Chain slipping Between Chainrings

[iandusud]

One thing that this highlights is just how inflexible these systems are. Give me bar-end levers any day!

[zenitb]

Unscientific ATM ..just measured one arm on each crank.

SPA 3.8mm Thorn 4.0mm

Spa and Thorn cranks ...

This is all sounding very similar to Ian's issue.

Next step is to unbolt the rings on the tandem and check the Thorn crank arm widths on that...its looking like its going to be the same 4mm that generated Ian's problem though...

I am going to tie all this up with a move to friction shifting so this could take a while

One thing to note is that I think the thicker Thorn cranks are much older stock than the Spa ones..on the SJSC site they talk about the stock being bought way back and it has now effecively run out. I bought the Spa crank a month ago (these are both 160mm cranks hoarded for a future tandem)

You could postulate that as time has gone on with the move from 7 to 8, 9, 10 speeds the manufacturers may have reduced the thickness??

[iandusud]

It would be worth checking the spacing between the tops of the teeth on the outer and middle rings and then adding that to the thickness of the spider lugs to calculate the exact effective spacing. Based on my experience you should be aiming for 7mm.

[zenitb]

It would be worth checking the spacing between the tops of the teeth on the outer and middle rings and then adding that to the thickness of the spider lugs to calculate the exact effective spacing. Based on my experience you should be aiming for 7mm.

Hard to do this accurately but squinting down with a steel ruler lined up it is certainly more than 7mm... pretty much 7.5mm I would say which does tie up very well with your theory.

I will get the file out next weekend, slap on the old Ultegra barcon as a shifter, and post back. Again, thanks for sharing iandusud..very timely.

[Mick F]

Slowster.
Thanks for that with respect to the differences.
For the sake of comparison, here's the two together.

Not a huge amount of difference.

[zenitb]

It would be worth checking the spacing between the tops of the teeth on the outer and middle ..
...Based on my experience you should be aiming for 7mm.

Lots of debugging today. Big to middle ring change fairly consistently resulting in ring riding...

Riding the middle ring..

Have filed crank tabs from 4mm to 3.5mm and with limited testing it hasn't "ring rode" since. Big ride tomorrow though so let's see how it goes...

Look away now...

[zenitb]

It would be worth checking the spacing between the tops of the teeth on the outer and middle ..
...Based on my experience you should be aiming for 7mm.

....
Have filed crank tabs from 4mm to 3.5mm and with limited testing it hasn't "ring rode" since. Big ride tomorrow though so let's see how it goes...

20220702_170241.jpg

1800 feet of climbing on the ride yesterday and many changes from big to middle front chainring.

No "gliding" at all. Early days but this is looking like a good fix.

Filing the outer side of the chainset bolt hole spider down to 3.5mm (from 4mm in my case) is the charm - as Ian has said. This is a much better spacing for Shimano 10 speed chain.

Cheers all

George (aka zenitb)

[iandusud]

1800 feet of climbing on the ride yesterday and many changes from big to middle front chainring.

No "gliding" at all. Early days but this is looking like a good fix.

Filing the outer side of the chainset bolt hole spider down to 3.5mm (from 4mm in my case) is the charm - as Ian has said. This is a much better spacing for Shimano 10 speed chain.

Cheers all

George (aka zenitb)

We got out yesterday for proper ride and likewise had perfect front gear changing with all three rings selecting properly and no chain rub on when using the outer ring with smallest sprocket.