Adjusting Axle Position in Track and Sliding Dropouts

Suppose we have a typical 18 speed, derailleur equipped drivetrain which we'd like to convert to a fixed gear design on BikeCAD. We can start by pulling up the Drivetrain dialog box (Drivetrain icon) and selecting a single front chainring. We'll then move to the second tab in this dialog box and choose a single rear cog. Because we'll be getting rid of the derailleur, we'll choose 0 extra links of chain. To actually get rid of the derailleur, we'll open the Rear dropouts dialog box (Rear dropouts icon) and uncheck the Derailleur hanger check box. Without a derailleur to take up the slack in the chain, we'll need to use a dropout that permits some sort of adjustment in the Chain stay length. One option is to use a track style dropout with a horizontal rear facing slot. We can change the orientation of the slot by modifying the input field labeled by the Greek letter Xi (ξ) from -90° to 180°. Because the idea is to allow the axle to be fastened at various points along the length of this slot, we'll want to make the slot longer by adjusting the slot depth dimension D. An alternative to using this sort of classic track dropout is to use a sliding dropout which we can achieve by clicking the check box labeled Sliding.

Whether we are using a sliding dropout, or a track style dropout, we can shift the position of the axle relative to the rest of the dropout by modifying the input field labeled Z. If we change Z to some larger value, it may seem surprising that the rear wheel has not moved further away from the bottom bracket. The reason for this is that in the Primary dimensions dialog box, we've specified a chain stay length of 405mm. Chain stay length is defined as the distance from the rear axle to the bottom bracket. This dimension must remain as it has been set, so the only thing BikeCAD can do is shift the dropout itself forward. In the early stages of a design this behaviour is likely quite desirable.

Besides chain stay length there are a number of other dimensions that locate objects on the frame relative to the position of the rear axle. For example, cantilever brake bosses, as well as chain stay and seat stay bridges are located with respect to the center of the rear axle. If we turn on the hidden line view to see the chain stay and seat stay bridges and also turn on the display of the dimensions used to locate these bridges, it is reassuring to see that as we shift the rear axle back and forth in the dropouts we are maintaining the desired amount of clearance between the edge of the tire and the bridges. However, there will come a point where you've finalized your design. You've determined the length at which you're going to need to cut your physical chain stays and seat stays and you've nailed down exactly where along those stays your bridges will be fixed. At this stage, you may like to see the effect that shifting the axle in and out of the dropout slot will have on chain length, wheelbase etc. We can do this by locking the frame using the Lock frame Lock Frame Icon icon. Once we click Lock frame we'll notice that all the input fields in the dropout dialog box are disabled except for dimension Z. We can change dimension Z and notice that the frame remains fixed, but the chain stay length (which again is the distance from the rear axle to the bottom bracket) grows to reflect how the wheel has been shifted backwards. Leaving the frame locked like this, we can play with values of Z to achieve the desired tension in the chain. Notice that as we gradually shift the chain backwards BikeCAD will make the chain tighter and tighter until it reaches the point where another link must be added, at which point the chain will be slack once again. By leaving the frame locked and playing with different values of Z and different sized rear cogs it is possible to determine the range of gears that can be used on a track bike without adding or removing links from the chain.

All points discussed above apply equally to the use of a belt drive which we can display by selecting Belt in the Drivetrain dialog box.