Wednesday, January 22, 2020

Concept Bike Revisited; Part 2

Last week I re-introduced my decade old idea and quest for the "perfect all roads" bike. (click here to see that post) Remembering that in 2010 there were no gravel bikes being marketed as such, (hard to believe now, innit?) we now have to adjust our viewpoint due to the current situation. So, let's see how far off I was from things which I spec'ed in 2010. This will be focused primarily on geometry.

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.
I need to address the fourth point because I wasn't very specific. I can say that we were thinking around 430mm there. So that's "shorter than a touring bike's chain stays" and definitely not as short as a cyclo cross bike's. Also, as a note on the tire clearance, that assumes plenty of mud clearance as well. In dry conditions, this ideal frame could run 45mm tires too, but my ideal, all-around tire width was thought to be a 42mm tire at that time. Keep in mind that there were no gravel specific tires at this point in history either. Yes- A lot has changed in a decade!

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
So, if we consider the geometry here, things are not far off from my list of desires. Here is the geometry of the Noble Bikes GX5 from their site. This is for the 58cm size, by the way.
  • 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*
Okay, so I'll contrast and compare starting from the top of the list. So, first is the head tube angle which is dead on what I was wanting here. The fork offset actually is a tick longer, which leads to a slightly quicker handling bike. That's maybe counter-intuitive to you, but that's how offset works given other parameters stay the same. Longer offset = quicker handling. Shorter is the reverse.

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.
These tires allowed for a wider, more voluminous tire to be fitted to a bike that might otherwise not be able to handle a 47mm-50mm tire in 700c. The slightly smaller diameter meant that your bottom bracket would end up being closer to the ground though. This could be a problem. It all depends on your bike and its bottom bracket drop.

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.
 *Well, Noble listed the GX5 as being good for only a 700c X 40mm tire. That said, this is based upon a very conservative industry standard. Noble cannot recommend anything larger per this requirement, but that doesn't mean bigger tires won't fit. 700 X 42's will work fine, and you can even shoehorn in a 650B X 47mm wheel and tire. There is not a ton of clearance with 650B X 47 on this bike though, so no wetter or muddy gravel or dirt.

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.......


Skidmark said...

A deep bottom bracket drop makes the actual chainstay length look longer. If horizontal length is measured it gives a more consistent comparison. Noble GX5 size 56 is 433.6mm.

Guitar Ted said...

@Skidmark - I understand that you are using a "virtual chain stay length". I was not going by "looks". I was using the figure Noble published, which is what people go by in terms of chain stay length.

Skidmark said...

My bad, by “looks” I meant the deeper the bottom bracket drop the more it adds to that (diagonal) measurement.