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RhynoCZ
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PostPosted: 15:26 - 24 Sep 2020    Post subject: Road bike gearing question... Reply with quote

Hi, this might be a simple one, but I've recently got a road bike with 2x9 gearing. Piror to that I was used to ride 3x7 (to 9) gears and knew that you can't cross the chain too much but this lead to griding noise and I just shifted the gears accordingly. Also, the middle front cog was universal for the full range of the cassette.

Now, the 2x9 setup, yesterday I went for a ride, having the small cog front and somewhere in the middle of the casset back, after a climb. Then I wanted to speed up, shifing the casset down to the lowest gear when all of the sudden, when putting more power to the cranks, without prior griding, the chain caught on the big front cog.

Now, do I have an issue or one just should never ride the small cog front with the smallest cog or the one before that in the back, on such bike? When using the big front cog I can use full range of the cassette.

I use the Shimano SORA R3000 Compact groupset.
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weasley
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PostPosted: 16:31 - 24 Sep 2020    Post subject: Reply with quote

Conventional wisdom says thou shalt never run big-to-big or small-to-small, as "cross-chaining" will ruin your drivetrain and open a space-time paradox.

I have to say that whilst I will generally avoid it, I always make sure that all my multi-ring bikes will run all of the cassette cogs in all of the chainrings. It is not ideal and the outcome will depend on the chainline and the size difference between the big and small front rings.
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Ste
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PostPosted: 17:30 - 24 Sep 2020    Post subject: Reply with quote

Sounds like someone needs to adjust the H and L limits on their derailleurs.

As weasley says, cross chaining is something that you shouldn't do but riding with the chain on the small front chainring and the smallest gear on the rear cassette shouldn't cause the chain to try and change to the bigger chainring at the front.

Once you've set H and L limits on the derailleur and indexed your gears properly you'll have silky smooth shifting and it won't try to change gears on its own. A quiet bike is a happy bike. Thumbs Up
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Easy-X
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PostPosted: 20:02 - 24 Sep 2020    Post subject: Reply with quote

Read this...

https://www.sheldonbrown.com/chainline.html

...there will be a test later Wink
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ThunderGuts
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PostPosted: 10:11 - 16 Oct 2020    Post subject: Reply with quote

Not convinced it's got anything to do with limit stops, more likely the bottom bracket is the wrong size bringing the chainrings close to the frame. The middle chainring, or the midpoint between the two chainrings in this case, should align with the centre of the cassette. If the BB is too short, the chainrings will be aligning closer to the large end of the cassette, so when heading for the smallest cassette cogs the chain will be at a pretty aggressive angle and could catch the big chainring.

If the chain is crossed at that much of an angle you'll be losing loads of power and destroying the chain quickly too.
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Ste
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PostPosted: 12:50 - 16 Oct 2020    Post subject: Reply with quote

The bottom bracket for the groupset only comes in one size so it's unlikely that the wrong size has been fitted. Razz

I'd expect that the frame width at the bottom bracket will be 68mm so there will be 2mm of spacers and I can't be arsed to check the manual from Shimano to see whether they all go on the drive side or if it's one 1mm spacer on each side.

Shimano manuals and instructions are good, they're really good.

I'm guessing that Rhyno has fixed it as he hasn't returned to this thread?
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ThunderGuts
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PostPosted: 13:52 - 16 Oct 2020    Post subject: Reply with quote

Ste wrote:
The bottom bracket for the groupset only comes in one size so it's unlikely that the wrong size has been fitted. Razz

I'd expect that the frame width at the bottom bracket will be 68mm so there will be 2mm of spacers and I can't be arsed to check the manual from Shimano to see whether they all go on the drive side or if it's one 1mm spacer on each side.

Shimano manuals and instructions are good, they're really good.

I'm guessing that Rhyno has fixed it as he hasn't returned to this thread?


Actually that's not accurate, at least as far as square taper BBs are concerned (probably external BBs too but I've not replaced any of those); they have a whole range of different spindle lengths ranging from around 107mm to 130mm. The shell is usually 68mm, which is what you're thinking off. If the groupset/frame combination calls for a 125mm spindle say and you fit a 110, it'll bring the chainrings in closer to the frame and make a mess of the chainline. Now, fit Campag cranks to a Shimano ISO spindle and you're into a whole other world of pain. Laughing
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Ste
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PostPosted: 14:00 - 16 Oct 2020    Post subject: Reply with quote

It's not a square taper, it's Hollowtech II. Razz

https://bike.shimano.com/en-EU/product/component/sora-r3000/BB-RS500.html
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ThunderGuts
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PostPosted: 22:21 - 16 Oct 2020    Post subject: Reply with quote

I stand corrected. Must be very specific to certain groupsets then.
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RhynoCZ
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PostPosted: 17:07 - 17 Oct 2020    Post subject: Reply with quote

Ste wrote:
I'm guessing that Rhyno has fixed it as he hasn't returned to this thread?

Well, yes and no. I went through the Sheldon Brown's site, watched some Park tool videos as well, and I did adjust the rear derailleur again. Very nice and quick shifting, no noise, the derailleur stops exactly where it should. Then, I had a look at the front derailleur. Fidling with the cable adjustment and L and H limit screws. I was succesful, the derailleur cage stops where it should, shifting is smooth and quick and there is no apparent rubbing.

Now, that being said and I think the bike well adjusted, I found out what probably caused my ''issue'', appart from forgeting how to used the gears on a bicycle. The frame (or something else) flexes under load, not very much but more than enough for the chain to rub against the derailleur cage when running the front big sprocket (50t) and the rear smallest one (11t) or the one next to it (13t). This doesn't happen when testing the bike in the stand with no actual load, so it's either the frame flexing or something is loose, I guess it's the frame.

So, my wild theory is, one of the magnets around the big front sprocket pulled the chain ''up'' just when I was putting some ''serious'' power down onto the cranks. As said before, there was no prior rubbing/grinding. But hey, I've learned my lesson and ever since then I just don't cross run the chain and I'm also more aware of what gears I use and when. The frame (or something else) flex remains though, so next I'm gonna try to set the 4th index position of the front derailleur shifter so there is enough room for the front big sprocket to ''wobble around'' under ''heavy'' load.

I've never had such gear ratios on a bicycle that would alowe me to flex the frame under load. Or am I just that strong these days? Perhaps the frame isn't the best, considering it's an entry level bike from Decathlon. Thinking

EDIT: And yes, Ste, the BB-RS500 bottom bracket is exactly what's on the bike. Thumbs Up
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Dave....
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PostPosted: 21:51 - 17 Oct 2020    Post subject: Reply with quote

Keep chain straight as possible and that goes for any set up.
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Ste
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PostPosted: 12:50 - 18 Oct 2020    Post subject: Reply with quote

If you've been following what's said in Park Tools videos then it's hard to go wrong. Thumbs Up

And here they are for anyone who's interested: https://www.youtube.com/c/parktool/

Other useful youtube channels for mechanical advice include...

GCN Tech for those of you with skinny tyres: https://www.youtube.com/c/gcntech/

GMBN Tech for those of you with knobbly tyres: https://www.youtube.com/c/gmbntech/
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RhynoCZ
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PostPosted: 17:20 - 18 Oct 2020    Post subject: Reply with quote

Ste wrote:
If you've been following what's said in Park Tools videos then it's hard to go wrong. Thumbs Up


Yep, altought the ''top gear'' is still not perfect. There is some flex in the frame, cranks, BB... not sure. I can't seem to make it perfect through out all the gears, so I've decided to just give up on the ''top gear'' for now. It's 50t/11t anyway, so downhill use only, for me at least. When I set the derailleur cage for perfect ''top gear'' performance, then the shifting isn't nice at all.

I might have a chat with a proper bicycle mechanic, my mate has a mate who could help. The trim function of the front derailleur shifter is a bit of a puzzle to me.
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'95 Mercedes-Benz w202 C200 CGI; At my disposal: MZ 150 ETZ, not anymore.
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weasley
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PostPosted: 20:41 - 18 Oct 2020    Post subject: Reply with quote

I was going to ask if it had a trim facility. This essentially means you can have the front mech in two positions for each chain ring. If you swing the up-shifter all the way in, as far as it goes, the FD should move to its outer-most setting - best for the outer cogs on the cassette. As you shift the cassette inwards the chain may start rubbing on the FD at which point you can ‘trim’ it; a light touch on the ‘down’ shifter should result in a light click and the FD should move just a little bit inwards. A full-blown tap on the down shifter should move it fully inwards (and change chainring) - when you use the outer cassette cogs again the chain may catch the FD and you can swing the up-shift lever just a little bit which should trim the FD out a touch to compensate.
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Ste
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PostPosted: 16:12 - 19 Oct 2020    Post subject: Reply with quote

Unfortunately derailleur trim functions is all new to me due to not being a road bike person.

Having watched a few youtube videos about front derailleur trim adjustment it does look like it could be the answer to the problem your having. As it's a topic I know nothing about, I'm reluctant to recommend any particular videos therefore all I can say is start at the top of the page and work your way down. https://www.youtube.com/results?search_query=derailleur+trim+adjustment

It's going to be something that's really quite simple to fix as the setup is so nearly perfect, it's just one combination of gears that's unhappy. Personally I'd do some reading up / youtube video watching on trim adjustments and then try playing around with those. Thumbs Up

There is flex in your frame (and other components) but there would have to be something seriously wrong for for the flex to be the cause of the problems you're having. Laughing
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