Chain sag

[FAF]

Afternoon all - thoughts (or knowledge!) please. For many years I rode Gixxers, then saw the light and am now on my second s1k. It's a Gen 4 and I have puzzled over the chain sag requirement since my gen3. The Manual advises a sag of 45-50mm. All other bikes I have ever owned have always been 25-30, maybe 35mm. A sag of 50 has always seemed waaaay too much?! so I (and the tyre place I use) have always set it at 30 - 35mm. The Manual identifies the measurement to be taken from the TOP of the mid-point chain link to the BOTTOM of that chain link when the chain is pushed up. It's this distance which is 45-50mm. I've always measured chains from the TOP to the TOP - which on the gen4 would give me 35mm.

Does anybody know the specific requirement/interpretation of this technical point please?

 

[Fraggle]

BMW dealer will set it to 45-50mm. Think it's based on the swingarm movement, so it'll be too tight in places with how you set it

 

[alex]

This was my understanding too. It's the relationship between the chain and the way the swingarm moves in relation to it. The length of the swingarm will be a big factor.

I've always measured middle to middle. (same as top to top)

 

[FAF]

BMW dealer will set it to 45-50mm. Think it's based on the swingarm movement, so it'll be too tight in places with how you set it

Thanks Fraggle - but is that measuring Top of chain to Bottom of chain or Top to Top (or as Alex below says, Middle to Middle)!

 

[FAF]

This was my understanding too. It's the relationship between the chain and the way the swingarm moves in relation to it. The length of the swingarm will be a big factor.

I've always measured middle to middle. (same as top to top)

Cheers Alex

 

[Wull]

Set it to the recommended spec as it is correct, my Motool slacker doubled up as a tool for measuring chain slack, it was brilliant, made it very easy and quick to check and set.

As with regards to what the manual says it is misleading, it doesn’t actually state what you’ve said it’s just the picture seems to show that and imho they haven’t taken that into consideration that people might interpret that as top and then bottom, as that would make absolutely no sense at all. You are measuring the amount the chain moves ( deflection as they call it ) so you either have to take your measurements consistently from the bottom/bottom top/top etc etc to give you an accurate reading.

 

[p0peye]

On side stand...no weight.

spin wheel until lowest sag (from 2014 manual), that is where you measure.

I use a 6" steel rule with 50mm marked with a bit of tape.

I then push down to find lowest sag and use 50mm mark on rule against top of lower chain. Then push up and read your sag. Adjust and check.

(pic in manual is from pushed up middle to pushed down middle of a pin, same as top to top in my World)

 

[FAF]

Set it to the recommended spec as it is correct, my Motool slacker doubled up as a tool for measuring chain slack, it was brilliant, made it very easy and quick to check and set.

As with regards to what the manual says it is misleading, it doesn’t actually state what you’ve said it’s just the picture seems to show that and imho they haven’t taken that into consideration that people might interpret that as top and then bottom, as that would make absolutely no sense at all. You are measuring the amount the chain moves ( deflection as they call it ) so you either have to take your measurements consistently from the bottom/bottom top/top etc etc to give you an accurate reading.

Thanks Wull - yea agreed it’s the picture not the text, and it’s the picture which got me puzzled! I will continue to use the same reference point (ie top) and set to BMW spec.

It will be interesting to see if this has any effect on my long-term problem of the quickshifter not downshifting at anything but slow speed - and the occasional lurch upshifting at high speed/revs as per other threads on here.

 

[alex]

QS is affected by chain slack in my experience