Hi there,
I have a 2024 Cannondale Synapse with 105 DI2 12 speed. .
When I coast downhill without pedaling, the chain comes off the front ring. Obviously this can be dangerous.

When this first happened, I replaced the freehub body and it fixed the problem for a few months. But the problem has come back. This time I got a new rear wheel, but the problem persists.
Here's what I've tried so far...

1. Adjusting the limit on the derailleur
2. Removing a link from lthe chain.
3. Replacing the rear wheel

The bike tech I go to - who iis excellent - tells me the problem is the rear derailleur arm does not pull back with enough strenth to take up the slack in the chain. He has a couple of ideas.

1. Upgrade the 105 DI2 rear derailleur to Ultegra. This would be expensive and I'm not sure it would work.
2. Change the 105 DI2 rear derailleur to a SHIMANO's GRX Di2 12-speed rear derailleur which has a clutch and a much stronger spring. This too, is expensive.

Anybody have a idea how to increase the spring tension on the derailleur without replacing it?
Please help. This bike was expensive, at least to me and I don't feel safe riding it.

Thank you,

How does the chain derail when you’re not turning the crank?


Even if there is / was a bit of chain sag from a dodgy freehub, I’m struggling to visualise how the chain wrapped round a 50 or 52 gets dislodged.

Does this happen immediately when you stop pedalling or after a while?

The fact it stopped after the freehub replacement for a while suggests the mech spring can’t be entirely to blame.

I can't speak to that particular RD but most any Shimano RD I've have has two spring tension holes in it. The default setting is the weaker of the two. I've not done it, but it can be changed if you have some mechanical aptitude. The Park document shows it. I don't know if it will help or not, but it's worth a try. I noticed long ago how the modern indexing RD have a softer spring setting than they were, say in the 80's. I never experienced any chain suck in my life until I started using a Deore LX 58something RD. .

https://www.parktool.com/en-us/blog/...lleur-overhaul

I have both 105 and Ultegra Di2/12 speed systems and have never experienced what you are describing.
My first step would have been inspecting the drivetrain for correct setup and adjustment...always starting at the beginning with der hanger alignment, proper B and limit screws settings (using the etube app where applicable), proper chain length, etc.
It sounds more like a chain length problem...but you don't indicate what the gears are...11-?.
It is also possible that the 'auto centering' of the front der. may be pushing the chain off depending on the gear you are in...50/52/? and the 30/34/?...have you checked the front/rear der alignment/adjustment using the etube app?

I'd be suspect of the bike shop if they are recommending replacement of the freehub and wheel for what could be a much simpler and less costly remedy.

There are steps to take before just replacing parts and hoping for the best that are more likely the reason than the steps you and the shop have taken...

Your freehub isn't freehubbing. When you pedal, the top span of chain tightens and drives the rear wheel. When you coast, that span goes slack. When your freehub iis bound by friction, it pushes the chain forward, causing ti to derail from the chainring. Your hub needs adjustment or a complete overhaul. I suggest you don't try to do it yourself.

He already replaced it

I'd take it to a different mechanic and ask them. Also make certain the person is also a authorized Shimano dealer or service center.

I just don't buy the thing about the RD. Otherwise why isn't everyone else griping about it here on BF and elsewhere.

The issue will almost have to be your free hub isn't doing it's job. However, you say you put another wheel in it. And that supposedly was with it's own free hub and still had the issue.

Perhaps it might be something else that is allowing the cassette to rub ever so slightly and that slows down the free hub. Maybe a spacer behind the cassette that shouldn't be there or something. But if it is, then that says even more reason why you should find another mechanic.

Or just learn to pedal going downhill. It keeps your legs warmed up and ready to pull at the bottom.

Thank you, everyone for your suggestions. I've got them all.

And please keep them coming.

Casey

You mentioned chain length.
If you have more than 2" "slack" when on BIG:BIG, reduce it to <2" but at least 1". (maybe fudge 7/8")
Maybe that's what you already did?

i've recently seen a BADLY Over-tightened Quick release cause extreme drag to the axle....

a bent up, reversed, or too-deeply installed, seal can also cause extreme drag troubles.......

also... if the Freehub was removed and re-greased... Too Much or too thick of a Grease can cause excess drag...
heck, i've seen a completely backfilled rear Hub nearly Lock Up because of too much stupid-thick grease.

I'd check out a different mechanic before I spent another penny. Just seems to me as though you've spent way too much money on what should be a simple problem.

Substituting parts in place of competence does not always work

I wonder why Cannondale support hasn’t been involved since this seems to be a safety issue and in my experience they seem to be pretty responsive?

.