Wednesday, March 29, 2017

Avanti Inc 3: Bushnell Eccentric BB Comparison

The recent upgrade done on the Avanti Inc 3 was a major change, where the drivetrain was changed to an electronic shifting, Alfine 11 Di2 version. Also, the controls and brakes were changed to a drop bar set up with road hydraulic disc brakes.

It was a good change as the drop bar set up makes it easier to cycle faster, as compared to on a flat handlebar bike. The road hydraulic disc brakes also works well, especially when riding this bike in the rain. It is my version of a road hydraulic disc road bike, that is also an all-weather commuting bike.

However, changing to a drop bar set up also means that the cycling geometry and fit is different from before. I noticed that the pedaling stance seems rather wide, as compared to my other bikes. In other words, the Q-Factor of this bike is wider than my other bikes, such as the Merida Scultura 5000 road bike and the Wheelsport Fantasy mini velo.

Q Factor on a bicycle

This is due to the Deore crankset, which was designed for a triple chain ring. As such the Q factor of this MTB crankset is wider than road cranksets. There are two reasons why I had to use a MTB crankset on this bike.

1) I wanted to use a 42T Gates front sprocket, which is only available in 4 arm, 104mm BCD. This restricted my choice of crank arms to MTB types.
2) The eccentric BB that came with the bike has a 73mm width, which does not allow the installation of road cranksets (68mm BB width required).

When using the flat handlebar setup, this wider Q factor was not an issue for me, as it rides more like a MTB that a road bike. However, after using a drop bar setup, this wider Q factor becomes more obvious. The ride feeling is quite strange and a little uncomfortable when the narrower drop bars are paired with the wide Q factor crankset.

I did some research and found that this Deore crankset has a Q factor of about 182mm, while a road crankset has a Q factor of less than 150mm. This is a huge 30+mm difference in Q factor that will definitely be felt. Reducing the Q factor will make this Avanti Inc 3 ride more like a road bike instead of a MTB.

In order to use a road crankset, I had to overcome the two limiting factors as explained earlier. First, to use a road crankset, I need to change the BB width from 73mm to 68mm. This requires the eccentric BB to be changed.

Secondly, the Gates front sprocket that is available for road cranksets are only available in 5 arm, 130mm BCD. The smallest sprocket available with this specification is 46T, which is larger than the 42T that I currently have. This means that the gear ratio will be changed again.

Stock Gear Ratio: 50T / 24T = 2.08
1st Mod Gear Ratio: 42T / 24T = 1.75
2nd Mod Gear Ratio: 42T / 28T = 1.5
Proposed Gear Ratio: 46T / 28T = 1.64

Once I change the front sprocket to 46T, together with the 28T Alfine Di2 rear sprocket, the gear ratio will be 1.64, which should be fine for my usage.

First of all, I need a new eccentric BB. The Bushnell eccentric BB seems to be the best eccentric BB available on the market, which I why I decided to get it to replace the stock eccentric BB on the bike. However, there were some issues with the order...

I ordered the Featherweight version, which has cutouts to reduce weight. However, they sent the normal (Retro) version without cutouts.

Credit to them, once I notified them of the error, they immediately sent out a Featherweight version of the eccentric BB. Which is why I ended up with two Bushnell eccentric BB. Might as well make a comparison then.

Featherweight version on left with many cutouts, normal (Retro) version on the right.

This Bushnell eccentric BB works differently from my stock eccentric BB. The Bushnell design has two wedges that will push open the two "wings" on the BB, expanding the diameter of the eccentric BB. This design applies a symmetrical load and should distribute the stresses better within the frame.

Strictly speaking, both orders were actually wrong as I ordered a black coloured, featherweight version. Instead, I got a black, retro version and a silver, featherweight version. However, I will just make do with what I have as they have already sent over another eccentric BB at no extra cost to me.

Available in silver or black

Featherweight version on left has cutouts at non-essential areas to reduce weight

Featherweight version is 154 grams

Normal (Retro) version at 211 grams. 57 grams heavier.

Now I have a choice to make. Should I use the silver coloured featherweight version, or the black normal version? It is a choice between weight and appearance.

In the end I decided that the weight difference was insignificant compared to the whole bike weight. The black version would look better as it will blend in well with the black coloured frame.

Black, normal version of the Bushnell eccentric BB will be used

Let's do a detailed comparison of the Bushnell eccentric BB vs the original eccentric BB. Weight, working principle and dimensions will be compared below.

Original eccentric BB. Uses a wedge on one side, similar to old school type quill stems. Also has cutouts to reduce weight.

Good weight of 177 grams, lighter than the Bushnell normal version

Side view

Top view. See the different wedge mechanisms.

Original eccentric BB disassembled.

Bushnell eccentric BB. More parts in this design.

One other thing to take note is that the frame needs to be compatible with the Bushnell eccentric BB. The important dimensions are the internal diameter of the frame BB shell, and the overall width of the BB shell.

BB shell internal diameter on the frame is about 55mm

Width is about 66.6mm

Original eccentric BB outer diameter, fits into the 55mm BB shell nicely.

Width of original eccentric BB. 66mm at area that contacts BB shell.

About 73mm wide at the area that the BB screws into. This is the dimension that limited my choice of cranksets to MTB type.

The Bushnell eccentric BB is designed for 53.8-54mm internal diameter BB shells, so the BB shell on the Avanti Inc 3 is too big. Luckily they also provide shims to allow usage of 55mm or 57mm BB shells.

Bushnell eccentric BB outer diameter, too small to fit securely into the 55mm BB shell.

Shim of about 0.5mm thickness is used, to reduce the internal diameter of the 55mm BB shell to 54mm

Bushnell eccentric BB, also about 66mm at area that fits into frame BB shell

About 68mm wide at the area where the bottom bracket cups screw into. This allows the use of road cranksets.

The main objective of this modification is to change the BB width from 73mm to 68mm, so that I can use a road crankset. Next, we will need to use a suitable road crankset, and also get a new front sprocket. Since the sprocket size has changed, a new Gates belt is also required...

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