laundromatt

Hi, I have a sekai bike with a somewhat unusual (at least to me) crankset. From reading here and elsewhere, it seems to have a detachable spider / chainring setup.

I would like to swap out the current setup with a single chainring. Can I use any crankset + chainring with this bottom bracket, or am I stuck with finding a spider/chainring that'll work with non-spidered crank arms?

Also, how do I take apart the current crankset? Do I need a crank puller tool?

I've attached some pics below. If I need to provide more info, let me know. Thanks!

tellyho

That seems like a funky combination of a swaged crankset (arms pressed onto spider, instead of one whole forged unit) with removable chainrings. The chainrings will come off when you undo the bolts.

HillRider

The large chainring is swaged permanently to the crank arm and can't be removed or replaced. The smaller ring can be unbolted and replaced.

Basically, this is a very low-line crank and the nutted bottom bracket further indicates this entire thing was pretty much bottom of the product line. If you want to use this cranks as a single chainring, you've got to use the existing big ring as is.

You could replace it with any JIS square taper crank if (and this is a big if) the current spindle is the correct length. Frankly, I'd get a complete new crank and matching bottom bracket.

laundromatt

Thanks for the replies. I was hoping to avoid it, but I think I may go with a new bb + crankset combo. Is it possible to get something like that for $100?

By the way, the current crankset is a Sugino Idol 170 mm.

Jeff Wills

Harris Cyclery sells several cranks for about $60, so a crank + BB should be within easy reach: https://www.sheldonbrown.com/harris/c...30-single.html

Some Origin8 cranks in the same color as the frame would be killer, though:
https://www.eighthinch.com/mm5/mercha...=cranksets_bbs

laundromatt

thanks for the link. i'll look into it. those red ones are striking, but i think i prefer silver...

so if i go with a 103 bb and a standard crankset, i'll be okay?

meb

I have 2 presently installed BBs like Laundymat has-one working fine and another needing replacement as it no longer holds the crank very long.

But I also have 2 unusally long ones I got because of the spindles-one is a 123 offset with a 128 drive side equivelant, the other a 134 length. Thought these might come in handy when I need extreme chainlines such as a mod I made replacing the 125 mm dropout on a French wheel with a 140 dropout from a tandem or whenever I build something with quadruple or quintuple cranks.
Are these crank retention problems typical of this nut type BB axle? Or is there another reason for the disparagement?

HillRider

"Crank retention problems" usually mean the crank arm itself is damaged. That is the square taper hole in it has been hogged out or distorted because the crank bolts (or nuts in this case) weren't torqued sufficiently tight and the arm moved on the spindle. The crank arms are aluminum and the spindle is hardened steel so guess which gets damaged? And, once this happens, the crank is not repairable by any reasonable method.

There is nothing inherently wrong with nutted bottom brackets although they tend to be cheaper, low-line ones. Installed properly and the nuts torqued sufficiently, they will hold the crank arms as reliably as any other type. That assumes the crank arm itself is in good condition.

operator

Yeah it's just more annoying to remove the nut than a standard 8mm hex wrench. Small profile of the nut doesn't make putting torque on it easy at all.

HillRider

I dunno. I was always able to use a 14 mm thin wall socket to remove/install the nuts. They worked just like the older bolted bottom brackets that used a 14 mm hex head bolt before the current 8mm allen hex design came along. No problem getting the proper installation torque with either type.

meb

That was my thought till I had the problem with multiple cranks. The BB clearly has a nut retention issue, lock washers didn't even solve it.

HillRider

Any chance you are trying to use an ISO bottom bracket with JIS cranks? The ISO crank could slide too far up an ISO taper and the end of square taper bottoms out the nut before the torque is adequate.

laundromatt

Hi, can I get an answer to my 103 mm / chainline question, notwithstanding the thread-hijacking?

I'm looking to just replace the existing bottom bracket and to go with a new crankset, but I'm just concerned the chainline won't match up to the rear. Almost everyone on the board seems to be after the 42 mm chainline. Will I get this with a 103 mm bottom bracket + any single chainring crank set-up?

Thanks!

HillRider

Probably not since cranks designs and alignments vary. You will have to match the bottom bracket length to the crank you actually buy.

meb

Well the installed version of that crank that was working fine just went into crank retention mode. Should I throw another left crank on there and risk destroying it if the BB taper is bad like the 3 I destroyed on the other bike or should I just change out both the BB & crank. FWIW, this is an old Raleigh Super Grand Prix 1977 I believe, Raleigh SR cranks. This is the only installed set of 165mm cranks I have, I may have another uninstalled set somewhere, but the 165mm cranks were to small for me anyway. This is an undersized frame for me by about 4 inches, and even with the 165mm cranks I have a front tire toe strike issue with my size 13 feet.

HillRider

If that bb has damaged 3 cranks, it's probable that the taper that was improperly made to begin with. As I said above, nutted bottom brackets tended to be cheapies so its precision may be suspect.

At this point, I'd certainly cut my losses and buy a complete new crank and matching bottom bracket set and be done with it.

meb

The question being asked relates to this weeks new BB failure on the one which has thus far only had 1 damaged crank Monday. The concern was given the bad history on the earlier similar problem with that style spindle if putting another crank on there would just be a repetition of the earlier history of destroying good cranks if it merritted a single replacement try.

meb

How would one distinguish between the two different square tapers?
Sheldon's site implies I might find JIS tapers would measure shorter but he left no quantitative data:
https://www.sheldonbrown.com/bbtaper.html

I've looked at the brand list on Sheldon's site, there is a strong chance both bikes had JIS cranks originally.

The bike that went through several cranks was a Raleigh Technium 440 from the late 80s.

The bike currently exhibiting it's first crank retention failure is a 77 Raleigh Super Grand Prix.

meb

I put another crank arm on and the nut worked its way off within 4 miles. So much for that spindle.

I guess I'll replace the cranks.