Tuesday, 21 January 2020

Bike chain gap gauge

For the setup of the 12 speed gears on Shelley's and Dean's bikes, it was necessary to adjust the chain 'B' gap closer than that recommended by SRAM.

I ended up drawing an additional curved line on the SRAM guide.  During a spare few minutes, in the evening, I created my own variation and 3D printed it.

My version has steps for 5mm, 10mm and the SRAM suggested 15mm. All of our bikes worked best closer to 10mm.

The tool fits the SRAM 50mm chain guide cog and it works the same way as the SRAM guide. With the chain in the lowest gear, on the largest sprocket, the tool is held by hand round the upper cog of the derailleur and aligned with the tip of the nearest tooth of the largest gear. The B-tension screw is rotated to change the gap to the desired distance.


Download 3D models:
Chain gap gauge, Fusion 360, STEP and STL (Zip)
Licence attribution - small business exception

My designs in the Fusion 360 Gallery.
My models in the GrabCAD library.
My designs on YouMagine.com


Monday, 20 January 2020

Shimano XT and SLX brake pads

The brakes on all our bikes now use the same styles of Shimano pads. The G-series, G03A, G03S, G04S or the J-series J04C, J03A etc.

I've been trying to work out the most appropriate pads to buy. My problems started when I was trying to find out what the difference was between G03A pads and G03S pads!

Shimano have a brake pad selector page, that is intended to guide you through to get the most appropriate pads:
That goes so far but often throws out multiple choices with no explanation of the differences.

Working the other way round. It is fairly easy to get generic information on resin, sintered or semi-metal but translating that in to which Shimano part number that relates to, can be confusing and is often contradictory.

As far as I can work out, in summary:
Resin = Better for dry or rainy days. Quick to bed and more control.
Metal = Better for muddy, wet, gritty conditions. Last a lot longer. Take a lot more bedding in.
One other thing that I found out quickly about metal, they can be noisy. They Squeal.

In addition, some disc rotors are not compatible with some brake pad compounds. Most, if not all, discs are compatible with resin pads but not all are compatible with the metal compound pads. Many rotors have 'Resin Pads Only' engraved on them. Some of our disc rotors have that engraved on them.

Shimano simply state the following for the use of each type:
Resin: silence and controllability
Metal: durability and braking power

The symbols on the packaging provides some details to expand on that but that is not easy to search for on the internet. I have interpreted the information I have found, as best I can, and presented it here for my own use in the future. There is no reason for you, the reader, to trust my list any more than any of the hundreds of others out there on the internet.

Shimano Markings:
First letter = Shape. Both the G and J fit the XT and SLX brake calipers.
J has cooling fins
G does not have cooling fins

Middle numbers = brake pad compound.
01, 02 and 03 appear to be updates of the resin formula. 03 being the most recent.
04 are always metal pads

Last letter = backing material:
A = aluminium (heat transfers to the caliper and hydraulic fluid, risk of the brakes locking on)
S = steel (heat transfers to the rotor, risk of brake fade)
Ti = titanium (heat transfers to the rotor, risk of brake fade)
C = combination, stainless steel with aluminium fins
The heat sink fins, fitted to some pads, are obviously intended to mitigate the disadvantages of each of the above materials.

J03A, J02A 
Resin pad with anti-fade heat sink 
Metal pad with anti-fade heat-sink 
G03A, G02A 
Resin pad with aluminium back plate 
G03S, G02S 
Resin pad with steel back plate 
Metal pad with a steel back plate 
Metal pad with a titanium back plate 

I can't find enough reliable information on other makes of brake pad. I'm sure there must be good alternative pads but I have no way of knowing, so for the time being, I'm sticking with the genuine Shimano pads.


I still don't know if I should buy steel or aluminium backed.
Metal pads are probably longer lasting for the muddy riding I do in the winter but I am less likely to be comfortable with the feel. On top of that, my disc rotors are not compatible with metal!

The following look like the best choices, which are compatible with both SLX and XT brakes:
J03A resin pads with heat sink
Alternatively, J02A, G03A or G03S would be perfectly adequate.



The source, the Shimano web site:

Brake Calipers
We have XT and SLX brakes in use. These are or are similar to the current M8000 and the M7000 brake calipers:


Sunday, 19 January 2020

Bike brake piston expander

When bleeding Shimano and other makes of bike disc brakes it is necessary to fit a bleed block.
This is nothing more than a 10mm thick spacer that prevents the pistons closing during the bleed process. It avoids overfilling the system with too much hydraulic fluid.

It is common for the pistons to be slightly too close together to fit the bleed block. Most people use a screwdriver or similar handy tool. This is fiddly and risks damaging the pistons.

There are specialist brake piston presses available, which are like a thin spatula. These only press on one side at a time. These serve multiple purposes but they can still be fiddly when trying to open to the full 10mm width.

I've come up with a simple design that can be quickly pressed between the pistons to separate them by the required amount.

It's not an essential tool by any means, often a chamfer on the bleed block is all that is needed. The  tool I have designed is just a little time saver.


Saturday, 18 January 2020

Fitting a 12 speed group set

I had a chance to see a 12 speed group set being fitted to a Giant Trance bicycle, which helped me when updating Shelley's Merida to 1x12 from 3x10. In both cases the set being fitted was from the SRAM eagle range. Both with an NX derailleur and GX shifter. One with an NX cassette and the other with a sunrace cassette.

As usual, SRAM have a very good video for fitting and adjusting the gears, however we had to fettle to get a better end result.

Shimano style driver

Both the SRAM NX and the Sunrace cassettes fit on the common Shimano style driver. The SRAM GX, and above, cassettes fit on a SRAM specific driver.


The SRAM derailleur fits in a different position to where a Shimano would usually fit.
The SRAM does not use an extension and, more importantly, it must not be offset away from the hub.

Shimano, offset and set back

SRAM, no offset and almost straight down


To get the length of chain. Wrap it round the largest sprocket and the chain ring. Add two links, one outer and one inner. Both ends of the chain should be an inner link.

The master link fits with the curved surface outwards and the arrow facing the direction of travel of the chain.

End stops

The end stop for the smallest sprocket can be done before attaching the cable.
On a SRAM, the limit screws are furthest away from the end they adjust.
Align the outer edge of the smallest sprocket with the inner edge of the guide cog.

To adjust the lowest gear end stop, attach the cable.
First turn the barrel adjuster on the shifter, full clockwise, then turn it back two full turns anti-clockwise. This is so there is enough scope for later adjustment.
Click the down shifter and pull the cable to ensure it is fully extended, then attach the cable to the derailleur.
Once attached firmly, click the other lever to pull the derailleur to the other end and adjust the end stop.
Turn the screw so the centre of the guide cog, on the derailleur, aligns with the centre of the largest sprocket.


This adjusts the gap between the guide pulley and the largest sprocket on the cassette.
It is the closest the tops of the teeth get to each other.
In the past it used to be fairly common to set this to 5 to 6mm. As cassettes have got more sprockets, this gap has increased.

For SRAM 12 speed they recommend a gap of 15mm. They supply a tool to help set that gap.
This was too wide for both the Giant Stance and the Giant Trance. Both of those worked better with a gap closer to about 10mm.
The Merida ONE-TWENTY would work at 15mm but was more reliable at about 11mm.

If the B gap is wrong it will affect how quickly or slowly the gears change and it might miss a gear. Except in extreme cases, the gear change will still work, even if the gap is not ideal.

Giant Stance

The mechanics who setup my Giant Stance must have ignored the SRAM tool. I would guess that they had some prior experience of the smaller gap required because it came from the shop working efficiently, with the smaller 10mm gap. It was measuring that, against the setting on the guide tool, that helped setup the Giant Trance to work.

Gear Change Adjustment

To get the gears to change up and down quickly and smoothly, it is necessary to adjust the cable tension. Typically there is a barrel adjuster at either the derailleur or the shifter end of the gear cable.
SRAM have the adjuster on the shifter.

When changing gear:

  • if it is slow moving from Large to Small sprockets turn the barrel adjuster clockwise.
  • if it is slow moving from Small to Large sprockets turn the barrel adjuster anti-clockwise.

If it won't adjust to give good up and down shifting, consider adjusting the B gap.



All the adjustments to get gear changing to work properly.

More about the B-tension adjustment.


Sunday, 12 January 2020

Danbury Common

This is the first time I had been to Danbury Common to ride. Roy and Dean went with me to show me the ropes.

The video is of my second run down the main track. As can be seen in the video, I have a few problems negotiating some berms.


Saturday, 4 January 2020

Bike steerer tube bung

This was requested by Dean to stop the grub and water filling up the steerer tube while cleaning the bike. He's used various things in the past but with the power of a 3D printer and some design work using Fusion 360, I came up with something that fits tightly.

This is not intended to stay in place while riding. In fact I would think that keeping it in place would trap moisture and encourage corrosion. If you are looking for something to keep the mud out while riding, I would suggest some air filter foam stuffed up the tube. That will keep the mud out and allow any moisture to evaporate.

The bung is designed to be pushed in to a tapered fork stem. It just pushes up, just inside the steerer tube, and holds in place while the bike is being cleaned. Best to clean out the tube with a rag before inserting the bung.

Despite taking lots of measurements from three bikes, the end result needed trial and error to get the bung to sit just inside the tube. If it goes too far in, it leaves too much tube to fill up with water and it is difficult to reach to pull back out.

They need a loop of strong string to help get it back out.

The differences in the diameter between an SR Suntour, Fox and Rockshox forks were only about a millimetre in total between the three. I still had to make them to fit specific forks. Only 0.5mm diameter difference between each!

Of the forks I have tried, the smaller fits the Suntour, the medium fits the Rockshox and the larger fits the Fox. I modelled them with a dot in a different location on the handle, to help identify which is which.

They all use two metric rubber o-rings. An o-ring with an inside diameter (ID) of 25mm and cross sectional diameter of 3.5mm fits all of them. I would guess that imperial o-rings with ID 1" and cross section of 1/8" would work just as well.

They 3D print as two halves with two alignment keys. I printed it all without any supports and just a skirt. It needed no cleaning up and can be pushed together. It's a tight fit. I also used a dab of superglued (CA) but that is not essential.


Download 3D models:
Steerer bungs, Fusion 360, STEP and STL (Zip)
Licence attribution - small business exception

My designs in the Fusion 360 Gallery.
My models in the GrabCAD library.
My designs on YouMagine.com