Trim function, front derailleur

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Cyckelgalen
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Trim function, front derailleur

Post by Cyckelgalen »

Does anyone know why road triple front derailleurs have a trim function, that is intermediate positions between the 3 main positions, whereas MTB triple derailleurs don't ? I miss that function when I use MTB shifters, even higher end ones.
Brucey
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Re: Trim function, front derailleur

Post by Brucey »

there are several reasons, I think the main two are

1) MTB chainsets are smaller and stiffer than road chainsets
2) MTB chainstays are longer than those on a road bike, so the chain is not at quite the same angle

cheers
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MorningDew
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Re: Trim function, front derailleur

Post by MorningDew »

Some Gripshift shifters have intermediate positions for the front mech. I'm not sure about the recent 10 speed, 11 speed, etc. but a lot of those are run as 1x anyway. However, if you're looking at an older MTB with 21 or 24 speeds then some older SRAM SRT-600 have intermediate clicks on the front shifter and the newer SRAM MRX Comp shifters have them too.
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foxyrider
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Re: Trim function, front derailleur

Post by foxyrider »

Cyckelgalen wrote:Does anyone know why road triple front derailleurs have a trim function, that is intermediate positions between the 3 main positions, whereas MTB triple derailleurs don't ? I miss that function when I use MTB shifters, even higher end ones.


Its because it would confuse the average ATB rider :lol:
Convention? what's that then?
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Sweep
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Re: Trim function, front derailleur

Post by Sweep »

Apologies if I am being thick, and am particularly nervous as I am posting after brucey, but isn't the trim function a feature, if provided, of the shifter rather than the mech?
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Cyckelgalen
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Re: Trim function, front derailleur

Post by Cyckelgalen »

You are right Sweep, I should have said "front derailleur shifter" to be more precise.

Long chainstays will certainly help to to prevent wicked chain angles, but it is two different touring bikes with XT trigger shifters that I find faffy to adjust with no chain rub at all, and touring bikes have even longer chain stays that MTB's. I would welcome being able to trim the front derailleur.

Both have touring triple cranksets with 26-36-48 chainrings. Could it be that modern XT derailleur/shifters are designed to work perfectly only with smaller MTB chainrings?

I'm not really convinced that stiffness is an issue (although Brucey will probably manage to convince me), the reason being that chainrings are mounted on a spider with arms that are quite thick and stiff, I would think. Larger chainrings will be mounted on slightly longer spider arms and I can't imagen them flexing a lot more. Furthermore, if stiffness is an issue, why are most modern crank spiders four-armed rather than good old stiff five-armed ?
Brucey
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Re: Trim function, front derailleur

Post by Brucey »

the apparent lateral stiffness of the chainrings is also sensitive to the stiffness of the frameset, the BB spindle as well as the chainset spider arms and the chainrings themselves. How hard you push on the pedals and how straight the parts are to start with also make a difference of course.

You don't say what exact parts you are using but IME an 8s 110mm BCD triple (with medium sized chainrings for touring) with a ST BB mounted on a steel frame is of marginal stiffness to work with a (trimless) MTB shifter. The same setup on a (stiffer) aluminium frame is more likely to be OK. I find I can't run 9s (or higher) MTB stuff unless the frame is stiff and/or the BB spindle is oversized; there is too much rubbing otherwise.

But I should mention that I've had all kinds of FD rubbing problems on nearly every derailleur geared bike I've ever owned, when giving it 'full gas'. It has always been worse on road bikes. I still have several practically unused Dura-Ace FDs from BITD; I just couldn't use them without getting bad rubbing (even with a chain that was narrower than the one recommended), I had to use old campag FDs (with a wider cage) instead. By 'rubbing problems' I mean rubbing both sides of the FD in one gear, sat in the saddle when giving it a three-minute effort. When it is like that, you practically have to trim the FD for each sprocket at the back. To this day I'm not sure how much of the movement was due to the frame, BB spindle, chainset and chainrings, but better quality chainrings were usually better than cheaper ones; even if the cheaper ones seemed to true to start with they often flexed more under load.

FWIW there are shimano flat bar shifters with trim positions, and matching FDs to go with them. I've not messed about with them much (flat bars and derailleur gears = MTBs for me) but I certainly wouldn't expect a strong rider to be able to use MTB shifters on a flat barred road bike without encountering rubbing problems. The chances of such problems I would expect to increase if more of the following apply;

- flexy seat tube on the frame
- skinny/flexy BB spindle
- slack in the BB bearings
- large chainrings
- cheaply made chainrings/chainset
- chainrings that are not straight to start with
- lots of sprockets at the back
- short chainstays
- flexible chainstays of any length
- FD meant for 'more sprockets than the chain' (eg using a 10s FD with a 9s chain)
- seat tube to chainstay included angle outside the specifications for that FD.

When I use 'modern' indexed shifting on a road bike the adjustment that most often gives me gyp is the front shifting; if you are lucky there are two trim clicks on the big ring and this almost invariably goes out of adjustment before anything else does. Oftentimes I can't use more than a few sprockets at the back on the big ring, not with any pressure and without rubbing, anyway. I much prefer to use systems with a friction shifter (or lots of tiny clicks) on the FD instead. Perhaps ironically the framesets which give me the least rubbing problems under reasonable pressure tend to be the ones I like riding least otherwise; when they are stiff enough around the BB area they tend to be too stiff all over, and ride too much like a plank for my tastes.

So anyway I don't mither endlessly about this, I know I just have to be careful about the kit I use. I don't think I'm alone by any means, but there are plenty of people that just put up with the rubbing or perhaps don't even notice it if it occurs mainly under duress.

I've mentioned this before, in relation to frame stiffness, but it worth mentioning again because it relates to this as well; in a simple cantilever bend, increasing the length of the cantilever by 20% increases the deflection at the tip by over 70%. This relates to chainsets and chainrings only obliquely, because they don't have uniformly distributed stiffness, but the bottom line is that it may mean that if a 42T chainring doesn't flex too much, don't expect a 50T one to do likewise; it might be flexing about 70% more, laterally, under similar conditions.

cheers
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Brucey
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Re: Trim function, front derailleur

Post by Brucey »

I tried to round up as much information as I could about trim clicks a couple of years ago in this thread;

https://forum.cyclinguk.org/viewtopic.php?f=5&t=124598

cheers
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colin54
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Re: Trim function, front derailleur

Post by colin54 »

It's a nuisance isn't it ? That's my recent experience too. I have a 7 speed triple, 42 tooth large ring on my Saracen MTB with STX Rapidfire (no trim function I didn't even know such a thing existed until a recent thread on the subject) little problem with rubbing. Quite a sturdy frame,Tange MTB tubing.
I also have a 7 speed triple 46 tooth large ring on a 531c Dawes Audax with an RSX STI (no trim), which is virtually impossible to set up well to avoid cage rub, I think I read that 8 speed RSX have a trim function, so maybe it was found to be a problem in the early days of STI. I'm going to convert this bike to down tube levers and aero brake levers at some point because of this very problem, not too fond of the feel of brake levers on the STI's come to that.
Edit : RSX 8 speed only has trim feature on the double STI ST-A416, Triple is ST-A417 which as Mercalia says down thread, has no trim.
https://si.shimano.com/pdfs/si/SI-87G0C-000-00-ENG.pdf
Last edited by colin54 on 9 Apr 2020, 9:00pm, edited 1 time in total.
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Sweep
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Re: Trim function, front derailleur

Post by Sweep »

Do check out brucey's link to that other thread OP.
I have several shimano trimmable flat bar front shifters for triples. Use them on my hybrid/tourer builds. I think they are great.
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Norman H
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Re: Trim function, front derailleur

Post by Norman H »

Some downtube cable stops have cam action lever adjusters with indents which can be used to adjust trim. I've only ever seen this feature for the rear dérailleur but I don't see why it wouldn't work on the left hand boss, albeit the other way up.

https://sprockets.uk.com/downtube-cable-stop-adjusters-resin/
Last edited by Norman H on 9 Apr 2020, 8:46am, edited 1 time in total.
Brucey
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Re: Trim function, front derailleur

Post by Brucey »

Following up Norman's suggestion, it occurs to me that you could use the trimmable RH ergo boss for the FD provided you are happy to have the gear inner wires crossing over under the down tube. Some folk do this anyway, because it gives the gear cables a nicer route around the headstock.

IIRC the lever/cam type bosses were originally used with high end shimano kit when STIs were still a fairly new thing.

Image
Image
SM-ST74 Dura Ace bosses as used with 7400 series STIs

cheers
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mercalia
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Re: Trim function, front derailleur

Post by mercalia »

not all road triple have trim. The 7 & 8 speed RSX never did and none the worse for the lack thereof. Trim is a silly idea covering a failure of real world design?
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Sweep
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Re: Trim function, front derailleur

Post by Sweep »

mercalia wrote:. Trim is a silly idea covering a failure of real world design?

or my lack of bike-building/set-up skills.
Not sure I see the problem to be honest - no great problem to cure a little rattle with a simple click and pedal on.
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Cyckelgalen
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Re: Trim function, front derailleur

Post by Cyckelgalen »

Trim is a silly idea covering a failure of real world design?[/quote]

I'd rather put it this way: trimming is a necessary faff because having indexed front shifting at all is probably more of a silly idea.
Indexing is a great idea for the rear shifter, because a very precise chain position is desirable for every cog, and there are too many cogs to manage manually, but at the front there are massive variations on the chain angle on every single chainring and straight basic 3 position indexed shifting doesn't account for that. Trimming does help, and as mentioned upthread, friction shifters are a great simple solution for front mech ailments and constant need of adjustment. I own a touring bike with bar end shifters and friction shifting front mech and I can do many thousands of miles without any readjusting.
Last edited by Cyckelgalen on 9 Apr 2020, 3:45pm, edited 2 times in total.
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