I have finally completed assembly of both bikes, so here they are. First, the 58 cm bike that I will be riding.
I will eventually put SPD pedals on this before doing any serious riding. The flat pedals are for purposes of riding around the neighborhood while I get the shifting and brakes adjusted. This bike has the narrower than expected Continental Contact tires that I wrote about previously. They will probably be a good tire for riding on roads, but that's not the primary purpose of this bike, so I will probably eventually get something wider. For those who are interested in such things, I weighed the bike and it came in a little under 22 pounds as pictured. The handlebars were wrapped with Bike Nashbar's handlebar tape. I used two rolls of it (white and "nuclear green") to get the striped pattern, spacing them fairly close together so that I used about 2/3 of each roll. That has the effect of making the padding about 50% thicker than normal. The green color turned out to be remarkably close to the green color we used in the frames.
Here is the 48 cm bike that Heather will be riding:
The two notable differences on this bike are the fatter tires and a big set back on the seatpost and saddle. I wanted to go with carbon seatposts because they are supposed to be more comfortable (I don't have personal experience--I'm still using an aluminum seatpost on my carbon road bike), and I knew this frame was going to require significant set back because of the steep seat tube angle. The geometry chart for this frame can be seen at Miracle Bikes web site. The 58 cm frame has a 73 degree seat angle (the same as my road bike), but the 48 cm frame has a much steeper 75.5 degree angle. That has the effect of moving the seat forward about 2.5 cm. So, for the 58 cm frame I bought a zero set back seat post and for the 48 cm frame I bought a post with about 2 cm of set back. I originally got a different saddle for this bike, but I was not able to get it back far enough, so I eventually swapped it out for this one that came stock on a Giant road bike and has longer rails. I understand these kinds of compromises are necessary on shorter frames, but in this case I don't think the seat tube angle needed to be this extreme.
This bike has a pair of 700x40C WTB Nano tires. They were tried with some success by someone on the forum discussing this frame at mtbr.com, so I thought I would get a pair and try them out on one of the bikes. So far I've only bought one pair, but if we like them after using them a bit, I'll probably get a pair for the other bike. Miracle Bikes specifies the maximum tire width as 38 mm, so this is technically exceeding that by 2 mm. The fork is clearly not the limiting factor here, as it appears to have more clearance than in the rear.
In the rear, things are a bit tight behind the bottom bracket, but I think it will work.
The seat stays also offer barely enough clearance, which I find strange because there is no particular reason not to make the clearance in this part of the frame more generous.
So, I think this frame will work with a 40 mm tire in dry conditions. Mud might be another story. The frustrating thing about the clearance in the rear is that there is plenty of clearance between the seat tube and rear tire, implying that the unusually steep seat angle on the 48 cm frame was not actually necessary. I think it could have been 1/2 to 1 degree less steep without making the gap between seat tube and rear tire the limiting factor in clearance, which would offer much more flexibility in choosing saddles and seatposts.
Based on my experience assembling the bikes, here are some things that I would change about the frame design:
- Reduce the seat tube angle by 1/2 to 1 degree in the smaller frame sizes.
- Make more tire clearance between seat stays. This is easy and doesn't compromise anything else in the frame.
- Make a little more clearance between chain stays. This may not actually be possible without some very creative engineering such as that being done by Open Cycle, but I'd love to put an even fatter tire on this bike if it were possible.
- Move the exit hole in the left chain stay for the brake cable forward a bit. Right now the cable has to make a bit of an awkward bend to get to the rear disc brake (Avid BB7 on these bikes).
- Taller head tubes. Nearly everyone on the mtbr forum thread discussing this frame that has posted pictures of their completed bikes has had 3-5 cm (or more) of spacers between the top of the headset and stem. I think this is a sign that the head tubes on this frame are probably shorter than they need to be. However, this frame is designed for cyclocross, and I don't really have experience with cyclocross frames, so it's possible that this is normal geometry for cyclocross racing, and that I and many others who are buying this for dirt road riding just have higher handlebar preferences than your average cyclocross racer.
Having said that, none of these are serious problems, and I fully anticipate that we'll have a lot of fun riding the bikes.