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Thursday, December 15, 2022

Thinking about Frame Geometry

 I spend too much time thinking about bike geometry. Not just that, specifically, the geometry of a single "road" bike. I put "road bike" in quotes because unlike many others, I ride my road bikes every where --- road, off road, loaded, unloaded, and I'm not opposed to carrying my bike if I have to and walking. I suppose I could use Jan Heine's "All Road" terminology. In any case, what I'm coming to realize is that the geometry of your bike probably needs to be customized to a degree most bike builders don't consider.

When I first got a bike that fit me in 1992, Terry Shaw put me on a 55cm Bianchi Eros. I used that bike for commuting, touring, and running errands. It was a nice, neutral handling bike that was perfect for beginners. Once I tried a Bridgestone RB-1, however, all the other bikes just feel wrong. I loved the responsiveness of the geometry (73.5 headtube with a 45 off offset for a 54mm trail with a 25mm tire), and the wide tire clearance meant it was truly versatile. Sure it wasn't designed for carrying a heavy load, but what I've since realized is that touring with as light a load as possible is far more fun anyway. One of the interesting things about trail nobody thinks about is that when you increase the tire size, the trail increases, which is exactly what you want whenever you need those wide tires --- slower steering.

What's funny about bikes is how fashion driven it is. I regularly rode 700x25 tires, and from 1995 to about 2008, I'd get people commenting about how I was being unnecessarily conservative in choosing "wide" tires. Everyone else was riding 700x23 (I did for a while, since the Michelin Hi-lite comps were $12 each, which was a bargain). Nowadays, you see people riding 700x42mm tires, and when touring in Europe, I'd get people commenting that I couldn't possibly be riding a particular bike path since my tires weren't wide enough.

I recently purchased a Rivendell Roadini to serve as a backup bike, and it's easy to see how different it is. It's got a significantly longer trail than my custom touring bike, but still manages to feel responsive. The BB, however, is higher (13mm higher), and I definitely feel that when descending. This comes from 2 places: first, the wider tires, and secondly, Grant Petersen designs his bike for people riding platform pedals, which don't have as much ground clearance as clipless pedals. There'll be lots of people who write about how BB height doesn't matter, but then I read Dave Moulton on the bike he designed for himself and how low BBs were in the early days of his riding:

While out training after dark, and coasting down hill; we would sometimes lower our heel so the steel tip made contact with the road, sending out a shower of sparks. A pretty spectacular visual effect, especially if several riders did it together.

Note that these were racing bikes, people who would certainly pedal around corners. Notice how by contrast, modern "gravel bikes" have gone the other way, with much higher BB heights (and reduced BB drops). Now some of those bikes are designed for 650B wheels, which are smaller and hence need to have higher BBs to clear obstacles, but the Specialized Diverge, for instance, has an 80mm BB drop and can run both 650B and 700c wheels. (Note that people do complain about the low BB) Another interesting titbit from Moulton's blog is that he off-handedly mentions that he managed to convince Reynolds to let him brass braze 753 frames (which are supposed to be silver brazed for lower temperature) because of his developed brass brazing technique.

Again, a lot of this is dependent on the kind of riding you do. If you frequently have to climb obstacles and jump them, I think a higher BB makes sense. But to be honest how many people do you see do that? I'm a decent bike handler, and many times ride trails that rarely see gravel cyclists. An occasional hit to the pedals doesn't do any damage (I've done it a lot on my single and my tandem --- just keep pedaling --- it's not the end of the world!), and while I've broken frames, none of those broken frames have ever been attributable to off-pavement riding.

To my mind, you have to design a bike's geometry for the size of tires you plan to run. If an 80mm drop is useful for 25mm tires, then raising the size of the tire to 40mm, you'd need the drop to be increased by about 15mm, so a 95mm drop. Now you might argue that a manufacturer can't stop someone from putting 25mm tires on a bike designed for 40mm tires, so maybe you can't go that low, but I'd argue that an 85mm drop wouldn't be extreme given that fewer riders are now using anything less than 28mm tires. Again, this doesn't apply to cyclists riding 650B wheels --- you'd have to do different computation.

Note that the lower the BB, the longer your chainstays will be for a given wheelbase. That also affects handling --- Dave Moulton will say that you'd need to stiffen chainstays so they don't flex as much. In any case, there's not much experimentation with frame geometry nowadays --- some of that is attributed to the same factory churning out frames for many different manufacturers. The other is that for better or worse, many people just get used to the bikes they buy off the shelf and don't think hard about how they could be improved.

I guess one of the big reasons why Grant Petersen has been so successful is that he's one of the few who's actively experimenting with frame geometry over the years, something that almost no one else is doing. His bicycles don't ride like anyone else's as a result, and even his imitators like Soma frequently miss the point of his bikes.


Sojka's Call said...

My main focus has been stack height at the stem/handlebars. I found that riding for a couple decades in the aero position had contributed to chronic back pain and uncomfortable hands and shoulders. The Rivendell Bleriot I bought for touring was the first bike with the handlebars deliberately set higher. The 3-4 inch higher handlebar-over-seat easily achieved with a quill stem and steerer tube left long, was a revelation.

When I purchased a custom steel IF Club Racer a few years later I specified comfort as a primary consideration. The bike shop used a Retul bike fit system and the IF ended up with the handlebars about 1" higher than the seat. This bike has served me well but tire size is limited to 32mm max.

Now, I am looking for a bike that can take wider tires both for comfort and to better handle dirt roads. While the Bleriot can do that with the 44mm tires, it is a heavy bike in the 25+ lb range. I would really like something sub 20 lbs to make it easier for me on climbs. When you start looking at anything with a carbon fork which is just about all bikes these days, you find that the max steerer tube anyone will allow is 35mm under the handlebars and 5mm above. This is a safety standard that has evolved that I didn't know about. It makes sense, but really limits bike setup.

Now, you have given me something else to consider. Yikes! I need to check the BB frame spec on my IF since that bike handles very well.

Piaw Na said...

I think you're going to end up with a custom. To be honest I don't think I need anything more than 32mm tires.