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- Dec 25, 2017
- Messages
- 141
No argument from me. Two of our club riders have a gates belt timing chain. It is clearly a nice way to go. They love it.
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Disc brake modification on tandem bike
- Thread starter AllenV
- Start date
- Joined
- Dec 25, 2017
- Messages
- 141
@bulgie I am honored to have a builder of Santana credentials here in this corner of hobby machinist. Please post every step of your intended frame build.
Back in the late ‘70s or early ‘80s a member of my riding group in my home town of San Diego bought a new Santana. What a machine! Super nice. A few years later my new wife and I came down from Santa Barbara, where we were attending graduate school, to see my folks. We managed to borrow that Santana for a ride. Even though that was our first tandem experience I could feel the design quality in the ride and handling.
I think that sparked my father’s scheme to build a tandem for us. He was an aerospace engineer and I was in engineering graduate school. We began a collaboration in designing a bike that he would build and I would supply components and assemble. The design had to reflect a student’s low budget. That meant no drag brake and no high-end super wide tandem rear hub. Then, to keep the rear wheel as strong as practical we planned to build it without dish and made the rear of the frame asymmetric to center the rim in the frame. We discussed the chain line quite a bit in this context. Recalling how much I liked the feel of that Santana, and that our local bike shop had a Santana on the showroom floor, we went there and measured everything so as to copy that geometry with minor changes to adjust the size of the front and rear cockpits. Then Dad acquired a Columbus Tandem Tubeset and set to work. The result is pictured below (after a strip and repaint that I did about a dozen years ago.). It was a 3x5 gear set-up. Sugino crankset and early 40-hole Specialized hubs.
We enjoyed that bike in Santa Barbara. Then one day a fellow male grad student and I decided to take it out. We were hammering pretty hard when the rear end began to feel soft. An inspection stop revealed that the joint between the rear BB shell and seat tube had let go. The bike still rode okay so we went home. The frame went back to Dad’s home shop in San Diego where he repaired it and added a partial lug to the connection. It has been trouble free since then. Although my wife retired from riding shortly after that, I continued to recruit stokers for occasional rides. The Canondale came along after I found a regular stoker and we began targeting fast centuries. The Canondale’s longer rear cockpit and modern drivetrain and brakes made a lot of sense.
After that frame, Dad built a few Aluminum frames. Then he went on to build a couple of Magnesium frames. He rode one of the Magnesium frames for the next couple of decades. I have a couple of his Magnesium tubing sets here, though I doubt I’ll ever use them.
Back in the late ‘70s or early ‘80s a member of my riding group in my home town of San Diego bought a new Santana. What a machine! Super nice. A few years later my new wife and I came down from Santa Barbara, where we were attending graduate school, to see my folks. We managed to borrow that Santana for a ride. Even though that was our first tandem experience I could feel the design quality in the ride and handling.
I think that sparked my father’s scheme to build a tandem for us. He was an aerospace engineer and I was in engineering graduate school. We began a collaboration in designing a bike that he would build and I would supply components and assemble. The design had to reflect a student’s low budget. That meant no drag brake and no high-end super wide tandem rear hub. Then, to keep the rear wheel as strong as practical we planned to build it without dish and made the rear of the frame asymmetric to center the rim in the frame. We discussed the chain line quite a bit in this context. Recalling how much I liked the feel of that Santana, and that our local bike shop had a Santana on the showroom floor, we went there and measured everything so as to copy that geometry with minor changes to adjust the size of the front and rear cockpits. Then Dad acquired a Columbus Tandem Tubeset and set to work. The result is pictured below (after a strip and repaint that I did about a dozen years ago.). It was a 3x5 gear set-up. Sugino crankset and early 40-hole Specialized hubs.
We enjoyed that bike in Santa Barbara. Then one day a fellow male grad student and I decided to take it out. We were hammering pretty hard when the rear end began to feel soft. An inspection stop revealed that the joint between the rear BB shell and seat tube had let go. The bike still rode okay so we went home. The frame went back to Dad’s home shop in San Diego where he repaired it and added a partial lug to the connection. It has been trouble free since then. Although my wife retired from riding shortly after that, I continued to recruit stokers for occasional rides. The Canondale came along after I found a regular stoker and we began targeting fast centuries. The Canondale’s longer rear cockpit and modern drivetrain and brakes made a lot of sense.
After that frame, Dad built a few Aluminum frames. Then he went on to build a couple of Magnesium frames. He rode one of the Magnesium frames for the next couple of decades. I have a couple of his Magnesium tubing sets here, though I doubt I’ll ever use them.
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Yikes, magbesium? That's pretty exotic, what's the joining method? Welding I assume, but what process, and what extra steps/precautions does mag require?
Dad's tandem looks amazing. Of course a modern frame now would have much larger diameter tubing, but I trust you found the rigidity adequate. My main concern is the 1" steerer — not even single bikes use such a small steerer anymore. I have broken two steerers (on singles) while riding, not fun. Do you know, did Columbus supply the same steerer they sold for single bike forks, or does this one have additional butting (thickening) at the bottom? The bottom is not only where forces are highest, it's also got weakening from the HAZ. If you're not sure, measuring the inside diameter at the bottom usually isn't too difficult, though Columbus complicates it slightly with their goofy raised helical ridges inside.
The helical ridges are an unfortunate feature, because they prevent you from strengthening the steerer by pressing in a doubler at the bottom. If you have a single-bike steerer and it has the ridges, then I'd recommend retiring the fork, replacing it with one with a real tandem steerer.
Back when tandems had 1" steerers, there were extra-thick butted steerers available from Reynolds, Ishiwata, maybe others. Santana took one of those and added a pressed-in doubler, so their steerers are reliable as far as I know. Never saw or heard of one broken. Adding a doubler is recommended for any 1" tandem steerer as long as it has a smooth bore inside.
"We rode it a lot and it didn't break" is not good enough — you know about fatigue, right? It's very difficult to inspect for incipient cracks there, so steerers usually snap with little to no warning. For this reason I strongly suggest steerers should be considerably stronger than the fork blades. If something's going to fail I'd prefer it to be the blades.
Here's an example of a tandem where they used a single-bike steerer with tandem blades:
BTW they were Just Riding Along (called "JRA" in the bike biz) when this happened. Claims of JRA are commonly made up to get warranty coverage after crashing into a parked car or driving it into the garage with the bike on the roof of the car, but in this case there were credible witnesses corroborating the story. He was applying the brakes when he hit a pot-hole.
See how the brake cable is out in front of the head tube? So the fork flexing back lengthens that path, which increases the application of the brake, which causes more fork flexing, which causes more increase in the braking, a runaway cascade. The brake lever snapped shut, nobody's hand can hold it against those forces, but by then the cascade was irreversible. It probably took a few milliseconds at most.
All single-bike tubing on this tandem, nothing "oversized", which I consider to be just short of criminal negligence. The maker, Bob Jackson, is out of business now, good riddance. The Hetchins on the downtube is just a resurrection of a long-defunct name with a storied history. Bob Jackson probably bought the IP, or the owner of the IP contracted BJ to make these.
-Mark
Dad's tandem looks amazing. Of course a modern frame now would have much larger diameter tubing, but I trust you found the rigidity adequate. My main concern is the 1" steerer — not even single bikes use such a small steerer anymore. I have broken two steerers (on singles) while riding, not fun. Do you know, did Columbus supply the same steerer they sold for single bike forks, or does this one have additional butting (thickening) at the bottom? The bottom is not only where forces are highest, it's also got weakening from the HAZ. If you're not sure, measuring the inside diameter at the bottom usually isn't too difficult, though Columbus complicates it slightly with their goofy raised helical ridges inside.
The helical ridges are an unfortunate feature, because they prevent you from strengthening the steerer by pressing in a doubler at the bottom. If you have a single-bike steerer and it has the ridges, then I'd recommend retiring the fork, replacing it with one with a real tandem steerer.
Back when tandems had 1" steerers, there were extra-thick butted steerers available from Reynolds, Ishiwata, maybe others. Santana took one of those and added a pressed-in doubler, so their steerers are reliable as far as I know. Never saw or heard of one broken. Adding a doubler is recommended for any 1" tandem steerer as long as it has a smooth bore inside.
"We rode it a lot and it didn't break" is not good enough — you know about fatigue, right? It's very difficult to inspect for incipient cracks there, so steerers usually snap with little to no warning. For this reason I strongly suggest steerers should be considerably stronger than the fork blades. If something's going to fail I'd prefer it to be the blades.
Here's an example of a tandem where they used a single-bike steerer with tandem blades:
BTW they were Just Riding Along (called "JRA" in the bike biz) when this happened. Claims of JRA are commonly made up to get warranty coverage after crashing into a parked car or driving it into the garage with the bike on the roof of the car, but in this case there were credible witnesses corroborating the story. He was applying the brakes when he hit a pot-hole.
See how the brake cable is out in front of the head tube? So the fork flexing back lengthens that path, which increases the application of the brake, which causes more fork flexing, which causes more increase in the braking, a runaway cascade. The brake lever snapped shut, nobody's hand can hold it against those forces, but by then the cascade was irreversible. It probably took a few milliseconds at most.
All single-bike tubing on this tandem, nothing "oversized", which I consider to be just short of criminal negligence. The maker, Bob Jackson, is out of business now, good riddance. The Hetchins on the downtube is just a resurrection of a long-defunct name with a storied history. Bob Jackson probably bought the IP, or the owner of the IP contracted BJ to make these.
-Mark
- Joined
- Dec 25, 2017
- Messages
- 141
@bulgie
On Magnesium (Mg):
Back at that time Mg was getting some attention by high-end builders. The strength-to-weight ratio is very attractive. I found one manufacturer (Vaast) using it today. My father managed to find a few tube sets. Perhaps as many as 5 sets total. He built two frames and gave me two left-over sets some years later. He likely had a fifth set consumed for various prototype joints and weld setups. Joinery was, indeed, TIG. In the ‘60s he, for a while, was lead engineer for welding at that aerospace company. This involved development of production processes for aircraft and defense systems subassemblies. The man was a font of welding knowledge.
The TIG welds were done with a Miller Dynasty 200DX. This is a TIG machine with variable frequency AC and Pulse capability. I don’t know the details of how he set up for Mg. Very likely he used pulsed mode to balance the melt penetration with oxide removal and get good control of the bead quality. Shield gas was likely Argon, but it may have been an Argon/Helium mix. Back-purge was used.
The great thing about that Dynasty (and I am sure other high-end) welder is that it can give nice control of the arc and weld bead all the way down to 1A. It is really nice for thin-wall tubing. Then, in pulsed mode, the duration and current of the tops and bottoms of the waveform can be set independently to get just the right oxide removal.
If any of you are considering using and welding Mg, it has flammability and corrosion properties that you should be aware of. This is a decent introduction.
https://yeswelder.com/blogs/yeswelder/how-to-weld-magnesium
This review of the Vaast bike has a short mention of modern Mg alloys for bikes and comment about a plasma electrolytic deposition process used to protect the frame from corrosion, inside and out. My father used a zinc-chromate primer on the outside and left the inside bare. Thus now, 30 years later, the inside might warrant some inspection for corrosion.
https://velo.outsideonline.com/road...ravel-bike-review-magnesium-makes-a-comeback/
On Steering tubes:
Thank you for your concern in bringing this to my attention. As far as I can recall the tubing set and this fork steerer and blades are tandem-proper. But, there is no paperwork on what was used and the builder is no longer with us. When I did the strip and respray, around 2008, I did not spot any cracks or unusual condition with the fork. Yes, I can measure the wall thickness. The current status is that that bike is retired from service and hanging upside down from a 12 foot ceiling, wrapped in old bed sheets. If I ever pull it down from there, I’ll have a look.
On Magnesium (Mg):
Back at that time Mg was getting some attention by high-end builders. The strength-to-weight ratio is very attractive. I found one manufacturer (Vaast) using it today. My father managed to find a few tube sets. Perhaps as many as 5 sets total. He built two frames and gave me two left-over sets some years later. He likely had a fifth set consumed for various prototype joints and weld setups. Joinery was, indeed, TIG. In the ‘60s he, for a while, was lead engineer for welding at that aerospace company. This involved development of production processes for aircraft and defense systems subassemblies. The man was a font of welding knowledge.
The TIG welds were done with a Miller Dynasty 200DX. This is a TIG machine with variable frequency AC and Pulse capability. I don’t know the details of how he set up for Mg. Very likely he used pulsed mode to balance the melt penetration with oxide removal and get good control of the bead quality. Shield gas was likely Argon, but it may have been an Argon/Helium mix. Back-purge was used.
The great thing about that Dynasty (and I am sure other high-end) welder is that it can give nice control of the arc and weld bead all the way down to 1A. It is really nice for thin-wall tubing. Then, in pulsed mode, the duration and current of the tops and bottoms of the waveform can be set independently to get just the right oxide removal.
If any of you are considering using and welding Mg, it has flammability and corrosion properties that you should be aware of. This is a decent introduction.
https://yeswelder.com/blogs/yeswelder/how-to-weld-magnesium
This review of the Vaast bike has a short mention of modern Mg alloys for bikes and comment about a plasma electrolytic deposition process used to protect the frame from corrosion, inside and out. My father used a zinc-chromate primer on the outside and left the inside bare. Thus now, 30 years later, the inside might warrant some inspection for corrosion.
https://velo.outsideonline.com/road...ravel-bike-review-magnesium-makes-a-comeback/
On Steering tubes:
Thank you for your concern in bringing this to my attention. As far as I can recall the tubing set and this fork steerer and blades are tandem-proper. But, there is no paperwork on what was used and the builder is no longer with us. When I did the strip and respray, around 2008, I did not spot any cracks or unusual condition with the fork. Yes, I can measure the wall thickness. The current status is that that bike is retired from service and hanging upside down from a 12 foot ceiling, wrapped in old bed sheets. If I ever pull it down from there, I’ll have a look.