welds were contaminated, and failed. In order to weld titanium, you have to keep oxygen away from it when it is hot. The simple way to do this is to use vacuum or an inert atmosphere. It isn't cheap. Other methods have been tried with variable success. Commercially, people use MIG welding and lots of inert gas. It may work. Some well known frame makers are Litespeed (expensive) and Habanero (less expensive).
Carbon fiber frames have been available for a while. They are really carbon fiber in an epoxy matrix. There are basically two ways to make a carbon fiber frame. The first is to make carbon fiber tubes and then join them. The second is to make the whole frame (or large parts of the frame) as one piece. Using tubes is more flexible, but there is the problem of joining the tubes. Making big pieces is simpler, but harder to make different frame dimensions. Carbon can be lighter than steel, aluminum or titanium. It will likely be more expensive. The material is easier to damage than metals. If you bike is likely to get banged around, carbon isn't for you. Well known brands are Trek, Cannondale, and Klein (who popularized oversized aluminum frames.)
I read about problems with disk brakes on bicycles at least 20 years ago (written mainly by Jobst Brandt). Being a fool, I ignored what I read, and purchased a bike with disk brakes. (Amp Research). It was a big mistake. There are many good things about disk brakes, but there are many bad things about disk brakes as well. Some good things: Brakes work well when wet, due to very high pressure. Slightly untrue rims won't limit braking. They won't heat up your tires and make them pop. Now for the bad things:
A disk is generally very small and light compared to a rim, so it will get hot very quickly and stay hot, as it can't radiate nearly as well as a rim. Of course, you could get a vented disk, or a big heavy disk, but nobody offers such for bicycles. Think about how much area to radiate heat your rim has, and how much area your disk has. Now realize that your rim is aluminum which is a pretty good conductor of heat (cpu heatsinks), and your disk is steel which is a much worse conductor of heat (ever seen a steel cpu heatsink?).
Lets do some math. Disk rotors are commonly available in 140, 160, 170, 180 and 200mm. Santana has a 10 inch rotor (254mm). Now lets consider a rim. Road wheels are now 700mm (622mm rim) as are 29 inch mountain bikes. For a disk, I will assume it has 20mm of braking surface. If you have a 180mm rotor, it has about 201 sq cm of radiating area. For the rim, lets consider one with 30mm high sides and 10mm wide at the bottom, giving a surface of 70mm. So the entire rim has a surface area of 1363 sq cm. This is 6.8 times more surface area than the disk. This means that the rim can radiate heat 6.8 times faster than a disk. Now if you are riding in the rain and have water to cool your disk this may not be a problem. But if you are riding down a hill or even a mountain, your disk will overheat. Santana did a test (with a tandem and no doubt 2 people) and their 10 inch brake caliper got up to 526.5F, which was enough to melt the plastic parts. If you have a carbon frame where the caliper is attached, it will likely melt too. Now I don't weigh as much as a loaded tandem, nor do I go quite as fast, but the test was only on a 15 percent grade dropping 528 feet. I do go down long steep hills and I have gotten air bubbles in my hydraulic cylinder, which really sucks for braking, much more than a bit of rain and rim brakes.
Disks require high pressure and therefore either need a mechanical system with very high leverage which will put lots of force on relatively small parts and likely have lots of friction, or a hydraulic system. There is nothing wrong with a hydraulic system, except hydraulic fluid expands when hot. If you don't have a reservoir for your hydraulic fluid, you will end up with air bubbles in your hydraulic system, and your brakes won't work well. I have yet to see a hydraulic system on a bike with a hydraulic reservoir.
If you look at the where the caliper is placed on the front fork, it is always placed behind the fork tube. This looks pretty, and the fork tube will shield the caliper from brush and the like. Unfortunately, this placement will cause a minor problem, which is it will try to eject the wheel from the bottom of the fork with roughly 2000 newtons of force on the side with the caliper. That is why there are huge lips on the bottom of the fork. Despite these lips, the force will loosen the quick release, and can still eject the wheel from the bicycle. See Disk brake and quick release problem for details. If you put the caliper in front of the fork tube you will likely be safe, but nobody does that. It isn't a problem generally for rear wheel disk brakes, as the caliber will end up back in the drop-out (unless you have drop-outs that are open in the back.)
Disk brakes have a lot less leverage than rim brakes. The amount of leverage is a function of the rotor size. So if you have 180mm rotors and 622mm rims, that means you have 3.4 times leverage. That means you will have to exert 3.4 times as much braking force with disk brakes vs rim brakes. This is not news. See Santana They said a factor of 3, but they were talking about 8 inch (200mm) rotors. Maybe I am a wuss and you are 3 times stronger than I am. Last time I checked I had a grip strength of 200 lbs. Still, less hand strength needed for braking is a good thing. Another fun fact, since there is much less leverage, it puts much more force on the frame and fork. This means the frame and fork have to be strong enough to deal with the force. I have seen a few forks that were not, and failed. Making the frame and fork strong enough for disk brakes adds weight, not to mention the weight of the disks.
Now if you have a tandem, you may need more braking power than two rim brakes provide. Drum brakes are highly nonlinear, and generally not recommended. A long time ago, Phil Wood had a big mechanical disk brake which was designed for tandems. I remember seeing them on tandems, and they were massive. It seems to be long out of production. I will defer tandem braking to Santana and other good tandem makers. Here is an interesting article about a disk brake failure on a road bike.
I decided I wanted to get a new mountain bike, and I decided to go with 29 inch wheels. The problem is nobody makes a decent bike that doesn't have disk brakes. I am baffled by this. I suspect people think disks are cool. I think bike makers think they can sell new bikes to people so they can have cool disk brakes. However, disk brakes are heavier, disipate hess poorly, and have require more force. They are also harder to service which is good business for bike shops. I hope in a few years people will come to their senses and I will be able to get a decent mountain bike with rim brakes.
9Niner is a company that primarily makes mountain bikes with 29 inch wheels. You would think they would understand stuff relating to that. The larger wheel offers increased rotational inertia, which in turn improves the bike's stability. While this is true it is a bit misleading. If the goal is increased rotational inertia, that can be achieved by adding weights to a 26 inch wheel. If someone suggested that, people would laugh at them. It would make a bit more sense to say people should use really heavy wheels in orde to increase rotational inertia. Generally, you want to reduce rotational inertia. The extra three inches also increases the amount of tire contacting the ground at any one moment, meaning enhanced tire bite.Wheel size has nothing to do with the amount of tire contacting the ground. The amount of tire contacting the ground is a function of weight on the tire and tire pressure. It doesn't matter if you have 20 inch, 26 inch, or 29 inch wheels. This is plaing wrong. Since all we do is 29ers, we had to get it right.Well, 9Niner also makes road bikes with 700c wheels. So it looks like 'all we do is 29ers' is false.
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