I am considering both of these tires. I want to go wider than 38mm if possible, and I wonder if you could tell me what you think of my clearance measurements. I know you have a lot of experience with this... is my bike too tight for a Hertre? This is a late 70s(?) Colner road racing frame, measured at 320mm from axle centers:

Fork: 46mm, Seatstays: ~65mm, Chainstays: 44mm if axle is set back in drop outs. Maybe 45mm if all the way back.

I've posted a few photos of it elsewhere, but here it is again, my 650b Trek 820:

http://bikeforums.net/attachment.php...hmentid=294485

Older MTBs (especially from the 80s, before the frame geometry became more aggressive, more like enduro motorcycles) are pretty good frames to convert since they have very long wheelbases (+ 100cm) whose proportions & slack geometry will overcome the routine 45mm BB drop to maintain a stable ride and handling even with the larger wheel size. I've tried a 650b conversion on a shorter-framed MTB (97cm) and it was proportionally too high-centered for city riding; the handling became tricky, awkward in the turns.

Bear in mind that most 700c road frames have deeper BB drops and work better with 650b wheels; even with a compact frame, the ride is stable due to the bike's overall lower center of gravity.

My current winter/ summer project is an (apparently) native 650b hybrid frame, a Panasonic from the early '80s- 26" frame w/ 60mm BB drop & 107cm wheelbase; 366mm fork w/ low-mounted canti bosses: smells like 650b to me.

I'm north of that building my 650b Terraferma. But how is this (or the $300 to build 650b wheels) different than building a high-end conventional road bike on a custom steel frame with custom spoked wheels? The rim costs are not different. Synergy rims cost about the same (in context of a $4k bike) in 700c and in 650b.

If you're going to built with Hetres and fenders, you need a frame that is wide enough between the chainstays near the chainstay bridge to clear the tire and the fender. We can argue about the right number and the right fender, but I think you need at least 6 mm on each side of the tire (42+6+6=54), and a total clearance between that and 60 mm to handle fenders not larger than 60 mm Grand Bois. Similar width is needed near the top of the fork blades.

That requirement alone makes a 42 mm fendered bike hard to make without a specialized frame. Vintage Treks don't have nearly that much width in either position. Between the chainstays both my 1984 610 and my 1983 620 measure about 47 mm. So there's an available 2.5 mm clearance for the inflated tire alone. Whether or not its adequate depends on correct rear-triangle alignment, wheel trueness, wheel dish, accurate tire mounting, and accurate wheel positioning based on the dropout adjusting screws. Break a spoke and you might not be riding home. Needless to say, adding fenders in this case is a non-starter.

I'm interested in converting one of these to 650b, but I think the opportunity is to go for 650x32b with fenders, on a vintage trek. But this only speaks to lateral wheel and fender clearance, not to trail adjustment or brake choice. I'd probably go for Mafac CP pivots front and rear and install Mafac Racer brake arms. Racers work on 27" tires that are 32 mm (1 1/4") wide with fenders, so they should work on 650x32c.

Why would I do this, why not just use 700x32c with Honjos? Well, I've already tried it with 45 mm plastic (Zefal) fenders, and that bike just does not have enough radial clearance for that tire/fender combination. It does work with 28s, which I use now.

So for a vintage Trek, it's an expensive way to go to 32 mm tire width with good fenders, and more if you're going to adjust the trail to either make it like Trek had it for 27's or 700c's, or you're going to adjust it for a front load. But these fork issues are significant no matter what frame you are going to do it to.

So I'd say if you really have to have fenders and 32 mm cushiness, one of these Treks is a good choice. I don't think it can get you "home" if you want 42 mm with fenders.

I'm assuming no frame work (replacing/recontouring chain stays) beyond adding brake pivots and possibly rear cable housing hangers to convert to centerpulls or cantilever brakes. If you're going to remake major sections of a bike frame, anything is possible, but it just gets more expensive.

Whatwolf, I think you would be best off targeting a 650x32c tire for your conversion, based on your 45 mm max internal chainstay clearance and your fork clearance. You are exactly on-target performing that measurement at 320mm from the planned axle center, at least for a Hetre. It might be a little shorter (315?) for a 32 mm tire. The seatstays are wide enough for a Hetre, FWIW.

On second thought, I guess 38s would not be bad, but the margin is getting lower.

That was the color chosen by McLaren Cars during the Can-Am days, just so everyone would see them coming up from behind and move over.

As to a conversion, J.P. Weigle has stated he likes the Raleigh Competition (before the G.S.) as a suitable conversion frame, he does not like the International for some reason, but the geometry between the two is not very different. I never read a follow up to as to the why, there have been a number of public inquiries of late that never get answers.

Isn't Weigle moving the brake bridge? I have a '72 Competition, and could not get enough reach with any brake available....

Weigle installs cantilever posts on frames originally equipped with sidepulls. The Competitions have a higher BB than the Internationals as I understand it. And not every Competition is a good candidate for the conversions he does, according to him. As we all know, Raleigh set the bar for inconsistency.

I have recently discovered that 1982-85 Lotus' (Loti?) make excellent conversion cannidates.