Causes of deformation of the “eight” type
When creating modern bikes, manufacturers use the latest technologies, but the design of the discs still remains outdated, spoke. Small curvatures do not lead to tangible troubles when driving, they can be tolerated for a long time. Ordinary deviations can lead to dangerous situations, such a malfunction must be eliminated.
There is an opinion that the “eight” on a bicycle is a consequence of active and careless riding. Driving over bumps, uneven surfaces, unsuccessful jumps, dashing turns lead to deformation. These factors inevitably affect the geometry of the rim, but they are not the only reasons for the formation of the “figure eight“. During operation, the driving style has the least effect on the appearance of distortions.
The vertically directed load occurs already when you get on the bike. Abrupt braking at speed leads to deformation, the “figure eight” begins to form. Biking and cornering at high incline speeds only exacerbate the problem.
This is especially true for those who like jumping on a bicycle. Not all bike models are equipped with special shock absorbers that partially compensate for the load upon landing. The jump irreversibly changes the geometry of the wheel of the bicycle, it is not easy to eliminate such a defect.
Careless driving cannot explain all the reasons for the emergence of the “eight”. Even if you ride very carefully on a perfectly smooth road, deformation will be observed over time. this is a characteristic feature of bicycle operation. The more kilometers traveled, the more often there will be a need to adjust the wheels of your bike.
A high-quality rim, made technologically correct, significantly reduces the possibility of forming a “figure eight”. On cheap models of bicycles, defects may already be present at the time of purchase, respectively, repairs will need to be done almost immediately after the start of operation. On a poor quality rim, curvature develops after the first few rides.
Rear wheel specificity.
On the rear wheel there is a cassette with gears and it takes up a lot of space. Therefore, on bicycles with a Parallelogram Derailleur, the rear wheel is asymmetrical. The opening angle of the umbrellas on the side of the bit is greater (dumber) than on the opposite side and the tension of the needles on it is twice as much. The weight of a cyclist on a 36-spoke wheel is actually based on 36 and 18 spokes. And that’s the problem.
The rear wheel is weaker than the front. The need to transmit torque makes it impossible to use a radial spoke pattern, although exotic combined tangential-radial patterns such as the Campagnolo G3 are sometimes found. The diameter of the flange in the rear hub is always larger than in the front. this requires the presence of a drum on the drive side. The increased diameter of the flange helps to simplify the lacing process, so as not to resort to dismantling the drum. And even with at least 30 mm of advantage in the width of the hub, the rear wheel is still weaker than the front.
Sometimes people try to solve this problem by using bushings with asymmetric flanges. such bushings are called Hi-Lo. In them, the flange on the side of the bit has a larger diameter than on the opposite.
Increasing the diameter of the flange decreases the effective spoke tension. Guitarists can give a current example of the scale length of a guitar: the longer the scale, the more you have to pull the strings and vice versa. The oversized flange hub also transmits twice the torque. It is for this reason that track bushings have a large flange diameter and that is why Hi-Lo bushings can sometimes be found on bicycles: this design allows, if not to compare the tension of both umbrellas, then at least come closer to this equation.
But the problem of asymmetry on a bicycle with a rear derailleur has not yet been solved. Most planetary bushings do not have such a problem, because the transmission of torque is carried out through one sprocket, it allows you to increase the width of the bushing and make both umbrellas symmetrical.
Wheel deformation or why the wheel became a figure eight on a bicycle?
Deformation is the loss of a derived wheel shape. Deformation can be reversed and irreversible. The main types of deformation are the so-called figure eight (lateral deflection of the rim segment), egg (ovality of the rim), umbrella displacement (when the wheel is not in the center of the fork), transverse and longitudinal indentation of the rim, and the like. Eight, egg, and umbrella offset refer to reverse deformation, and the latter is almost always a mechanic’s fault. All others are examples of catastrophic failure that requires the replacement of one of the main components or the entire wheel as a whole. They are caused by impacts, the force of which exceeds the threshold of elastic deformation of the materials from which the wheel is made, bursting of the spoke and nipple, seam disengagement, concentric, longitudinal or transverse bursting of the rim, bursting of the hub flange caused by static and / or dynamic loads, and the like.
Sometimes wheel deformation is caused by irreversible stretching of the spoke (poor quality). Also, due to the prolonged action of a large static load, the spoke can pull out the surrounding area around the hole visually similar to the caldera of an active stratovolcano. This type of deformation caused by the mechanic’s failure to follow the recommendations for tensioning the spokes and / or overloading the bike with camping equipment (user error).
Within the framework of this article, we will not consider catastrophic (irreversible) deformations of the wheel and its components and will focus on what can be corrected without replacing components.
Determination of wheel curvature
Important! Before starting to eliminate the “eight”, you need to determine in what place on the rim the defect was formed.
We start by turning the bike over, setting it in a stable position. Then it is necessary to determine the degree of curvature. There are several ways to do this.
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The first method is visual, it allows you to determine the area of the largest deviations. It is necessary to spin the wheel of the bicycle with all your might. The movement of the wheel rim should be maximum; at low speeds it is quite problematic to determine the damaged area with your eyes. Rear discs can be spun using pedals.
Damaged areas move away from the plane of rotation. The segment of maximum deflection must be grasped with your hands and fixed. You can only use the visual method if you have some experience with the bike.
If the bike model is equipped with V-brakes, you need to bring the pads to a minimum distance, before contacting the rim. Then we turn the wheel and observe in which direction the curvature went.
The contact method involves marking the deformed area. A marker or a piece of chalk is used as a tool. It is advisable to attach the crayon to the feather of the fork or hold your hand strictly motionless.
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Slowly rotating the bicycle wheel, we bring the indicator directly to its surface. First, the chalk marks the largest bulge. Next, you need to spin the rim harder, continuing to bring the indicator closer. Thus, the mark of the curvature zone is formed.
When the crayon runs along the entire circumference, the marking can be considered complete. The G8 zone will be clearly outlined with a bold chalk line.
There is another way to define curvature. Take sticks for the ears, fix them with tape on the feathers of the front fork of the bicycle perpendicular to the rim. Slowly rotating the wheel, we bring the sticks closer to the rim. Mark the touch points with chalk.
If the wheel has lost its correct round shape, that is, a curvature of the “egg” type is observed, the same marking is made: the indicator is brought to the outside of the rim with the tire with the camera removed.
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Defect elimination tools
To repair your vehicle yourself, you will need the following tools:
- marker or chalk;
- plastic assemblies for dismantling the tire;
- spoke wrench.