mstateglfr 

I got a 90 Diamond Back Ascent EX frameset for cheap and wanted to build it up with Deore Deerhead components I had.
I started a thread a couple months ago about this bike- the crankset I wanted to use wouldn't clear the chainstay. That was a 48-38-28.

I have a 42-32-22 crankset I can use which will clear the chainstays.
But the front derailleur won't fit the arc of then large ring now since the rings are so small.
And the front derailleur will run into the chainstay if I lower it more on the seat tube. As is, it's way too high above the top of the largest ring.


So-
- would a modern front derailleur fit the smaller crank better? any suggestions on what to get that has a compact curve?
- any workarounds anyone can think of? As is. The derailleur is resting on the top of the lowest water cage hole...not sure if I could place a derailleur that fits the curve below that brazeon point and still have the derailleur cage not contact the chainstay.



I thought it'd be $150 and id have a rigid mtb in my size for once and with a cool old group on it.
I didn't figure this many headaches!

dsbrantjr

It'a not so much a modern derailleur you need but one which is shaped to fit the diameter of the rings. Microshift makes a few you could try as does Shimano. Examples: microSHIFT -The best control system
Amazon.com : Shimano Altus FD-M190 Front Derailleur Max 42 Cap 18 Clamp Dia 28.6-34.9 Dual, Low Clamp : Front Bike Derailleurs : Sports & Outdoors this one has a low clamp which should clear your bottle boss
or search on "42 t triple front derailleur shimano" or similar

mstateglfr 

So I can use a low clamp in place of a high clamp? Everything I've ever done to front derailleurs has been while using the original or replacing it with an identical style.

If these can swap(hi or low clamp) without issue, that could solve the issue.
Thanks

Bill Kapaun

The correct thing to do would have been to use a longer BB.
Changing the crank & FDER costs how much vs a new BB? That's a totally backwards approach.

mstateglfr 

I have a 127.5mm spindle in the frame right now, it wasn't long enough to keep the first crankset from striking the chainstay.
I agree, a longer bb would have been easiest. I would have loved to just swap it for a longer one, but 130 is it.


As for cost, the current small crank was $15 and a new FD would be $12 probably. Not a huge hit, though it means I am not getting to use all the original components I had hoped for.

Bill Kapaun

Is this a loose bearing/cup BB?
If so, is the spindle in correctly?
Many have a shorter end intended for the NDS.
What's the "code" on the spindle, such as 3T, 3S etc.

mstateglfr 

It's a loose cup n cone.
I pulled the spindle, it's installed to correct way. Appreciate the suggestion, itd be great if it were just an issue of the spindle being on wrong.
Here is a pic of the spindle code.

Bill Kapaun

Not sure if the 7 is the code, but from Sheldon-
7" series spindles fit "oversized" 73 mm ISO/British bottom brackets.
I don't know if you have a 73mm BB shell, but I thought they were all cartridge???

mstateglfr 

Yeah, I've read that too- the Sheldon comment.

Its a 73mm shell. Also, this thread from years ago references the same model bike and spindle. Looks like it was what DB spec'd the bike with. https://www.bikeforums.net/bicycle-me...m-bracket.html

And they are for sale on eBay
Vintage Bottom Bracket Spindle MTBS108- 73mm x 127mm -NOS CrMo JIS Square Taper

FastJake

Pretty much any MTB FD from the 1990s should work with that crank. Low clamp/high clamp shouldn't matter on that frame. What you need to make sure of is top pull versus bottom pull. Some FD's from that era are both. And the clamp diameter (28.6, 31.8, or 34.9) but if it's too big it can always be shimmed.

The local bike co-op around here has boxes of them. I have quite a few myself so if you need something send me a PM.

I'm not sure why your other crank didn't work (was it original???) but that's a different issue.

JohnDThompson 

I'm not sure that would help much. From the OP's pictures, it appears he cannot lower the derailleur any closer to the rings without it hitting the chainstay.

The Deore XT derailleur was designed to use larger rings than the OP wants. I think the best solution is a modern derailleur that would both follow the profile of the rings better and allow the cage to be lowered closer to the rings for better performance.

mstateglfr 

Yeah, this frustrating frame is a mystery. Whats brutal is that the 1990 catalog isnt available online. the '89 and '91 catalogs are available, but not the '90. If I could see what crank was originally used, I would buy it right away and be done with this.
The '89 and '91 cranks are both 48-38-28, so its safe to say the '90 was also that setup. The larger crank I have tried is a Shimano FC-B124 with biopace rings. Its from 1987, I believe. I have tried other 48-38-28 cranks though, 2 others, actually- a Sakae XCT, and another one that was at my local collective. Both had the same issue.

Total mystery.




I will look for a 90s or later FD which is made for a smaller crankset and is both top and bottom pull compatible. Hopefully that will allow the FD to be mounted without interfering with the cage brazeons and also avoid contacting the chainstay.

Thanks for the suggestion.

mstateglfr 

This is for sure what I will do. Thanks for the confirmation on it being a good option to try next.