Friction Drive Introduction and FAQ

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9:59 PM
Jan 11, 2008
Mount Vernon, Ohio, USA
Installation: Friction drive motorized kits offer far and away, the simplest installation, of any bicycle drive approaches. After all, all you have to do is to do is to get the friction roller in contact with the tire at a right angle. The installation will include a mounting bracket (typically, a 'U' shaped bracket) clamped to the rear seat stays, bolting the drive channel to the mounting bracket, attaching the support rods to the bike in the vicinity of the rear axle, and to the drive channel, installing the throttle and (optional) kill switch, and routing the throttle cable/kill switch wire from the motor to the handlebars. Kits which are shipped unassembled, or motorless (BMP) will require more assembly than those which ship pre-assembled (Staton and Dimension Edge.)

This simple installation also means that a friction drive is the easiest drive type to switch from one bike to another.

Tire Wear and Roller Slippage: A properly installed and adjusted friction drive will not wear the tire much more than other drive types. There will be <i>some</i> additional wear, but, running a tire in excess of a thousand miles is not unusual. When the road surface will be wet, some care must be taken when using a friction drive. You do not want 'jack-rabbit' starts, for instance. You want to accelerate more slowly. More than with other drive types, the tire pressure impacts drive performance and use - for a standard, 26 inch, 2-1/8 inch 'balloon' tire, you should keep the driven tire at about 50 psi. When the road surface is wet, you will need to add additional roller pressure, so that the friction roller presses 'into' the tire approximately a quarter inch, or even a little more. When the road surface is dry, the roller pressure can be less. There's a trade-off involved with roller pressure. Increased roller pressure reduces slippage, but, it decreases performance (and fuel economy.) In a nutshell, you will want to adjust your tire and roller pressure to allow maximum performance, with the least slippage for the conditions. Time, and experience, will soon teach you what 'settings' you will need to use. There is one note of caution though - IF the roller is slipping, let off the gas, otherwise, a spinning drive roller could chew a hole in the tire...

  • How fast will I go with a given drive roller? The maximum theoretical speed depends entirely on the engine speed and the roller diameter. Use the GearRat gearing/speed calculator to determine the maximum possible speed for the engine/roller combination you have. In the real world, other factors influence the top-end. The first of these is the engine power; if you are trying to spin a 2 inch roller with a 1 horsepower motor, you probably won't have enough power to get you up to the theoretical maximum speed. Other factors which can impact the actual top-end speed, are the tire type, tire pressure, and roller pressure. Smooth tires have better performance (and do NOT result in reduced traction, even when wet, on the road.) Tires with lower rolling friction will have better performance than those with higher rolling friction. Increased tire pressure results in lower rolling friction, which results in better performance. And, of course, wind resistance plays a factor, as does the weight of the bike and rider.

    Finally, there is one additional point to remember: Even though a larger roller diameter increases the top speed, it also reduces the acceleration (and hill climbing ability) by exactly the same amount.
  • Wait a Minute! Doesn't tire size play a factor in calculating the top speed? With friction drives, No, it does not. With all other drive types, tire size does matter. Here's why tire size doesn't matter with a friction drive: The speed of the tire rotation in RPM is related to bike speed, but, it is really the speed of the circumference of the tire, in miles per hour, which is directly related to bike speed. After all, the tire is in physical contact with the road, and if there is no slippage, the tire circumference speed and the bike speed HAVE to be equal. Likewise, if there is no slippage, the tire circumference speed and the roller circumference speed MUST be equal, because THEY are in direct contact. Think of it this way - Essentially, the tire is just a transfer roller (or idler wheel,) between the drive roller and the road... A smaller tire would spin faster than a larger tire, but, since the circumference of the smaller tire is proportionally less, (by exactly the same ratio as the tire diameter, and the RPM increase,) there is no difference in bike speed.
  • My friction Drive is vibrating a lot! What's the problem? It could be several things. First, check the bearings, to make sure that the drive roller itself isn't vibrating. Make sure that the wheel is round - an out-of-round wheel can bounce you around. While an out-of-true wheel (side-to-side) could introduce a little vibration, it will definitely wear faster, and will soon become out-of-round because of this. The third common factor in vibration is the tire type - a smooth tire will have virtually no vibration, but a knobby mountain bike tire can cause a LOT of vibration. Really, any repeating tread pattern in contact with the drive roller can lead to vibration. If you occasionally need to go off road, and don't want a slick tire, get a tire with an inverted tread pattern which has the center portion smooth. The Continental Town and Country tire or Serfas Drifter tire offer low vibration and good off-road performance. Refer to this thread for more information.
  • I would like to use a slick/semi-slick tire like the Innova Swiftor, the Kenda Qwest, or the Bontragger Hank, but wouldn't the lack of tread lead to hydroplaning and/or poor wet traction? Definitely not. Concerned with hydroplaning when airplanes take off and land, the FAA has performed many experiments in this area. Their findings clearly show that the speed at which a smooth tire can hydroplane is based only on tire pressure. The higher the pressure, the greater the speed needed to hydroplane. At 50 PSI tire pressure, the minimum hydroplane speed is nearly 70 MPH! Tread channels CAN route water away, to increase the hydroplaning speed, but, they do so by reducing the surface area of the tire that is in contact with the road, and thereby REDUCE the traction the tire can provide under all other conditions, including a wet or icy road surface. Since a friction drive bike, with tire pressure of 50 PSI could never exceed the minimum hydroplane speed for that pressure, any tread pattern will reduce the tire's grip on asphalt under wet or dry conditions, or even icy conditions. About the only road condition where a tread might help, is wet snow. Ref this thread.
  • My friction drive roller is spinning against the tire. How can I improve the 'grip' between the drive roller and the tire? As mentioned above, first, be sure that your air pressure is sufficient, and the roller pressure is good. Once you've checked both of these two items, the only other approach that would work is to increase the roughness of the drive roller. Essentially, the 'high spots' of the roughened surface press into the rubber of the tire, increasing the traction between the roller and the tire. Some users have reported success with the welder by adding many small bumps of weld/weld splatter on the roller. Others have roughened the surface by gluing sand or other similar small aggregate to the roller with epoxy or JB Weld adhesive. Still others have glued rough, fabric backed sandpaper to the drive roller. Several threads discuss epoxying sand or sandpaper to the roller. Ref this one as an example.
  • I Know - I can make a 'curved' roller that wraps around the tire. That will increase the surface contact with the tire. That should help, right? No, this really isn't a good idea. On the surface, it seems as if it should help, but here's the rub (pun intended): When the motor is spinning the roller, the speed that the tire (and thus, the bike) will reach is dependent only on two factors - the engine RPM and the roller diameter. At any one time, the roller MUST spin at the same RPM at all points. (It's rigid, after all...) But, since the roller diameter is changing over its entire length, this means that the speed that the motor is trying to push the bike will be different at each and every point along the length of the roller!

    Let's suppose that you have a curved roller with a narrowest diameter of 1 inch, which gets increasingly larger, so that near the edges, it is two inches in diameter. And, it makes contact with the tire at all points along this curved face, as it 'wraps around' the face of the tire. You rev the engine and are running along with the motor spinning at 7000 RPM.

    At the narrow part of the roller, a point on the circumference of the roller is whipping around at 20.8 MPH, and, of course, it is trying to push the bike along at that speed as well. BUT, at the edge of the roller, where the diameter if the roller is 2 inches (and the circumference is also twice as great) a point on THAT circumference would be moving at exactly twice the speed as at the narrowest point - 41.6 MPH!

    Since both points are in contact with the rubber of the tire, this means that there is a difference in roller circumference speeds of 20.8 miles per hour!!! And, that speed difference will be spent in heating up the friction roller and the tire rubber, as the roller rubs against the tire, at virtually every point along the roller... :(

    In all probability, the bike will be traveling somewhere between 20.8 and 41.6 MPH, and the tire will be getting eaten up at BOTH the crown of the tire, and the tire sidewalls, simultaneously. And, you will have a large power loss and much shorter tire life.

    I intentionally used a large diameter difference in this example. But, even if the diameter difference is relatively small, the net effect will be the same. You will have power being wasted at the roller/tire interface, and you will experience power loss. Both the roller and the tire will get hot, and you will experience increased tire (and roller) wear.
  • I have a small lawn mower engine with a vertical shaft. Couldn't I put a roller on that motor, and use it as a friction drive, with the roller pressing against the side of the tire? Well, you could... There have been at least one documented case of this approach, usually paired with an idler roller on the opposite side of the tire, used to keep the drive roller from pushing the tire right off the rim. However, there's a pretty good argument that this approach will also lead to long-term problems with tire wear. Here's the problem: as with a curved roller, the drive roller is at a fixed RPM along it's length, because it's a rigid roller. But, it is pressing against a tire radially, so that at the bottom portion of the drive roller contact area, the effective wheel diameter is smaller than the effective wheel diameter at the top of the drive roller contact area... This means that the drive roller will be trying to make the rear wheel move at a different RPM along the entire drive roller/tire contact area.

    As an example, suppose that the roller is in contact along one inch of the tire sidewall, with a 26 inch wheel. We'll assume that the radius of the tire contact area ranges from 11.5 inches to 12.5 inches from the axle. 12.5 inches is 8.7% greater than 11.5 inches. This difference in radius means that, for a given Wheel RPM, the tire at the outside of the roller contact area will be moving 8.7% farther than the tire at the inner portion of the contact area. And, this difference in distance moved means that the roller will be scraping or moving against the tire at most of the contact area. Movement of two surfaces means friction, which generates heat and wear. This leads to shortened tire life. Also, keep in mind that this wear is on the tire sidewalls, which are much thinner than the tire face, so, you can expect a MUCH shorter tire life. Lawn mower engines typically range from 3.5 to 6 horsepower, so you may not notice the power loss that occurs, but, your tires will. There's a good chance that you would experience a sidewall blowout, and since this would be more likely to occur when the sidewall is being flexed more than normal (when you're going around a turn, for instance,) the blowout has a greater chance of happening at just about the worst possible time, from the rider's viewpoint... :( Bottom line - I would avoid this friction drive approach...
  • My Friction Drive is really chewing up the tires! What's the problem? It could be several things. First, slick or semi-slick tires work well with friction drives; mountain bike tires do not. Friction drive rollers tend to eat the edges off the nubs of mountain bike tires. Of course, the tire pressure and roller pressure also have a lot to do with this, as a slipping roller chews up the rubber pretty fast. Assuming that these are eliminated, be SURE that the drive roller is properly aligned. The drive roller center line should be parallel to the axle of the wheel it's spinning. The most important alignment is, when looking from the top, that the drive roller is parallel to the axle. If you are misaligned when looking from the top, on every rotation of the drive roller, you're tending to scrub off a bit of the rubber, in a side-to-side motion.The alignment when looking from the back is not as important as from the top. Refer to the attached sketch. One tool you can make and use to help get the roller aligned is a sort of elongated "L" square from thin wooden stock. It should be about 3/4 of the tire diameter in length, and about the length of the drive roller on the short leg. Assuming that the wheel is fairly well trued, you can lay the long edge against the edge of the tire. (If the tire is warped or twisted on the rim, let out the air and lay the inner edge against the wheel.) Before gluing the two pieces together, make a mark on the short leg, so that it will be approximately centered on the roller in use. Make sure the inner face of the two legs are square, add glue and clamp. Just make sure the stock you use is fairly thin, and straight. .


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    Friction Drive ALignment.jpg
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