For a belt drive or single speed bike, it is necessary to tension the belt or chain without a rear derailleur. The usual methods are to use a horizontal dropout, a sliding rear dropout, an eccentric bottom bracket, or an external chain tensioner. Regardless of the method, the fundamental idea is to allow adjustment of the chainstay length, or more precisely, the distance between the centre of the front chainring and the centre of the rear sprocket. Each method has its pros and cons, let's take a look.
Horizontal Dropout:
- Nutted axles are usually recommended, as quick release axles are sometimes not tight enough to prevent rear hub movement during strong pedaling.
- Realignment of the rear wheel is needed whenever the rear hub is removed from the dropout.
- Probably the most lightweight method as there is no additional part required, unlike other methods.
- Not suitable for disc brake bikes as the rear brake caliper (which has a fixed position on the frame) cannot always be aligned properly with the disc rotor.
Horizontal dropout allows the whole hub to slide forward/backward for chain tensioning.
Sliding Rear Dropout:
- Suitable for disc brakes as the disc brake mount slides together with the dropout, ensuring proper alignment between brake caliper and rotor.
- Relatively heavy and looks bulky.
- Need to align the left and right side dropouts properly, to prevent the rear wheel from slanting to one side.
- Wheel removal or installation is easy as it is just a horizontal dropout.
- No problem with using quick release axles or thru-axles.
Sliding rear dropout, where the brake caliper moves along with the rear hub.
Eccentric Bottom Bracket:
- Suitable for any types of brakes as the rear dropout is just a standard vertical dropout.
- Clean appearance as the large eccentric bottom bracket is not so obvious compared to other tensioning methods.
- Relatively heavy as the additional eccentric bottom bracket needs to be quite solid.
- Consistent belt/chain tension, as the tension is not affected when the rear wheel is removed or installed.
- Easy to make fine adjustments, just by rotating the eccentric bottom bracket.
- Bottom bracket height will vary, depending on how it is adjusted.
- Can cause creaking if not greased or installed properly.
Off-centre hole is where a standard bottom bracket is installed. This was from the Avanti Inc 3.
Chain Tensioner:
- Not so neat appearance, as it looks very much like a conventional rear derailleur.
- Can also be made by converting an old rear derailleur.
- DATT dropout mounting point needed, and this is most commonly used when converting a normal bike with derailleur to single speed/internal hub.
- Adds some weight as compared to horizontal dropout.
- Chain length is not really important as the chain tensioner will just take up the slack.
- Wheel removal is easy as it works on a standard vertical dropout.
Chain tensioner will be used, when all other methods are not available.
For me, my preference is to use an eccentric bottom bracket, as it looks the neatest, and is the easiest to use. Once it is set properly, there is no need to adjust it anymore. Wheel removal or installation is also easy as the rear dropout is just a normal vertical dropout.
I first came across an eccentric bottom bracket on the Avanti Inc 3, and I want to have the same design on this new Fabike C3 frame. The previous C2 frame was using a sliding rear dropout, but this new C3 version is now using an eccentric bottom bracket.
Looking inside the bottom bracket shell. Can see the rear brake hose which is routed internally, which was already installed when the frame was delivered.
This eccentric bottom bracket shell design is actually quite clever. Instead of using a non-standard, large 55mm diameter hole like on the Avanti Inc 3, this Fabike C3 frame uses a more standard size. The hole on this Fabike C3 frame has a diameter of 46mm, which is actually the same size as that used for a Press Fit 30 bottom bracket! In other words, if you don't need an eccentric bottom bracket, the same frame can accept a Press Fit 30 bottom bracket. Clever way to make sure that the frame design is not limited to using a less common eccentric bottom bracket!
Of course this requires an eccentric bottom bracket that has an outer diameter of 46mm to fit this frame. It is quite easily found, and is similar in design to the 55mm diameter eccentric bottom bracket.
Fabio from Fabike was kind enough to include this eccentric bottom bracket at no extra charge, and it seems to be an OEM version as there is no brand logo on it. Wheels Manufacturing also has it available if you need one aftermarket. The version that I have converts a Press Fit 30 bottom bracket shell to an eccentric type with 24mm Shimano crank spindle size.
Press Fit 30 to 24mm, eccentric bottom bracket. One good thing is that this weight of 168 grams already includes the sealed bearings for the bottom bracket.
Clamping area should be at the bottom as advised by the eccentric bottom bracket maker. Notches at the side allow adjustment by rotation.
I am pleased with this bottom bracket design, as it allows the usage of an eccentric bottom bracket, and is also quite lightweight as the bottom bracket bearings are already included. The only tricky part is aligning the left and right side, when rotating the eccentric bottom bracket. Moving on, let's take a look at the headset.
The headset design is quite standard, which is no problem at all. It is of an integrated headset cup design, which means that the cups for the bearings are already molded/machined into the frame. There is no need to press in any headset cups which means lower weight and also less work for me. This design is the same as that used on the Canyon Endurace.
Integrated headset cups for the sealed bearings. Just place the bearings into the cups and it is done!
Headset parts, also included with the frame. The blue part is the top headset cover, which comes in a few different colours for you to choose. The two sealed bearings are of different size, with the larger one at the bottom for more rigidity.
Weight of headset is 70 grams which is quite standard. Not possible to save weight here when the headset cups have already been integrated into the frame.
Weight of top cap and spacers. Just an rough estimate, as the exact spacer height has not been measured or determined yet.
Finally, the most unique part of this frame is the rear dropout design. This is an exclusive design which was created by Fabike, and I have never seen this design before. Fundamentally it is a vertical dropout, but it is different as the non-turn washers for the internal hub is integrated into the dropout design.
Normally, the non-turn washers for the internal hub are fitted outside of the dropout. This means that the coloured non-turn washers will be visible from the sides of the bike.
Blue non-turn washer visible at the rear dropout area, in between the dropout and the axle nut.
Usual installation steps for non-turn washers:
Install hub into dropout > Slide non-turn washers onto hub axle > Rotate hub axle + washers until washers can slide sideways into dropout > Tighten axle nuts
For this unique Fabike design, I think there are two main purposes for designing a new non-turn washer. First, it is to hide the non-turn washers on the inside of the rear dropout, for a cleaner appearance from the outside. The other reason is to make the dropout design interchangeable, so that the same frame can also accept normal rear hubs with quick release axles or thru-axles.
My initial thought was that this non-turn washer slides upwards into the corresponding slot on the frame rear dropout. This steel non-turn washer can be flipped if the angle of the flat needs to be reversed.
One problem with this design is that the installation method is quite unusual. The slot on the dropout (on the carbon frame) is only wide enough for the flat portion of the hub axle to slide through. This angle is different from the angle of the steel non-turn washer. In other words, it is not possible to slide the hub axle and the non-turn washer vertically into the frame at the same time.
The official method from Fabike is to first widen the dropout temporarily by manually widening the frame slightly, then adjust the hub axle angle to slot through the frame. The non-turn washers should already be on the hub axle. After that, rotate the hub axle, to align the non-turn washers with the shape on the frame. Once aligned, the non-turn washers can be slid sideways into the slots on the frame. It is quite difficult to visualize here, will try to elaborate more in a later post.
While testing this method using another hub, I found a problem with this non-turn washer design. For internal hubs, the flat area on the hub axle does not extend all the way to the lock nut! In this case, the special non-turn washers cannot be slid all the way in to touch the lock nut, there will be a gap remaining.
Flat area on the hub axle does not extend all the way to the locknut, which prevents the non-turn washer from touching the locknut.
This is a big problem, as the non-turn washer needs to rest flat against the lock nut of the hub. There cannot be a gap, or the clamping force from the axle nuts will deform the threads on the hub axle. The only way to solve this is to file the hub axle, so that the flat area extends all the way to the lock nut.
Filing the flat areas of the hub axle is quite straightforward, as long as you don't damage any other parts of the hub. The only point to take note is to avoid filing away too much of the lock nut. This needs to be done 4 times, twice on each side.
After filing the hub axle, the flat area has been extended to reach the lock nut.
The non-turn washer should be able to rest flat on the lock nut of the hub axle, once done correctly.
As described earlier, the non-turn washers need to slide into the frame sideways, instead of vertically upwards from the bottom opening. The fitting between the non-turn washers and the shape on the frame is quite tight, so it is difficult to align it properly for installation. In order to make this a bit easier, I decided to file a small chamfer on the outside edges of the non-turn washers, to make it easier to slide the washers sideways into the frame.
Manually filing the chamfers onto the edges of the non-turn washers.
Weight of the axle nuts is 34 grams.
By now, it should be obvious that this Fabike C3 has a special dropout design, which take quite a bit of effort to modify, adjust and improve. I would have much preferred it to use just a conventional vertical dropout design with standard non-turn washers. That would make the installation so much easier and hassle free. The only downside are the visible non-turn washers from the outside, but that does not bother me at all.
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