Classic & Vintage - Miyata 1000 triple crankset problem

I rescued a 1985 Miyata 1000 from a trip to the tip. I am now using it for my commute to work and am very impressed with its performance, with the exception of the transmission on the smallest chain ring.

Spec:
Crank: Sugino AT 170mm triple
Chain rings: Sugino 50, 40, 28, with Sugino chain ring bolts. Granny ring secured to spider with chain ring bolts "inside out" ie allen head portion of bolt faces non-chain ring side of bike.
Rear der: Shimano 600 SIS
Front der: Shimano, don't know model
BB: Tange
Freewheel: Shimano 6 speed

Problem

When using smallest chainring the chain "jumps" slightly when on any rear sprocket other than the largest sprocket. In static position turning cranks and shifting it is noticeable that when in rear sprocket 2 + the chain is fouling the inner part of the chain ring bolts that secure the middle chain ring to the spider. Obviously I do not expect to be able to use smallest chainring, smallest rear sprocket combo, but I would have though that sprockets 1 to 5 or at least 1 to 4 would work ok.

On a 21 yr old bike for which I have no history it is unlikely the transmission is original, although the chain rings may well be. Note the chain rings are pretty worn and have lots of nasty marks on their inner and out edges from chain fouling (wonder what caused this).

Any suggestions?

Perhaps the chain has ben replaced with one that is too wide.?
Is it possible that at some time the chain rings have ben re-asembled incorrectly (is it critical that hey align in some way???)

This really is a great bike. It rides with more "feel" than many of the modern bikes have tried. Living in hilly Sydney, Aus, I'd like to get the transmission working properly.

The fact that you indicate the chain is fouling on the chainring bolts indicates that the middle chainring has probably been installed inside out. Chainring mounting holes normally have a countersink on one side that permits the flange on the bolts to sit flush with the surface of the chainrings. Reversing the chainring should improve the situation.

However, it sounds like your problems may go a bit deeper, extending into a chainline issue. This could be due to frame alignment or component issues. However, since the 1985 model 1000 was spec'd with a Shimano crankset, the most likely cause is a replacement Sugino AT that moved the chainline too far inwards. Since you have a triple crankset with a 6 speed freewheel, the theoretical chainline extends from the middle chainring to a point between the 3rd and 4th cogs. Put the chain on the middle chainring and sight down it. It should angle slightly towards the hub when on the 3rd cog and slightly towrds the dropout when on the 4th.

Alternately, measure the distance between the inside of the dropouts and divide by two. This measurement should be the same as the distance from the middle of the bottom bracket shell to the centerline of the middle chainring.

To verify that frame alignment is not an issue, tie a piece of string to the rear dropout and extend it around the font of the head tube. Pull it taught and tie it the same location of the opposite dropout. No measure the distance from the string to the seat tube. If the frame alignment is perfect, the measure will be equal for both sides.

The fact that you indicate the chain is fouling on the chainring bolts indicates that the middle chainring has probably been installed inside out. Chainring mounting holes normally have a countersink on one side that permits the flange on the bolts to sit flush with the surface of the chainrings. Reversing the chainring should improve the situation.

However, it sounds like your problems may go a bit deeper, extending into a chainline issue. This could be due to frame alignment or component issues. However, since the 1985 model 1000 was spec'd with a Shimano crankset, the most likely cause is a replacement Sugino AT that moved the chainline too far inwards. Since you have a triple crankset with a 6 speed freewheel, the theoretical chainline extends from the middle chainring to a point between the 3rd and 4th cogs. Put the chain on the middle chainring and sight down it. It should angle slightly towards the hub when on the 3rd cog and slightly towrds the dropout when on the 4th.

Alternately, measure the distance between the inside of the dropouts and divide by two. This measurement should be the same as the distance from the middle of the bottom bracket shell to the centerline of the middle chainring.

To verify that frame alignment is not an issue, tie a piece of string to the rear dropout and extend it around the font of the head tube. Pull it taught and tie it the same location of the opposite dropout. No measure the distance from the string to the seat tube. If the frame alignment is perfect, the measure will be equal for both sides.

Thanks T-Mar. Will check. Could it also be the rear freewheel? Perhaps he bike was orignally supplied with a 5 speed freewheel, as opposed to 6-speed? Would that cause the problem??

Regards

nah. It would have come with 6 by 1985. Even the department store bikes were getting six by then. (Zoop Zotemelk tested a 7-speed freewheel in the 1978 Tour).

Thanks T-Mar. Will check. Could it also be the rear freewheel? Perhaps he bike was orignally supplied with a 5 speed freewheel, as opposed to 6-speed? Would that cause the problem??

Regards

Switching to a standard 6 speed freewheel would involve spreading the rear dropouts about 6mm. This could cause frame alignment problems, but should be caught by the described check.

Switching to a standard 6 speed freewheel would involve spreading the rear dropouts about 6mm. This could cause frame alignment problems, but should be caught by the described check.

T-Mar, I have performed the checks you suggested, an am now a bit concerned. :eek:

The chain wheels appear to be correctly installed, with all 3 having the stamped info on size facing out. There are countersinks, but they are very shallow.

The frame bottom bracket is 68mm. Distance between rear dropouts is 122mm / 2 = 61mm. Distance between centerline of middle chainring and middle of crank is 46mm. Variance 15mm. However, the RHS chain stay is deeply recessed to accommodate, I presume, the granny ring.

I performed the frame alignment check using the string method. Distance RHS drop out to centre seat tube = 430mm. LHS is 425mm. Variance 5mm. Is this mis-alignment significant? I measured 3 times, same result.

Any more ideas???

Dukerider

You could have a chainline problem, as T-Mar suggested, if the Sugino AT replaced the original Shimano crankset. I don't know if that's so, however - my 1986 210 (the low-end touring model) has a Sugino triple with approximately the same gearing (50, 44, 28 - which, oh by the way, is a terrible match for the 14-28 freewheel, but I digress).

However! I do see that you have 122mm frame spacing in the rear triangle. That sounds about right, though just a bit wide, for a five-speed freewheel, but definitely too narrow for a six. Did you remove the rear wheel for this test? If you did, when you pulled it out, was it tight, and did the rear triangle "spring" closed a bit? If so, you may have found your culprit. The solution could be to have the frame professionally aligned and cold-set at the proper spacing, or if you're really brave, to do the job yourself. Someone who knows more about this might be able to confirm, but 122mm definitely sounds wrong for a 6-speed, and so that may be your problem.

Oh, and is a 5mm misalignment significant? Almost certainly.

You could have a chainline problem, as T-Mar suggested, if the Sugino AT replaced the original Shimano crankset. I don't know if that's so, however - my 1986 210 (the low-end touring model) has a Sugino triple with approximately the same gearing (50, 44, 28 - which, oh by the way, is a terrible match for the 14-28 freewheel, but I digress).

However! I do see that you have 122mm frame spacing in the rear triangle. That sounds about right, though just a bit wide, for a five-speed freewheel, but definitely too narrow for a six. Did you remove the rear wheel for this test? If you did, when you pulled it out, was it tight, and did the rear triangle "spring" closed a bit? If so, you may have found your culprit. The solution could be to have the frame professionally aligned and cold-set at the proper spacing, or if you're really brave, to do the job yourself. Someone who knows more about this might be able to confirm, but 122mm definitely sounds wrong for a 6-speed, and so that may be your problem.

Yes, rear wheel was removed for test. The frame did not "spring" closed. When replacing the wheel the rear triangle was maybe 1 to 2mm too tight - had to apply moderate outward pressure on dropouts to slot the axle in. I did not think that the 6 speed would have been a later upgrade as the downtube levers are SIS with 6 incremental clicks. They are secured by a band - not attached to braze-ons. Suppose they could have been changed though. I did note on Sheldon Browne website that the '85 Miyata 1000 originally came with a 5 speed freewheel. The serial number on mine is MS44032 which "I Think" makes it an '86 model?????