Saturday, February 27, 2021

A Discussion Concerning Types Of Geometry For 'Gravel' Bikes

The Raleigh Tamland series, circa 2014, set a standard for gravel geo.
 While writing up my ten year review of my Black Mountain Cycles "Monster Cross" frame and fork it occurred to me that a discussion of where we've been to where we are now in terms of geometry for 'gravel' bikes might be interesting. But first, I want to take a moment to say something I feel is very important here.

The term which ended up becoming the name of bikes that were not limited to riding on smooth pavement but were designed for any road ended up becoming something rather unfortunate. While I've mentioned this ad nauseam  here on my blog, I will risk saying it again- "Gravel Bikes" is absolutely wrong for this genre' of bicycle. It should never have been the term adopted for a bike that is possibly the most versatile style of bike made in the last half century. But there ya go. I cannot change that unfortunate term anymore than you can now. So, please understand that I do not endorse the term 'gravel bike', but for reasons of communicating simply and clearly, that is the term I will use. Okay, with that out of the way.......

As I stated in my BMC review linked above, in the late 00's and early years of the last decade, there were no 'gravel bikes' at all. What we were using were adopted bikes from other cycling genres. Bicycles meant for mountain biking, cyclo cross, randonnuering, and other types of bicycles were all used on gravel roads and still can be. This is very important to understand.  Any bicycle you can ride most likely could be ridden on a gravel road, but it may not work all that well, or be satisfying in the end. Nuff said there.....

The point I wanted to make is that early gravel events after 2005 were populated with a cornucopia of bicycle types, but mostly they shook out to be hard tail 29"ers and cyclo cross bikes. Elements of both were necessary to end up with the 'gravel bikes' we all enjoy today. The big tires and more relaxed geometry of 29"ers were matched up with the sensibilities of cyclo cross bikes, namely the drop bars and lighter weight frames, to come up with where we are today. Add a dash of early 20th Century road bike design and you are pretty much there. A bike that can go from pavement to single track with aplomb. Any road available becomes a road you could undertake a cycling adventure on.

Now to undertake the rest of the discussion I have a couple of visual aids to discuss. 

A diagram showing pertinent dimensions for a bicycle's geometry. (Image courtesy of Nicolai Bicycles)

The above diagram shows measurement points on a bicycle's frame which most bicycle nerds know as 'geometry'. Not the subject in school, but yeah- related. The angles and distances all matter. It's best not to focus on one thing, but to take things in a systemic way, because one change makes a difference everywhere else on a bicycle. 

That said, there are four main areas of concern when I look at a bicycle design. I take them as a whole, but each has a very dramatic effect upon handling, in my opinion. So, I list them off as follows and I will give the letter designation which you can cross reference to the diagram above for a visual aid. 

  • Head Tube Angle (E), Fork Offset (L), and Fork Axle To Crown (K)
  • Seat Tube Angle (F)
  • Bottom Bracket Drop (G)
  • Chain Stay Length - Actual (Hr) and Virtual (Hv)

Okay, breaking it down, the first bullet point can be called as a group, "Front End Geometry". This determines handling in corners, but for my money, this determines, not 100% but to a great degree, how my bike will handle rough, loose gravel and dirt roads. Next I listed the Seat Tube angle which is important as it relates to rider positioning in relationship to the crank set, but in conjunction with Bottom Bracket Drop and Chain Stay Length, it helps determine how stable or unstable a bike might feel like in loose gravel and dirt. Speaking of those things, the final two bullet points are very important. Bottom Bracket Drop also makes a bike feel more secure and stable, or it can be adjusted to make a bike feel more nimble and easier to loft over obstacles. Chain Stay Length helps determine down hill stability, climbing prowess, or it could help determine a bicycle's ability to loft the front wheel or clear luggage carriers like panniers, etc. 

So, as you can see, all are interrelated and changing one thing can affect another. Change the Axle to Crown length on a fork and that affects Bottom Bracket Drop, Seat Tube Angle, and Head Tube Angle. (Which is why you want to be very careful when choosing a fork for a bicycle.) That's just one example. But for this post, I am not going to get way into the weeds on geometry, but I am going to cut to the chase and talk about what does and does not work for me. 

A geometry chart for a Kinesis Tripster AT (Image courtesy of Kinesis)

Now keep in mind that a bicycle frame geometry changes with size, most often anyway, as you can see by checking the geometry chart above. So, when I say "what works for me" I am talking about a size 57cm-58cm frame, or in modern-day parlance, a "Size Large". If you are a smaller statured person, you should note that a different geometry will likely be something you'd prefer than what I do. That said, certain things can be said to be 'universal' in this discussion. 

What I look for immediately to know if I am interested in a design is Head Angle and Bottom Bracket drop. These are the two most frequently missed points on 'gravel bike' design, in my opinion. Get those 'wrong' and I will not look any deeper into your bike's design-  for me anyway. I like a Bottom Bracket Drop of at least 70mm and down as low as 75 (using tires from 40mm- 45mm) If your design incorporates a big tire, lets say a 50mm tire, then I can see going as low as 80mm, although then that pretty much excludes using certain sizes of tires and wheels. (I'll maybe talk about that in another post) 

Head Angle is the other thing. I maybe could go with a 72°, but only if everything else is compelling to me. I'd much rather see a 71.5°- 71° head angle and maybe even a 70° - 70.5° angle would be appealing if other front end measurements align with that to give me decent handling. So, yeah- I'm not saying I don't look at everything, but if you list a 68mm bottom bracket drop? Yeah, I'm turning the page. Got a 72.5° head angle? Same thing there. At least for me. Your mileage may vary. And on THAT point......

Here's the thing: I got what I wanted in most everything I could demand when Raleigh called me up and asked me "What would you do if you designed your own gravel bike?" My answer became pretty much what the Raleigh Tamland was in 2014. 71.5° head angle, 72.5mm bottom bracket drop, and everything else to go with that. One of the engineers on that Tamland project went on to form Noble Bikes, and the GX5, which I use as a test mule for, is basically a carbon Tamland. So, your 'new fangled design' has a pretty high bar to clear when it comes to what I like. And furthermore; a LOT of other people agreed with me. Tamlands were a huge hit when they came out, and that geometry, or geometry very close to that, is where a LOT of other companies landed when they came out with gravel bikes. So, it wasn't just what I ended up liking. Apparently it works for a lot of people. 

To my way of thinking, there are then three distinct types of geometry for bikes in this genre'. One being the "Cyclo Cross Derivative". These bikes typically have a higher bottom bracket- or 'less drop', than most. 70mm or less is typical. They also may have a steeper head angle, but not always. Shorter chain stays are also a hallmark of this sort of bike. 420mm is roundabout where you'll see these types of 'gravel bikes' landing at. Tire clearances will be limited many times to 45mm or less in width.

Then you have what I am calling "First Generation Geometry" which is what my Tamland has. Tire clearances up to 50mm, 430-435mm chain stays, 72°-73° seat tube angles along with what I mentioned above. 

Then you have what I am calling "MTB Influenced Geometry". This usually is typified by a slacker than 71.5° head angle, a deeper bottom bracket drop, a longer front/center, and short-ish chainstays. There are not many bikes like this, but probably the best example is the Evil Bikes "Chamois Hagar".

Again- this is far to brief a discussion to cover everything. There are people about to slam my comments with "Hey! What about "X" and "Y", or THIS BIKE, which has....." You get my drift. I get it. Exceptions. They exist all over. This post is a big generalization and overview. Not a specific "this is the way it is" discussion of 'gravel bike' geometry. It is also my opinion. Not the "Rule Of The Land", so take it for what it is. Just a friendly discussion about my views on geometry. (For a deeper dive into my thoughts about 'gravel' bikes design here is a post from 2013  This being pre-Tamland, but post my talking with Raleigh) Maybe you've got a favorite? Let me know. Got questions? Of course, hit me up in the comments. Want to know more about what I think about "X" or "Y"? Again, make a suggestion. I'll follow up with another post if warranted. 

And as always- Thank You for reading.


tntmoriv said...

Hi GT, thanks for the geometry lesson, it is nice to see all the thoughts and explanations in one place! I am wondering if you ever did a straight-up geometry comparison of your original Raleigh Tamland and the Gen 1 Salsa Fargo, both wit his their factory forks. You also mention tire sizes affecting geometry, so how close or different are those two bikes when running what you may consider their optimum tire sizes? Would that be 40 mm on the Tamland and maybe 50 or 55 on the Fargo? Thanks for a great rainy day weekend post!

Guitar Ted said...

@tntmoriv - Thank you for all the comments! I am glad you enjoyed the post today.

next- Thanks to S. Fuller, a frequent commenter here and fellow Gen I Fargo owner for leading me to the dead link on Gen I Fargo geo.

Comparing the Tamland to the Gen I Fargo, we see that it was equipped with "29 X 2.1" WTB Vulpine" tires, (Some of my favorite tires- sadly out of production) Anyway, that's what? 53+ mm? Okay, quite a bit bigger tire then. Looking at bottom bracket drop, I see the Gen I Fargo had (in Size Large) a 75mm drop, so not pushing any limits there. Head tube angle was 70° and Seat Tube Angle was 72°, ( a bit slack on seat tube for today's bikes, eh?) Chain stay length turned out to be 465mm (18.5") so quite long, but this was billed as an off-road tourer, so....

The really interesting bit is fork offset, which for the Size Large was 50mm. The Tamland's being the same there.

So, there just are far too many dissimilarities to make any kind of true comparison there. The thing that stands out to me is how very different the first Fargo was from all the following versions. You can see maybe why it stands out as the better Fargo for gravel/dirt/paved roads.

GravelDoc said...

Your recommendationo have worked well for me!

MuddyMatt said...

I find my aluminium Cannondale Topstone sits right in the pocket of what you say here, in fact I did pay close attention to what you have previously said regarding geometry when I bought the bike.

Reach is maybe a touch long for me at 394mm for a size large, given I am 'just' 5' 10-and-a-bit but a shorter stem would be my final tweak to fix that.

It's a comfy place to be with a 75mm BB drop and 55mm fork offset but needs bigger tyres for off road really. I was originally thinking about having a second 650b wheelset, but aside from money that 75mm BB drop is probably pushing things a bit.

It all gets a bit complicated, but my best advice is the numbers GT advocates help you sit 'in' the bike, rather than being perched on it. Which makes a big difference over time in varied conditions because the bike feels like its working with you and not against you.

Skidmark said...

Greets GT, what was it seemed to hold the Salsa Vaya back from stardom?

Scott said...

I didn't realize that about the origin of Noble bikes. That's a fun link!


Guitar Ted said...

@Skidmark - The Vaya was a missed opportunity and also a bike that should have been one of the better gravel bikes for the grassroots guy/gal ever. Steel, GREAT handling, and looked good too with a brand that had cache' within the gravel community early on. In my opinion, Salsa did three things wrong with the Vaya.

One: They doggedly stuck to their "touring bike-lite" marketing in the first several years of the model. They never embraced it as a gravel bike until it was too late.

Two: They waited far too long to equip it with a carbon fork and a 44mm head tube.

Three: They never transitioned the frame from being built for touring (heavier gauge tubing) and tried a lighter, livelier tube set for it, especially after introducing the Marrakesh. And they still don't have through axles on the rear.

I think having the Vaya developed at the same time as Fargo Gen I kind of made it a weird bike in the line to begin with. Then the Warbird came out and kind of filled that gravel bike space. The Vaya became the odd man out, in a way. Titanium Vayas don't seem to be regarded in the same way though. Interestingly, you almost never see a Ti Vaya come up used. Ever notice that? That should tell you something.

I owned a Vaya for a time and it was a stellar handling bike that could take on big tires. It was just built too stoutly/heavily and lacked many of the small details/features I would have wanted in a gravel bike. Had Salsa made the right moves when the bike dropped originally the Vaya would have been a gravel road cycling classic, and the Tamland would maybe never have existed. Oh well.....