moxfyre

I finally finished reassembling the Nishiki frankenbike I've been working on. Most of the original components are of a good quality (SunTour, DiaCompe, Sugino), but one problem I've had is that the rear derailer is WAY TOO SMALL.

With a double crank (50,40 -ish), and a 6 cog freewheel, only 3-4 of the 6 cogs can be used with each chainring, without binding the chain or having the derailer bend back so far it can't wrap it all up. And this is after a couple hours of derailer tweaking/chain resizing, etc.

Has anyone ever seen a bike designed with such a woefully inadequate R Der? I'm thinking of getting a Shimano Sora 8 speed derailer from Nashbar, anyone know how well that'll work with friction shifters?

kerk

You sure you have the high and low stop screws far enough out? Your rear der will surely shift to all cogs. Chain slack may be a problem, but I've never seen a rear der not handle a 6 speed freewheel.

USAZorro

Your derailleur... Sun Tour? Which Model?

moxfyre

The derailer is Sun Tour AR, I think it's from the early 80s, but the capacity isn't marked on it...

And I don't think the limit screws are a problem because it has enough forward-and-backward motion to reach all the cogs. I also adjusted moved the wheel as far back in the dropout as possible (moving the cogs as far above the derailer as possible), and I adjusted the B-tension.

sydney

There are plenty that might not handle a 33 or bigger cog that was common on 6 speed freewheels.

sydney

What's the tooth count on the biggest and smallest rear cogs, and the tooth count on the big andsmall rings? A sora 8 speed will work with friction, but won't work if your big cog or wrap requirement exceeds it's capacity.sounds to me like you are currently exceeding the wrap capacity of your RD.

USAZorro

Actually, we ought to ask which cogs it isn't reaching. It could be size, spring tension, dishing, or a bad derailleur. I found a Nishiki 12 speed (Marina) a few weeks ago with the same derailleur. It was way too big for me to extensively test ride, but I noted the rear derailleur wasn't hitting all the gears. I passed it off to just needing adjustment, but maybe that's not it.

USAZorro

I had another though there. Where did you mount the derailleur? Is it behind the axle in the dropout, or below?

moxfyre

So, with the large chainring, it hits the 4 smallest gears, but the derailer gets sucked up against the cogs if I try to go to the two largest. With the small chainring, it hits the 4 largest gears, but can't wrap up enough for the two smallest :-( I'll count the teeth when I get home this evening!!

T-Mar

It is most likely a case of inadequate derailleur capacity, as Sidney suggests. However, it could also be insufficient spring tension for the cage pivot as USAZorro suggests. In the later scenario, I have seen cases where owners have shortened the chain trying to compensate for tension problems, resulting in a binding situation in the large/large combination and slack in the small/small combination. Ideally one should not be riding in these combinations, due to the severe chain angle, however the bicycle, as originally manufactured, would surely have handled these combinations without the chain tension problems you described. If not, there would have been huge potential for lawsuits.

FYI, the SunTour AR came in long and medium cage combinations. The medium cage (model RD-4200 - should be stamped on back) was rated for a 28T maximum cog and a 30T total capacitiy. The long cage RD-4400 was rated for a 30T maximum cog and 34T toal capacity. Once we know the derailleur model number and the sizes of the chainrings and cogs, we will have a much better idea of the problem and how to solve it.

John E

Yes, please do. If you have a short-cage derailleur, you cannot run more than 24, perhaps 26, teeth on your largest rear cog. The key is the pivot point of the cage. If the pivot point is lined up with the jockey (upper) pulley, you have a wide-range derailleur, which should be able to handle 13-32 or even 34 teeth, even with a 10-tooth drop in front. If the pivot falls between the two pulleys, you have a racing or narrow-range derailleur.

I have a tight-range SunTour Cyclone II on my PKN-10. It handles my 48-45-34 / 13-15-17-19-21-23 half-step-plus-Grannie combination nicely, but it's near its wrap limit.

sydney

No the question cannot be answered without knowing the chainring and cog sizes! Now if we knew the RD and chainset/freewheel were all original to the bike,we could THEN assume it was not a component compatibility issue.

sydney

that is just not always true either.

sydney

I think this depends on the make and vintage of derailer.It works for some old suntour stuff I have but doesn't apply to a newer shimano stuff. There is more to it than cage length too,as that is mostly about wrap. Parallelogram geometry is a serious part of ability to shift a big cog.

moxfyre

John E, Sydney, thanks for your help. It was in fact the medium cage model and I think I had set the spring tension too low when I cleaned the derailer. So to save time (I'm giving this to my gf tonight), I slapped an Altus long cage rder on there, out of the parts box. Doesn't quite match the other chrome components, but it shifts quite well...

Here's a photo of the finished Frankenbike, including Baby Blue paint job, which doesn't look half bad considering how I butchered it. I need to get matching handlebar tape eventually too. I think the Sugino cranks look real awesome, would love to find another set for my own use.

USAZorro

Very well done Moxfire. The Nishiki I rescued has the exact same derailleur, and a 13-27 six speed Suntour freewheel. If I had the larger one, I would have offered you a trade. If you're interested in the freewheel, let me know - I doubt I'll be using it.

moxfyre

Hey, thanks! My girlfriend is gonna be riding it and I don't think she cares too much about gear ratios