Adjusting a frameset to 650B Geometry Question
#1
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Adjusting a frameset to 650B Geometry Question
I have an old 531 tubing kit and the main triangle is already together. I hope to fit 42mm tires on 650B rims and wanted to know if I should trim the length of the fork blades (pre-raked) to keep the geometry correct, i.e. didn't want the head tube lifted up too much, or the chain stays to be too long. This is going to be an all around touring bike. With 42mm tires the total diameter may be not far from 700C wheels with narrow tires, so possibly no trimming would be necessary?
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Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Mongoose Tomac ATB, GT Bravado ATB, JCP Folder, Stewart 650B ATB
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Can we make assumptions?
Like what tire diameter tires (rim diameter and tire width) the frame (and hence the main triangle) was designed for? How about steering geometry or stay lengths? Can you indent stays, have you done this before? Have you drawn out any aspects of the original or modded designs?
By shifting BB drop fork height issues can be dealt with to a degree without seat or head angles shifting too much. But the devil is in the planning. I'd design a new frame and see how the old main triangle overlaps with the new. I don'[t think we can offer specific suggestions without a lot more info. Andy
Like what tire diameter tires (rim diameter and tire width) the frame (and hence the main triangle) was designed for? How about steering geometry or stay lengths? Can you indent stays, have you done this before? Have you drawn out any aspects of the original or modded designs?
By shifting BB drop fork height issues can be dealt with to a degree without seat or head angles shifting too much. But the devil is in the planning. I'd design a new frame and see how the old main triangle overlaps with the new. I don'[t think we can offer specific suggestions without a lot more info. Andy
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AndrewRStewart
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Using the chain stays that are with the kit? You might have to modify/indent them to fit 42s. Same with the fork, check the crown to see what the max tire size may be, maybe not 42s.
regards, Brian
regards, Brian
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Brian
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Theoretically, the outer diameter of a 700 x 23 tire is exactly the same as a 650b x 42 tire at 668mm.
In any event, if you are using the same size tires front and rear, none of the angles will change. If the tire diameter is different from the original, bottom bracket drop and trail will be the only things that change. Tire clearance is a different matter and 42's may not be possible with a road frame and fork.
In any event, if you are using the same size tires front and rear, none of the angles will change. If the tire diameter is different from the original, bottom bracket drop and trail will be the only things that change. Tire clearance is a different matter and 42's may not be possible with a road frame and fork.
#5
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Theoretically, the outer diameter of a 700 x 23 tire is exactly the same as a 650b x 42 tire at 668mm.
In any event, if you are using the same size tires front and rear, none of the angles will change. If the tire diameter is different from the original, bottom bracket drop and trail will be the only things that change. Tire clearance is a different matter and 42's may not be possible with a road frame and fork.
In any event, if you are using the same size tires front and rear, none of the angles will change. If the tire diameter is different from the original, bottom bracket drop and trail will be the only things that change. Tire clearance is a different matter and 42's may not be possible with a road frame and fork.
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you can fit wider tires if you make the stays longer than you might otherwise make them. Many people say that longer stays is imperceptible to them, and they certainly are for me.
Also, you can braze the rear dropouts on the stays and then check the fit before you braze anything else on the rear triangle. It's a lot better if you have a fork for that, but you don't need a fork for rear stay clearance checks
Also, you can braze the rear dropouts on the stays and then check the fit before you braze anything else on the rear triangle. It's a lot better if you have a fork for that, but you don't need a fork for rear stay clearance checks
#7
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you can fit wider tires if you make the stays longer than you might otherwise make them. Many people say that longer stays is imperceptible to them, and they certainly are for me.
Also, you can braze the rear dropouts on the stays and then check the fit before you braze anything else on the rear triangle. It's a lot better if you have a fork for that, but you don't need a fork for rear stay clearance checks
Also, you can braze the rear dropouts on the stays and then check the fit before you braze anything else on the rear triangle. It's a lot better if you have a fork for that, but you don't need a fork for rear stay clearance checks
Thanks Unterhausen,
I left the stays a bit longer and now I am pretty confident there will be no problem with 38mm tires, not positive about 42mm but I can be happy with 38s. The method I used for shaping the chain stays did not work as well as I would have liked. I was shooting for maintaining a convex surface on both the inside and outside and getting a nice regular "D" shaped cross section. I shaped a block of oak and placed the stay in that "U" shaped groove and used a piece of softer wood on the inside and squeezed it in a vice. An arbor press might have been better. Basically it wound up squeezing in on both sides despite my groove on the outside face. Consquently, it looks a little flattened on the outside face, and I can chalk that one up to a "first frame" lesson. I thought I could get the clearance without creating a concave crease on the inside of the stays, but maybe that is just not feasible.
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I have used a hammer. Don't go crazy with it.
#9
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Clearance looks decent for 42 mm tires without fenders...or mud. I need to shorten the fork blades about 30 mm. I would like to get a nice square cut and I'm a little unsure if I can do it with the tools at hand. I have an ordinary homeowner hacksaw, of course, and a dremel with a cutoff wheel. These are 531 fork blades with the "Imperial" shaped of oval which is flatter than most of what we see now the "Continental". Any suggestions on how to get a clean straight cut on those blades would be greatly appreciated.
Gyro Gearloose
Gyro Gearloose
#10
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Can be done with a hacksaw, but need to have good sharp blade and good control over the saw. Use two hands on the saw one on the front and other on the handle. Slow cutting motion, keeping eye on straightness of cut. Cut several mm longer than desired. Have a line marked all the way around the leg and judge cut by the line. If going off line, turn blade 90 degrees and start again. Eventually you will get through the leg reasonably well and take a file to make it straight. I have done all my tubes this way with good results.
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a steerer cutting guide will work
#12
framebuilder
You can use masking tape as a cutting guide. Use a small adjustable machinist square to set the top of the tape distance from the top of the blade. Obviously you move the square with the ruler set to your preferred distance to different spots to make sure the tape is equidistant all the way around. Next cut close but not exactly on the tape line. If you start to go crooked, you turn the blade over and finish the cut from the other side. Now file right to the tape line. This method insures the shortened blade remains square at the top.
#13
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Thanks to TiHabañero, Unterhausen, and Doug for your valuable suggestions. I have what I need to proceed. This may be the longest frame building project ever. Looking forward to starting another frame soon.
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how long has it taken you? If you bought the tubes retail back in the '70s, then you win. But my travel bike that I just finished was started in 2010. And I'm not quite done with a frame I started in 2008-ish. It's quite possible Andy has me beat though.
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I have had frames (that were completed) take a few years before. My 3-speed started as a hub scavenged from Geo Rennies Bicycle Shop in about 1980, the bike was on the road by 2006. Then there's the projects not even started but have been moved with me many times. Andy
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AndrewRStewart
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I bought two Reynolds 531 tube sets of the in either 1974 or 75 while working at Bumsteads Sporting Good in Ontario, California. One set was built up by Jack Davis as my road race bike, which I equipped with Campy Nuovo Record and I still own. I finished out some very rough lugs (Prugnant knock-offs) back about that time and then did not touch it again until the mid nineties. I had problems with keeping the brass flowing and did not have a much information to go on at that time. Since then, this forum, Youtube videos and practice, I managed to finally finish the frame with a new set of cast lugs. I found a new fork crown to match the "Continental" 531 oval fork blades at Rene Herse Cycles. Technically, I won't be done until I finish the fork and paint it the whole bike. That may not happen until later this year. I am shooting for my second frame to be done by next Spring!
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