bark_eater 

Scratchyscratch grade school math puts me at 25.8mm so 4 mm to spare..

Now if grade school math can predict the bottom bracket leangth...

DiegoFrogs

This may be a bit off-topic (700C!), but based on recent posts I think it'll be well tolerated.

I'm a bit curious about the impact on ride quality of the bend radius of the fork blades...

Aesthetically, I think I prefer a relatively long radius bend where the fork ends are inserted directly into the radiused length or only after an almost imperceivable tangent length (any surveyors/transportation engineers here? I tried that gig for a while...). My own '77 Centurion Pro-Tour isn't a bad example, but some of the high end touring and sports touring bikes from Lotus from the ~1980 era really sort of grab my attention...

The ride quality of that bike is really good - I like it a lot. But my '67 Carlton with Reynolds 531 blades is different. The blades neck down really quick from oval with what seems like a lower aspect ratio to round with a smaller diameter. The radius of the curve is rather short, and then there's ~1.5" of tangent to the insertion point of the dropouts.

Honestly, I don't know if it's the cross sectional differences in the blades, or the geometry of how they were bent, but the Carlton has some magic that I can't bottle up. If all else were equal, and one was only working with bending a prismatic tube (one that wasn't changing in its cross section along it's length), would one even notice the difference between bending radii and the second tangent length or absence thereof, assuming the relative position of the hub center and fork crown were the same?

southpawboston

There was a big discussion about this about eight years ago on the Rawland and 650B lists, after the Rawland Stag was supposed to be spec'd with thin-tubed, high-rake forks with a nice decreasing radius bend, aka French curve. The bikes ended up shipping with the really nicely radiused fork blades, but they were stout, and the number one complaint about the bike was the hard-riding fork, despite excellent frame dynamics otherwise. A group of us started a group buy on Jeff Lyon replacement forks that kept the same rake with nice decreasing radius, but with skinny Kasei tubes. I have that fork, and it transformed the ride. So I would say it has to do more with the diameter and thickness of the fork legs than the tangent at which the fork blades meet the dropout. I have some higher trail bikes with very little fork rake but skinny Reynolds forks that also ride really supple.

gugie 

I picked up this dingus awhile back for a song. Just like the McGuffin in the title, it had been painted over a few times, which disguised the full 531 frameset. Details from the eBay ad made me think it was an early 70's Raleigh Gran(d) Sport(s). A clue:


Black, white, red then finally some lagoon blue

Headbadge was intact, so it's definitely a Raleigh. Serial number stamped on non drive side, stamped dropout. Yeah, Gran(d) Sport(s).

Local Portlander wanted a 650b townie bike. We talked, this fit him, it was on.

Here's the list of mods:

1. New vertical dropouts and seat stays
2. Spread to 130mm
3. Cantilever bosses
4. Wiring guides for generator hub, internal downtube to non drive side wiring - tail light to be installed on non drive side seat stay near dropout
5. Fender bridges/fender hard mount points
6. Bottle bosses, cable guides
9. Slightly modified Haulin Collin rack, fitted to bike.
10, Oh, let's powder coat it candy apple red

Pre-powder coat:


All dressed up:

lonesomesteve

You may want to check your serial number to confirm the year and model because I don't think there was such thing as a 400t Elance in 1982, but I do know that the two 614s, two 710s and a 311 all from 1982 that I've owned all handled 650b x 38mm with 170mm cranks quite well. The touring and sport touring models in 1982 all had pretty similar geometry, though the racing models had slightly lower BBs.

nlerner

That’s a sweet little firecracker.

amedias

All else being equal (KEY phrase there) then where the bend is and it's radius can make a very small difference in the ride and flex characteristics of the fork.

BUT....all other things rarely are equal, any subtle impact from the geometry of the bend is normally dwarfed by differences in tubing diameter, wall thickness, butting profiles, and...the thing everyone seems to forget...the crown and steerer. Crowns can make a big difference to how a fork flexes, and also there's a fair degree of flex in the steerer. All those other factors come into play as well, individually and as a 'system', when one part flexes it only flexes so far before the next bit starts to as well, and playing around with the specs can induce (or tame) movement in other areas. Thin diameter blades will flex differently to large diameter blades, same with wall thickness, same again with where the taper from large to small begins. Degree of ovalisation can also play it's part too. Also, don't forget a simple thing like the length of the fork! A long fork built for 27in wheels has longer blades than a short one built for 700x23c and short brakes, that length affects the leverage and flex of the fork too, same with steerer length.

It's also worth remembering that whole thing works as a system of springs too, the type and size of impact force can mean that one or other part of the system is making the largest contribution to feel. Skinny ending blades may help with small vibrations, but it's how it responds to bigger bumps is likely to be determined by bits 'further up' the fork, and the crown. Stuff in the middle, ie: medium sized bumps is the hardest to pin down, some forks seem to send it all through to the rider, others seem to filter it all out, and it's all down to how the fork (and frame) responds as a whole, and the interactions between those different bits and spring rates, rather than the individual bits.
As an example a very sturdy steerer/crown and upper legs with tiny flexy tips may end up less comfortable overall than a less sturdy upper fork with slighty beefier tips, it's all to do with tuning the fork to the expected input forces.
Also, Put a hard narrow tyred wheel into a fork and it will transmit different vibrations than if you use a big fat squishy tyre, this can make a fork/bike feel different. With the big tyre it might only really flex the fork on big bumps as most of the chatter will be taken out by the tyre, so you might feel the 'thumps' more as they become noticeable. With the skinny hard tyre the fork may be doing more of the work on small and medium bumps so you notice the chatter.

Comparing two different forks from different builders there is likely to be more difference in all the 'other' bits making the difference than the geometry of the bend.

bark_eater 

That was a typo. The bike is a 1986 400t and has the same 7.2 cm bottom bracket drop. My math shows a loss of 6mm going from from 700x25 to 650bx38. Probably confirmation bias driven, but if that 6mm was added to the BB drop on a frame running 700x25 the drop would be 7.8 cm, still higher than the 8.0 cm Richard Sachs uses on his frames. The Soma Champs Elyse fork is a wild card, as I haven't been able to compare the exact leangth of the original fork and what difference the different headset will make. MKII eyeball sez I will gain 2-3mm of height forward.

PS. Chiming in on fork flex characteristics, I am a little disappointed in how heavy the Soma fork is. I looks like a mountain bike fork with out canti studs. I don't have the specs in front of me, but there is gobs of room between the blades, probably more than could be used by any crown mounted brake. Incongruous to the beefiness of the fork, there are no mid fork low rider mounts. Neither point is a big deal, but dose point to bean counter engineering.

noglider 

@bark_eater, I heard a (very plausible) rumor that because of liability concerns, steel forks are much heavier than they used to be. This could explain some of the magic that @DiegoFrogs mentions. I notice it, too. I compared weights of forks from my 1971 Raleigh International and a Surly Cross Check. The Raleigh fork was barely more than HALF the weight of the Surly fork. It might be very difficult to find a recently-made steel fork that is so light.

I imagine that fork design and construction is very complex, perhaps more so than of frames. It must be a bit of a black art.

amedias

Most current stock/off-the-peg steel forks are un-necessarily heavy* and over-engineered for obvious reasons, but custom built stuff can be built as light** as it ever was.
Some off-the-peg current forks can be decently light too though, a lot of it comes down to intended use and you need to build in a lot more leeway for any fork intended for bigger tyres or load carrying etc as it may be used in extremis, where as proper 'road' fork has a narrow design intent and can be built accordingly.

Also, the advent of the 1 1/8th steerer has a lot to answer for :-(

The current SOMA offerings are definitely at the burlier end, I've got a couple of the Champs Elysees in both 1 inch and 1 1/8th versions here in 'the pile' and they're obviously built conservatively and are going to be stiffer and heavier than custom (or a lot of classic) forks.

* A modern 1 1/8th 'road' fork being ~1-1.3kg uncut, and in the 800g-1kg area when cut for your average bike
** Vs ~600-700g for a quality lightweight steel 1inch steerer road fork.

(Obviously there will be outliers at both ends in the super-light and super heavy-areas below and above the weights mentioned above)

AJI125 

I have the same Soma Champs-Elysees fork waiting to go on my 650b'd LeMond after the current bike project wraps up. I agree with your observations of weight and stoutness (don't have ride observations yet). I'm coming from a Fuji HiTen fork I swapped on there (1.2 wall I believe). So should be interesting.

VintageRide

A number of changes to the 1981 Fuji S12S - replaced the Sugino XD 600 crankset with a more appropriately vintage Sugino Super Maxy 34/48T double. Apparently these TA Pro Vis clones are not all that common. I also went from Campagnolo 10 speed Ergo shifters to Suntour Power Shifters and along with those decided to go to an 8 speed 11 - 34T cassette from 9 speed, eliminating the ShiftMate pulley arrangement. I decided that an earlier Shimano rear derailleur was more appropriate than the M960 I was using, looks much nicer and shifting is quite good, the Suntour ratchet shifter design makes it almost a breeze.

At some point I will also install a pair of Tektro R340 brake levers, in silver with tan hoods of course.

The bike has been a great all around rider and a pleasure to ride for the last 5 years.

DQRider 

That is a very classy steed indeed!

.

southpawboston

I had read somewhere long ago that early carbon fork crash failures prompted development of a new testing standard, at least in the US, that made it difficult for pre-existing steel forks to pass. So to meet the standard manufacturers started making them more stout. Not sure if that was just internet rumor or not, but it's my understanding of why today's steel forks are so rigid.

Out of curiousity, I measured the diameter of some of the fork blades in my stable, close to the dropout (rounding to the nearest 0.5mm), the type of bend close to the dropout, and how stiff the fork feels. All the forks have 1" steerers. The caveat is that I do not know the blade thicknesses.

• Mercian Olympique, Reynolds blades - 11.5mm, high trail, large radius bend, fork gives a compliant ride
• Jack Taylor Tour of Britain, Reynolds blades - 12.5mm, medium trail, small radius bend ("French curve"), fork gives a compliant ride
• Jeff Lyon custom, Kaisei blades - 13mm, low trail, small radius bend ("French curve"), fork gives a compliant ride
• Jeunet 630, Reynolds blades - 13.5mm, high trail, large radius bend, fork gives a reasonable ride
• Shogun 1500, Tange blades, 14mm, high trail, large radius bend, fork is fairly stiff
• Rawland Stag, unknown blades, 16mm, low trail, small radius bend, fork was stiff as hell! (No longer have it, it was so stiff I replaced it! But I jotted down that measurement when I had it!)

So among the six forks the only correlation with ride quality is blade diameter. Three forks with blades < 13.5mm ride great. The Jeunet with 13.5mm diameter blades is somewhere between compliant and stiff, and the Shogun with 14mm blades is fairly stiff. My Rawland with stock 16mm fork blades was so stiff I ended up replacing it with a Jeff Lyon custom fork that replicated the fork on my Jeff Lyon custom. That bike now has a much more supple ride. There does not appear to be any correlation with trail or the radius of the bend close to the dropout.

gugie 

That makes sense to me. My most comfortable bike is my Weigle-ized Raleigh Competition with 531 blades, 12mm diameter near the dropouts. I've taken it on a few week long credit card tours, I can ride all day on it and not feel beat up. I found a set of 531 blades on eBay last year and snapped them up for a future project. Rene Here specs their Kaisei fork blades as 13mm OD, Columbus Chromor "Rando" blades are nominally 12.5mm.

52telecaster

I built 2 sets of 650b zac19s. Pretty nice wheels really.

DiegoFrogs

Thanks for all of that!
My Centurion is ~ 14.0 mm in the plane of the bike near the dropout, 13.8 mm laterally... so it's tapering the full length of the blades. All the radii, tapers and transitions are gentle, which is visually appealing. Elegant, and a bike built to carry a load.
The Carlton is ~10.9 mm round near the dropout, and appears to have necked down to that diameter long up the blades. That bike also has rapid taper R531 chainstays without the normal crimps and kinks one employs otherwise. That was normally what I mentally considered as the magic potion, but I may have been wrong...

Thanks to everyone for contributing good stuff!

AJI125 

To continue the fun. The Soma Champs-Elysees Road fork (newer version at least) is about 14.5mm near the dropout. A generic ~90s Tange CrMo replacement fork (Chrome with forged DOs) is more oval-ish, 14.2x15.1mm. A genuine 1982 Trek 613 Magny-X “death fork” is about 13.8mm. Fuji Hi-Ten about 14.5mm. Bianchi SLX fork 13-13.5mm.

The larger diameter should be stiffer, right? Just like OS tubing allows for thinner wall? Not counting all the other factors (thickness, curvature, etc)?

gugie 

Absolutely correct. Type of steel has nothing to do with stiffness. Tubing diameter is the dominating factor in stiffness.

gugie 

If you're interested in getting into low trail/650b on a budget, a vintage Fuji S12-S is as good as it gets. 650x42 tires fit with fenders, brake bridges with centerpull brakes are in just the right place, the frames are good quality, and they're reasonably readily available without breaking the bank.

greg3rd48 

Just a gorgeous build.

rhender 

Do you know if the Fuji S12-S LTD would be more or less the same? There are a few for sale near me, and I'm considering pursuing a project like this one.

gugie 

As far as 650b convertability is concerned, I would say yes. @bear_a_bug did one awhile ago that turned out very nice. Not sure what the frame differences are outside of the LTD version had chrome socks front and rear, at least for the years I've seen.

VintageRide

The only difference is basically that the three main tubes are double butted cro moly on the LTD. versus straight gauge for the S12S with the fork and stays on both being hi tensile. I can say that the standard S12S is a very nice ride, predictable and quite stable. I am using Grand Bois 42mm tires so they do fit with fenders which is not something very common among road bikes of this vintage. They are basically sport touring with 44 cm chain stays and yet the bike handles surprisingly well and while not a racing bike I find it agile and easy to steer. If you get the fit right it just disappears and you simply get out and ride, enjoying the scenery. Even when loaded with a few pounds in the front bag it does not feel heavy when moving along, the steering will get less responsive with enough weight but the bike remains stable. I have had over 10 pounds and it was not bad at all.



It is true as gugie suggested, not likely to find any other vintage non 650b bike that takes so well to being converted. I did have the rear stays cold set to accept a modern 9 speed 135mm hub and that was it. If sticking with say 7 speed the 126mm stock spacing should suffice. Having one modified to accept canti brakes as well as rack mounts would make it all the better. I still might have it done and get a repaint in the same factory color one day just to get away from the clamps on the fork, makes it much tidier. All one really needs are the Dia Compe 750 center pull brakes.



The only real "problem " is finding a good example in one's size, mine is a 56 cm. I check from time to time but they tend to be few and far between, so if anyone has a chance to buy one locally do not wait too long. The models from 1979 - 1982 will have the same geometry with a nice low trail, 1981 - 82 have the braze on cable guides on the top tube. It is possible that earlier S10S from the mid '70's and on are the same but I believe they have an all hi tensile frame. No doubt they ride well, just heavier which might translate to a less agile feel and handling. I really like the thin seat stays on the S12S as well.



I would suggest if mounting fenders to go with stainless steel, a bit heavier but not prone to cracking as the aluminum ones are. I have a pair from Velo Orange on mine and due to simply drilling a small hole and mounting it with an L shaped bracket at the rear brake bridge it eventually broke in two at that point after a few years. Mind you I made the mistake of not using a leather washer to isolate it. I rode the bike on quite a few gravel trails so there was most likely more vibrational stress introduced, the more damaging high frequency kind. Using a bracket that crimps around at both sides around the fender edges would be a better choice there, though less clearance at the tire side wall. Use larger washers as well at the back stays and front chain stay bridge.

rhender 

Thanks for the reply gugie Very helpful

VintageRide Do you mind if I ask how tall you are/what your inseam is? I have a bead on a 56cm, but I'm typically closer to a 58cm. Of course a test ride would sort it out, but it's a bit of a drive and I'm hesitant these days...