Ever modified a crank to add a granny?
#1
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Ever modified a crank to add a granny?
member merziac posted some pics of his merz mtb with a modified dura ace crank making it a triple. seems like a neat idea. certainly looks nice! but, it got me to thinkin'
i've got two early 80's SR super custom cranksets with a 118bcd and the smallest ring you can run is 36t. even with a larger cog of 34t, that's still not real low for big hills around here. so, since i have two, it occurred to me i could try modifying the spider of one to fit a 74bcd inner to make a triple
i have a drill press, tapping tools, good bits, a vice, metric measuring tools, ring spacers and bolts.....basically everything i would think i'd need to try it. thing is, i believe there's a probably a good efficient way to go about doing this since it's obviously been done. no need to reinvent, and i could hopefully avoid mistakes. for example, what's an effective way to secure the part and make sure the holes are drilled evenly/straight and properly centered? and, of course, if there's design considerations of the part that might lead to failure, i'll have to reconsider this attempt
as always, thanks!!!
ps. i know...it'd be much easier to use a same period triple crank to achieve desired gearing, and for now i'm doing that. but, it think it'd be swell to run the original SR. i've got several 118 rings for it, too
i've got two early 80's SR super custom cranksets with a 118bcd and the smallest ring you can run is 36t. even with a larger cog of 34t, that's still not real low for big hills around here. so, since i have two, it occurred to me i could try modifying the spider of one to fit a 74bcd inner to make a triple
i have a drill press, tapping tools, good bits, a vice, metric measuring tools, ring spacers and bolts.....basically everything i would think i'd need to try it. thing is, i believe there's a probably a good efficient way to go about doing this since it's obviously been done. no need to reinvent, and i could hopefully avoid mistakes. for example, what's an effective way to secure the part and make sure the holes are drilled evenly/straight and properly centered? and, of course, if there's design considerations of the part that might lead to failure, i'll have to reconsider this attempt
as always, thanks!!!
ps. i know...it'd be much easier to use a same period triple crank to achieve desired gearing, and for now i'm doing that. but, it think it'd be swell to run the original SR. i've got several 118 rings for it, too
#2
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I wouldn't "try" this out on a crank I wanted or valued. I'd do it (practice) on something else-
Then again- I wouldn't do this to a crank- first- I don't know what I'm doing. Second- I don't know how much tolerance the alloy has- even if all the drilling is perfect- I think the Super Custom lived at the lower end of the SR range and I don't know what that means as far as metallurgy and forging and quality. Third- I don't know if by drilling you're introducing cracks or stress risers that could fail catastrophically and suddenly.
SR made a Super Custom triple with an 86 BCD- I had one on my Trek 420. It's the same BCD as the Stronglight 99 crankset.
Then again- I wouldn't do this to a crank- first- I don't know what I'm doing. Second- I don't know how much tolerance the alloy has- even if all the drilling is perfect- I think the Super Custom lived at the lower end of the SR range and I don't know what that means as far as metallurgy and forging and quality. Third- I don't know if by drilling you're introducing cracks or stress risers that could fail catastrophically and suddenly.
SR made a Super Custom triple with an 86 BCD- I had one on my Trek 420. It's the same BCD as the Stronglight 99 crankset.
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You also need high angle drill bits. Aluminum is soft and will tear rather than chipping as you drill.
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member merziac posted some pics of his merz mtb with a modified dura ace crank making it a triple. seems like a neat idea. certainly looks nice! but, it got me to thinkin'
i've got two early 80's SR super custom cranksets with a 118bcd and the smallest ring you can run is 36t. even with a larger cog of 34t, that's still not real low for big hills around here. so, since i have two, it occurred to me i could try modifying the spider of one to fit a 74bcd inner to make a triple
i have a drill press, tapping tools, good bits, a vice, metric measuring tools, ring spacers and bolts.....basically everything i would think i'd need to try it. thing is, i believe there's a probably a good efficient way to go about doing this since it's obviously been done. no need to reinvent, and i could hopefully avoid mistakes. for example, what's an effective way to secure the part and make sure the holes are drilled evenly/straight and properly centered? and, of course, if there's design considerations of the part that might lead to failure, i'll have to reconsider this attempt
as always, thanks!!!
ps. i know...it'd be much easier to use a same period triple crank to achieve desired gearing, and for now i'm doing that. but, it think it'd be swell to run the original SR. i've got several 118 rings for it, too
i've got two early 80's SR super custom cranksets with a 118bcd and the smallest ring you can run is 36t. even with a larger cog of 34t, that's still not real low for big hills around here. so, since i have two, it occurred to me i could try modifying the spider of one to fit a 74bcd inner to make a triple
i have a drill press, tapping tools, good bits, a vice, metric measuring tools, ring spacers and bolts.....basically everything i would think i'd need to try it. thing is, i believe there's a probably a good efficient way to go about doing this since it's obviously been done. no need to reinvent, and i could hopefully avoid mistakes. for example, what's an effective way to secure the part and make sure the holes are drilled evenly/straight and properly centered? and, of course, if there's design considerations of the part that might lead to failure, i'll have to reconsider this attempt
as always, thanks!!!
ps. i know...it'd be much easier to use a same period triple crank to achieve desired gearing, and for now i'm doing that. but, it think it'd be swell to run the original SR. i've got several 118 rings for it, too
#6
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Triplizers are great, in lew of that id want an indexing head and a drill press minimum.
#8
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I wouldn't "try" this out on a crank I wanted or valued. I'd do it (practice) on something else-
Then again- I wouldn't do this to a crank- first- I don't know what I'm doing. Second- I don't know how much tolerance the alloy has- even if all the drilling is perfect- I think the Super Custom lived at the lower end of the SR range and I don't know what that means as far as metallurgy and forging and quality. Third- I don't know if by drilling you're introducing cracks or stress risers that could fail catastrophically and suddenly.
SR made a Super Custom triple with an 86 BCD- I had one on my Trek 420. It's the same BCD as the Stronglight 99 crankset.
Then again- I wouldn't do this to a crank- first- I don't know what I'm doing. Second- I don't know how much tolerance the alloy has- even if all the drilling is perfect- I think the Super Custom lived at the lower end of the SR range and I don't know what that means as far as metallurgy and forging and quality. Third- I don't know if by drilling you're introducing cracks or stress risers that could fail catastrophically and suddenly.
SR made a Super Custom triple with an 86 BCD- I had one on my Trek 420. It's the same BCD as the Stronglight 99 crankset.
lower end? i wouldn't say that at all! a lot of really high quality bikes came out with them. as well, it's a high quality alloy when you do some reading on them. super nice cranks, i think. polished up they resemble any of the "echelon" components of the day. it's just too bad they had this funky bcd...lol
i have considered finding an 86 bcd and it'd be convenient, but 28t is the smallest ring you can use. plus, i have this thing about trying to use what i have
#9
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i'm pondering making a (hand)drill guide using a sacrificial crank with existing 74bcd pattern. secure the two together at the spindle and pedal locations and drill through one into the other using the 74bcd holes to guide my bit. when drilling/tapping/installing inserts, the existing holes with buggered threads already act as guides in many cases. you just have to be patient and drill slow so you don't move off center. then you can rethread and/or install inserts, etc.
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Indeed. Only a knowledgeable machinist understands what is entailed to get a good result with high probability. Then there is the question of whether there is sufficient meat in the spider to allow the required material removal while preserving adequate strength.
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DO IT!!
Drilling and tapping an old aluminum crank is a piece of cake. If you screw it up or dont like it toss it in the junk drawer and try again.
You seem proficient on engine blocks and already have the tools so get out a 6inch scale, measure twice drill once and go for it. The issue would be frame clearance so a longer BB spindle would be definitely be needed.
Drilling and tapping an old aluminum crank is a piece of cake. If you screw it up or dont like it toss it in the junk drawer and try again.
You seem proficient on engine blocks and already have the tools so get out a 6inch scale, measure twice drill once and go for it. The issue would be frame clearance so a longer BB spindle would be definitely be needed.
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@jonwvara does this. Jon, want to talk about it here?
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You also need high angle drill bits. Aluminum is soft and will tear rather than chipping as you drill.
The derailleur will take up any out-of-round you introduce, and it's a granny gear, it's not like you're going to spend a ton of time in it. I would just go for it. I would rest the chainring bolt flats of the crank on some blocks to make sure it's parallel with the face of the crank. Depending on your drill press table, you may be able to stick an appropriate sized bolt up through the table and put that through the square tapered hole so that you can put all the holes the same distance from the center. Using the chainring as a template is also a good idea. I'd bolt it down to the first hole you drill/tap and then each consecutive hole. Do them in a cross pattern, probably. If the surface you're entering is at an angle, it may benefit you to buy a 1/4" or 5/16" flat endmill to throw in your drill press chuck and touch to the face you're drilling to make it flat so that the drill doesn't try to move as it's entering an angled face. Plunging an endmill should not be a problem for your drill press chuck, despite what others may tell you.
Oh, and you know about the trick to use a tap guide in the chuck of your drill press to align the tap and provide a little pressure, right? It requires your work to be held down to your drill press table, but it makes tapping a lot easier. Just don't move anything after you have drilled the hole, chuck the tap guide in your drill press, and pilot the tap with it.
Now that I have access to the tools, yeah, I'd program a mill to CNC drill it. Because I can. But if you have a good eye, you'll be surprised how close you can get with normal tools.
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#14
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and @52telecaster i've never used an indexing head in anything i've done, but i think needing one here might be unnecessary
i understand.... the crank and targeted spots for drilling would need to be flush to a horizontal surface (drill table) to be made plum to the vertical plane of the drill travel. i've come up with a couple of ways in my head to maybe be able to do that. it's a little involved and i'd have to experiment certainly with like a scrap disk of some sort to make sure i get distance and planar relationships accurate enough. tomorrow, when i have a little more time, i'll try to explain. not too complicated, but i haven't worked out all details, either. essentially, though, the crank would become it's own indexing head utilizing a bottom bracket installed into a shell cut from a scrap from and secured to some plate metal acting as a the table.
i understand.... the crank and targeted spots for drilling would need to be flush to a horizontal surface (drill table) to be made plum to the vertical plane of the drill travel. i've come up with a couple of ways in my head to maybe be able to do that. it's a little involved and i'd have to experiment certainly with like a scrap disk of some sort to make sure i get distance and planar relationships accurate enough. tomorrow, when i have a little more time, i'll try to explain. not too complicated, but i haven't worked out all details, either. essentially, though, the crank would become it's own indexing head utilizing a bottom bracket installed into a shell cut from a scrap from and secured to some plate metal acting as a the table.
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DO IT!!
Drilling and tapping an old aluminum crank is a piece of cake. If you screw it up or dont like it toss it in the junk drawer and try again.
You seem proficient on engine blocks and already have the tools so get out a 6inch scale, measure twice drill once and go for it. The issue would be frame clearance so a longer BB spindle would be definitely be needed.
Drilling and tapping an old aluminum crank is a piece of cake. If you screw it up or dont like it toss it in the junk drawer and try again.
You seem proficient on engine blocks and already have the tools so get out a 6inch scale, measure twice drill once and go for it. The issue would be frame clearance so a longer BB spindle would be definitely be needed.
oh, the longer spindle is already there on the bike. and, the crankset on it is already a triple at 52/47/36. take the other (identical) spider i have, modify, and swap. easy. not sure on how i'll gear....either half step with granny or just basic crossover. likely/ultimately a half step, though, if i can find a 46t ring with the 118bcd. there's one on ebay right now, but it's out of the uk and shipping is reflective. so, it may start it's new modified life with 47/36/24 rings
Last edited by thook; 08-24-19 at 11:26 PM.
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I've always machined aluminum with normal drill bits and tooling, whether it's castings, tubing, weird alloys, you name it. Nothing special in my machining experience, and I get beautiful chips. Use some cutting oil and change your feeds/speeds if you're shredding it. I like the way Tap Magic Aluminum works, but I hate the way it smells, especially when I smoke it up, parting things on the lathe on a taper. WD40 or 10W40 or even Pro Link wet chain lube or Fluid Film is also fine. Basically anything is better than nothing for most things. Then stainless and inconel often do better without cutting fluid as long as you don't let your tooling dwell and your workpiece harden, but I digress.
The derailleur will take up any out-of-round you introduce, and it's a granny gear, it's not like you're going to spend a ton of time in it. I would just go for it. I would rest the chainring bolt flats of the crank on some blocks to make sure it's parallel with the face of the crank. Depending on your drill press table, you may be able to stick an appropriate sized bolt up through the table and put that through the square tapered hole so that you can put all the holes the same distance from the center. Using the chainring as a template is also a good idea. I'd bolt it down to the first hole you drill/tap and then each consecutive hole. Do them in a cross pattern, probably. If the surface you're entering is at an angle, it may benefit you to buy a 1/4" or 5/16" flat endmill to throw in your drill press chuck and touch to the face you're drilling to make it flat so that the drill doesn't try to move as it's entering an angled face. Plunging an endmill should not be a problem for your drill press chuck, despite what others may tell you.
Oh, and you know about the trick to use a tap guide in the chuck of your drill press to align the tap and provide a little pressure, right? It requires your work to be held down to your drill press table, but it makes tapping a lot easier. Just don't move anything after you have drilled the hole, chuck the tap guide in your drill press, and pilot the tap with it.
Now that I have access to the tools, yeah, I'd program a mill to CNC drill it. Because I can. But if you have a good eye, you'll be surprised how close you can get with normal tools.
The derailleur will take up any out-of-round you introduce, and it's a granny gear, it's not like you're going to spend a ton of time in it. I would just go for it. I would rest the chainring bolt flats of the crank on some blocks to make sure it's parallel with the face of the crank. Depending on your drill press table, you may be able to stick an appropriate sized bolt up through the table and put that through the square tapered hole so that you can put all the holes the same distance from the center. Using the chainring as a template is also a good idea. I'd bolt it down to the first hole you drill/tap and then each consecutive hole. Do them in a cross pattern, probably. If the surface you're entering is at an angle, it may benefit you to buy a 1/4" or 5/16" flat endmill to throw in your drill press chuck and touch to the face you're drilling to make it flat so that the drill doesn't try to move as it's entering an angled face. Plunging an endmill should not be a problem for your drill press chuck, despite what others may tell you.
Oh, and you know about the trick to use a tap guide in the chuck of your drill press to align the tap and provide a little pressure, right? It requires your work to be held down to your drill press table, but it makes tapping a lot easier. Just don't move anything after you have drilled the hole, chuck the tap guide in your drill press, and pilot the tap with it.
Now that I have access to the tools, yeah, I'd program a mill to CNC drill it. Because I can. But if you have a good eye, you'll be surprised how close you can get with normal tools.
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+1
Remember Bio-Pace, anyone?
I’d be more concerned with getting the holes to line up with the holes in the chainring.
I’d definitely want to use a chainring as a guide/template, one way or another. If parts are difficult to clamp, I might do one first, get that screwed in. then make myself a center punch. Or a centering sleeve to a drill. Then do an opposite hole and get that screwed in. Then do the rest.
Remember Bio-Pace, anyone?
I’d be more concerned with getting the holes to line up with the holes in the chainring.
I’d definitely want to use a chainring as a guide/template, one way or another. If parts are difficult to clamp, I might do one first, get that screwed in. then make myself a center punch. Or a centering sleeve to a drill. Then do an opposite hole and get that screwed in. Then do the rest.
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Ever modified a crank to add a granny?
I haven't, but the previous owner of these two has:
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@jonwvara does this. Jon, want to talk about it here?
I didn't have a good photo of my triplized crank, but here's one I cropped out of a full bike shot.
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As others have noted above, a triplizer chainring is a quick, easy, and fully reversible means of adding a "granny" gear to a vintage crank. Red Clover makes a couple versions, the 144mm BCD ring @andy k shows above, and a 122mm BCD version for the Stronglight 93 crank:
#22
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Yes. Chainring. Blame mine and the translator. Sorry, i do not speak English. If you try, the appropriate screws and spacers for the third chainring, are these:
https://www.xxcycle.fr/visserie-pour...-mygal,,fr.php
https://www.xxcycle.fr/visserie-pour...-mygal,,fr.php
so, how'd you center the "plate" on the spider?
#23
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holy shorts!! i have a suntour road vx crank that's just like that. for years i've pondered making it a triple in that exact same manner! i am thrilled to see someone else has actually done it. now you know i'm gonna have to...lol! (at some point, anyway )
thanks so much for sharing!
thanks so much for sharing!
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As others have noted above, a triplizer chainring is a quick, easy, and fully reversible means of adding a "granny" gear to a vintage crank. Red Clover makes a couple versions, the 144mm BCD ring @andy k shows above, and a 122mm BCD version for the Stronglight 93 crank:
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"Progress might have been all right once, but it has gone on too long."
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www.redclovercomponents.com
"Progress might have been all right once, but it has gone on too long."
--Ogden Nash
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