Shims between Crank and Chain Ring

Question

I should like to move the chainring on my single speed bike a little further outward (fig 1). The reason is to have enough clearance between chain ring teeth and frame (fig 2).

At present I have a spacer between the left bottom bracket cup and the bottom bracket shell (fig 3). However, this leaves me with quite a bit less spline engagement of the left crank arm to the spindle (hollowtech 2) than what I should wish for. The stop plate at the crank just clicks into the gap on the spindle with no room to spare.

To summarize, I have to remove or reduce the bottom bracket spacer. This will leave too little clearance near the chain stay. Hence the need for spacers at the chainring.

I can reduce the spacer width as much as i like by grinding it down. Which is how I got that spacer in the first place. At present the spacer is about 1.3 mm to 1.5 mm wide.

Clarifications

The chainring has about 2 mm to 3 mm clearance at it's closest approach. It is an oval ring. The crank is roughly at the 4 or 5 o'clock position then.

The frame is aluminium. I'm a heavy rider (85 - 90 kg) and push quite hard in short bursts.

Chainring and sprocket size get the, so called, magic gear ratio for that frame. That is, the drivetrain length just fits the chain length. The dropouts and bottom bracket of this frame don't leave a degree of freedom for adjustment. In order to reduce chain wear I've chosen the largest ring/sprocket. The smaller mechanical advantage reduces overall drivetrain strain.

Questions

The actual question has several aspects:

• What should I use for shims between chain ring and cranks? Is a round washer ok?

• How thick may such a shim be without running into trouble at that interface?

• How close may I get the chain ring teeth to the frame?

• Can I get rid of the spacer at the BB, is it worth the bother?

Fig 1: Shimano CX-50 110 mm BCD 5 bolt crank with single aftermarket chainring and bolt.

Fig 2: present clearance between widest part of oval chainring and frame (with BB spacer)

Fig 3: BSA bottom bracket shell, spacer, Shimano cup.

Fig 4: full drivetrain.

Answer 1

Chainring spacers are available commercially, wheelsmfg making 1,2,3 etc mm ones. They are marketed to singlespeed and track riders wanting to optimise chainline. They have a good contact area and are cut square, maximising this. On my gates belt (which is used with a 3sp hub or singlespeed 55/22) I have used 5mm spacers from a pack of overlength chainring bolts to get my "perfect" beltline on a road 130mm spider. I use steel chainring bolts and I advise that you do the same as the alloy ones look pretty but cannot take a good torque value. I have used bolts with a shoulder, taken from a Shimano 6700 chainring, to get full contact and minimise shear stress.

It has to be noted that your chainset has 5 arms, but chainsets exist with spiders of 4 and even 3 arms, using the same type of bolts, so even if clamping force is reduced by 2/5ths, you should still be able to make a safe system! Certainly this option is better than reducing the clamp area of your left hand crank arm, which can squirm off if mounted too far outboard (and I have seen this problem enough times). I can't see any evidence of chainring movement (no bright marks or fretting) after using this style of setup myself, but I have taken care to set it up nicely as I initially had similar concerns to yourself, and 5mm is a big space to take up. The shearing forces are reduced proportionally when using the smaller spacers and when using less than 1mm, should be negligible. Even if you buy a Sugino XD2 chainset perhaps from New Albion, you will find washers between the crank arm and the middle ring, and noone seems to think this is a problem, they are a well regarded set of cranks.

As for clearance, leave a couple of mm as the frame and the crank arm can flex. If you see contact has been made, open up a little more space. I started with too little clearance (less than 1mm) on a square taper chainset with my setup and lost a very little paint. I then upgraded to a stiffer hollowtech chainset and increased the spacing by a further 0.5-1mm and haven't had a problem since. When making a bike like this, you have the freedom to do some experimentation and use helicopter tape to protect the frame if you judge it too finely.

The bike looks great btw, love the colour.

Answer 2

"Clearance is clearance" as they say, meaning that missing by a little is just as good as missing by a lot.

Round washers between your spider and chainring would work, but they won't have as much surface area in contact, giving a place for
dirt to collect
for aero losses
and most importantly, stresses to build up.

Ideally you'd cut some shims that are the same shape as the spider/chainring crossover area, probably a D shape and then punch a round hole through the middle just large enough for the bolt to go through.

I'd go with 1~3 layers of softdrink aluminium, or a similar soft metal. Remember softdrink cans are thicker than beer cans and that the thickness decreases with height up the can. Try cutting 5 columns up a can and folding them . Wash them first. Use a wad cutter to make a clean hole.

Your other concern is the length of your chainring bolts - by spacing it out a little then you;re loosing thread engagement, and they're not particularly long bolts in the first place.