A quick refresher for you non-clicker types that may not have seen, or remembered, last week's post then before we continue. This also will serve as a good quick reference anyway.
- Head angle 71.5° (I said 72° might be okay)
- 73° seat tube angle.
- Bottom bracket drop in the 75-80mm range.
- Moderate length chain stays.
- Room for 42mm tires.
Now let's take a close look at a "modern" gravel bike. My Noble Bikes GX5 will be our subject here. It has a carbon fiber frame, internal cable routing, disc brakes in the flat mount style, and basically could be thought of as a prototypical gravel bike circa 2020.
|The current state of the Noble Bikes GX5|
- Head Tube Angle: 71.5°
- Fork Offset: 52mm
- Seat Tube Angle: 72.5°
- Bottom Bracket Drop: 72.5mm
- Chain Stay Length: 440mm
- Tire Clearance: 40mm recommended*
The seat tube angle is a half a degree slacker on the GX5. Note that the rest of the size range has 73° seat tube angles. Pretty close here. But now we come to bottom bracket drop and something which we could not consider in 2010 affects this a bit. That being wide 650B tires for gravel travel.
|Fatter 650B tires allow for a different ride characteristic and performance.|
So, as an example, I had a Twin Six Standard Rando for a while. This bike has a 75mm bottom bracket drop. Right in the range I wanted for this "concept bike". The Standard Rando worked well with 650B tires and wheels, but I would clip a pedal now and again running the 47mm width tires. How close was it? With 172.5mm crank arms I could dip my heel in my size 46 shoes and drag my heel off the tops of the rocks on the road. Any lower in bottom bracket height and I'd be in trouble with 650B X 47mm tires.
So, I've modified my bottom bracket height requirements for a bike that would run dual wheel sizes. Now if you weren't ever going to run 650B wheels and tires, then dip that bottom bracket lower. It'll be fine. Same thing with bigger tires. If you'd never run anything smaller than a 700c X 45mm, then the bottom bracket could be made to be lower as well.
Note also that Noble Bikes GX5's in smaller than 58cm sizes are running 75mm bottom bracket drop. So, this bike would be a candidate for 650B wheels, but........ What about tire clearances?
|The Noble Bikes GX5 with 650B wheels and tires mounted.|
Finally, the chain stays on the GX5 are 440mm long, and 10mm longer than I figured necessary. Okay, I will admit that the GX5 is a really stable, really smooth riding bike, and no doubt, that extra chain stay length probably lends a bit to the tire clearances, but it is longer. Could this be done in a 430mm length? Probably, but perhaps at the expense of comfort. Maybe a compromise of 435mm? Maybe. I think a steel bike could be 430mm chain stay length, so I will allow that the carbon construction here perhaps is the main limitation. To get a shorter stay, you'd probably see that dropped drive side chain stay, like so many other carbon bikes have.
Now with that done, the next post will cover frame materials and things like water bottle bosses, fender mounts, and smaller details. Then I'll get on to my final vision for the concept bike. Stay tuned.......