Does this sound right to "compensate" a fat seat tube?
Shimano make that R443 front mech for use with road chainrings and trigger shifters. Because this front mech is a road mech to go on road bike frames, it seems that Shimano have made it to work best on a 27.2mm seat tube - and why not, since that is indeed the most common road seat tube diameter.
The problem is now people are making road frames that have a 34.9mm seat tube on them - which is the thickest diameter you can get even on mountain bikes!
So because of this, because that front mech is on a 34.9mm seat tube and not a 27.2mm seat tube - does it sound right to do this:
34.9mm - 27.2mm = 7.7mm
7.7mm ÷ 2 = 3.85mm
This means that front mech is poking out from the bike frame 3.85mm more than Shimano meant it to be.
I tried a 115mm bottom bracket with a triple chainset that said "recommended" bottom bracket spindle length should be 113mm. With the 115mm spindle, the front mech was all "tucked in" and to get the front mech to work I had to have the cable really loose when it was on the granny ring. This is because part of what would have been taking up some of the cable slack (by having the mech itself set more outwards), its unable to, because the mech has to be so close to the frame.
I knew it was wrong with a 115mm spindle, because the front mech had to touch the chainstay if it were to change down to the granny ring.
So I thought this chainline is 3.85mm too narrow - the 122.5mm bottom bracket would almost perfectly fix it (brings it out 3.75mm) so the front mech is "fooled" into thinking it is on a 27.2mm tube.
Yes, this fixed it, I could even fit my chain catcher thingy back on again.
I just wondered if all of this sounds normal? Shimano might have never expected a road frame to have a 34.9mm seat tube. Sheldon Brown mentions that if it is thicker than 31.8mm you might have problems with the R443 mech and need a wider BB spindle.
I was thinking about getting a 127mm bottom bracket just for the sake of trying it, but I think it would be going too far the wrong way then. There is a good amount of wiggle in the chain catcher now and although the front mech's cage is again nearly touching the chainstay on the granny ring, it doesn't touch. It looks right and changes better.
All I want to know is... is it even normal to need a bottom bracket thats 122.5mm as opposed to 113mm and is the only reason for that because of a thicker (the thickest) seat tube?
Shimano can't accommodate for everything lol. The fact that such a mech even exists is a godsend for commuters/touring. 8)
The problem is now people are making road frames that have a 34.9mm seat tube on them - which is the thickest diameter you can get even on mountain bikes!
So because of this, because that front mech is on a 34.9mm seat tube and not a 27.2mm seat tube - does it sound right to do this:
34.9mm - 27.2mm = 7.7mm
7.7mm ÷ 2 = 3.85mm
This means that front mech is poking out from the bike frame 3.85mm more than Shimano meant it to be.
I tried a 115mm bottom bracket with a triple chainset that said "recommended" bottom bracket spindle length should be 113mm. With the 115mm spindle, the front mech was all "tucked in" and to get the front mech to work I had to have the cable really loose when it was on the granny ring. This is because part of what would have been taking up some of the cable slack (by having the mech itself set more outwards), its unable to, because the mech has to be so close to the frame.
I knew it was wrong with a 115mm spindle, because the front mech had to touch the chainstay if it were to change down to the granny ring.
So I thought this chainline is 3.85mm too narrow - the 122.5mm bottom bracket would almost perfectly fix it (brings it out 3.75mm) so the front mech is "fooled" into thinking it is on a 27.2mm tube.
Yes, this fixed it, I could even fit my chain catcher thingy back on again.
I just wondered if all of this sounds normal? Shimano might have never expected a road frame to have a 34.9mm seat tube. Sheldon Brown mentions that if it is thicker than 31.8mm you might have problems with the R443 mech and need a wider BB spindle.
I was thinking about getting a 127mm bottom bracket just for the sake of trying it, but I think it would be going too far the wrong way then. There is a good amount of wiggle in the chain catcher now and although the front mech's cage is again nearly touching the chainstay on the granny ring, it doesn't touch. It looks right and changes better.
All I want to know is... is it even normal to need a bottom bracket thats 122.5mm as opposed to 113mm and is the only reason for that because of a thicker (the thickest) seat tube?
Shimano can't accommodate for everything lol. The fact that such a mech even exists is a godsend for commuters/touring. 8)
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Comments
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Manc33 wrote:So because of this, because that front mech is on a 34.9mm seat tube and not a 27.2mm seat tube - does it sound right to do this:
34.9mm - 27.2mm = 7.7mm
7.7mm ÷ 2 = 3.85mm
This means that front mech is poking out from the bike frame 3.85mm more than Shimano meant it to be.
Not quite right i'm afraid as you make the mistake with the diameter of 27,2 , that is the inside diameter.
The outside diameter at alu frames is minimal 31,6.....0 -
Oh yeah... so this means I already over-compensated, if anything. Still, the FD cannot go any closer to the chainstay, it ain't gonna get any better.
My FD clamps onto a 34.9mm seat tube but the top of it is 36.6mm. The fact that I can't find a 36.6mm quick release seat tube collar tells me these sizes are pretty unexpected.
I can't believe I have to undo a bolt every time to alter my seat height.
When you're tinkering all the time with different cranks, pedals and saddles, it does become tedious. 0 -
Sometimes you can "win "a few mm by filing off the FD on the right place....
The good news is that bike manufacturers come more and more to the conclusion that those fat seat tubes were a mistake and return to 27,2/31,6.
As a matter of fact even with my titanium frame ( 31,6) the FD is the limiting factor on the chainline as it has bended seatstays for maximum clearance of chainwheels and crank.0 -
I had a Triban 3 that was a 34.9mm seat tube and it was alright with a 115mm bottom bracket, triple chainset and Sora road FD.
Now using a R443 front mech on a 34.9mm tube, I need a 122.5mm spindle.
My shifter shifts the front mech a bit far, I mean I can't screw in the limit screw because the mech stops it. If I loosen the cable slightly to bring that in, then it won't change from middle to large. I think at the moment that might be because I have a 34/46 (middle ring is 74% of big ring). Swapping to 36/48 (middle ring is 75% of big ring) will help lol, by that tiny amount, but I think a large 46 isn't right for the R433 mech and having a bigger large chainring might help. After all what "large" road chainring would be only 46.
What gets me is that front mech spec PDF says have a 45mm chainline, but on a 34.9mm tube it needs to be a wider chain line. Why say 45mm! That same PDF also warns about tubes wider than 31.8mm - so it is coming straight from
themselves. 0 -
The specification is obvious for a 31,6 frame.....0
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Keezx wrote:The specification is obvious for a 31,6 frame.....
It would be if it mentioned the two figures together, E.G. "Use a 45mm chainline on a 31.8mm seat tube" or "Use a 50mm chainline on a 34.9mm seat tube" but none of that is mentioned. It just says you might have problems with the chain line beyond 31.8mm then in another column completely, use a 45mm chainline.
Most people just buy a complete bike already built, when it comes to replacing the bottom bracket, they know what size to get - whatever came off the bike.0
