Road vs Gravel Bike Geometry

Equipment
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Trail 101 info below.

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I bookmarked this for deeper dive later, but some quick thoughts. From my experience and the pending search on several gravel bikes, I have found the dimensions that matter most to me with respect to handling are as follows, grouped for common relationships:

  1. Wheelbase, Front Center, Rear Center (aka Chainstay Length)
  2. Head Tube Angle, Trail
  3. BB Drop

The above assumes I can hit my desired fit window within the other important dimensions like Frame Reach & Stack, as well as the combo of Stem, Handlebar & Hoods that all impact final rider position. Mix all that with the basic recognition that the Bottom Bracket (BB) is the origin for most of the dimensions that related to Fit AND Handling.

There are many other dimensions that may be worth a further look, especially if you are splitting hairs between seemingly similar options, but I think most of the big ones above have the greatest impact on handling.

SUPER broadly speaking (with all the unstated caveats that some values can be contradicted by alterations in other design elements):

  1. Bikes with shorter values in #1 will have the ability to change direction more readily and with less rider input & force. This attribute can be considered as good or bad depending on a riders preferences and goals. What someone might call “twitchy, nervous, scary” could easily be called “fast, agile, fun” by another rider. Roughly speaking, the longer you space the wheels, the more the bike will want to remain straight, and resist turning. This attribute comes from two main impacts, IMO.

    • First is the pure distance between the contract patches of the tire (Wheelbase).

    • Second relates to my new windmill to tilt against… Weight Distribution. There is a long history to review here with respect to MTB’s, but consider that the Rear Center (Chainstay) has remained relatively similar over time, the majority of change seen in MTB evolution in the last 15-ish years comes largely from Front Center. This coupled with Rear Center drive Wheelbase, and ultimately change the mass that each tire supports.

    • It’s possible to manipulate weight distribution with the same Wheelbase by altering FC & RC in equal parts. Or we can lock down RC (which is mostly true, and even changes here are minor when compared to FC & WB) that REALLY impacts how much weight we have on the front tire. This all assumes same rider placement and fit relative to the BB.

    • As the front wheel moves further out (away from the rider) it supports less direct weight, that effectively gets shifted to the rear. (I need to do a quick set of static mass distribution graphics to make this clear). This is not good or bad inherently, but can drastically alter the default handling characteristics felt by the rider. Long Front Center in particular may require greater rider weight shift forward for cornering, particularly in flatter turns and/or loose surface conditions.

  2. The article above covers Trail well. I think of it a bit like casters on a shopping cart.

    • Bigger the Trail (longer the caster wheels is behind the pivot), the more likely the wheel will stay straight without rider input & resist turning from the rider or terrain.
    • Smaller Trail allow for easier turning with less user or terrain input.
    • Here again, good/bad is in the bars of the rider. Some want a bike that is “stable” with longer Trail while others want a bike that “agile” with less Trail.

  3. BB Drop is not always considered, but it relates to the aspects of stability as well. Think of it a bit like how far “down into the bike” the rider sits between the wheels. This is particularly relevant when considering cornering, since we are usually applying larger force to the pedals (based on the BB) in an effort to control our center of mass.

    • Larger BB Drop means we are well down below the wheel axles. It also means we are usually closer to the ground (assuming equal wheel & tire setup) which means leaning the bike over is a bit easier. It also tends to make the bike more “stable” since the mass is “hanging down” below the wheel axles.
      • These two latter elements are somewhat contradictory… and something that I still struggle to understand and relay at times. I have notably different experience than many other people from my experience riding Observed Trials bikes. These started as regular bikes with legit BB Drop, but have morphed over decades to actually have BB Rise (BB higher than wheel axles). This is due to the unique evolution of Trials riding and ways to make the bikes perform better. As such, higher BB’s are applied to make riding on the rear tire easier. But this also makes the bike more “tippy” and less inherently stable when on both wheels.
    • Smaller BB Drop means we are more up and closer to the wheel axles. Pretty much the inverse of what’s stated above.
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I like your grouping of #1. I self-designed my Ti frame with a long-ish front center and lower trail (this is an example of me not knowing precisely what I was doing) and while it handles “snappy” as desired it also feels like I have to steer the bike a bit and manage the front end tracking through the turn; conversely, my Nature Boy has what I would describe as very neutral handling where I just kinda look and the bike goes there.

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@mcneese.chad thanks for this! It largely checks with my understanding of these measurements but the groupings really help.

WRT bottom bracket drop, I’ve noticed bikes with front end suspension tend to have shallower BB drop, which I think is to ensure sufficient clearance when the front end is fully compressed (Seigla 65mm, Revolt X 69mm vs Empire 72mm, Grizl 75mm, Revolt 81mm). @FergusYL (IIRC) has commented on the importance of a low BB for his chunky riding in the northeast. Definitely want to discuss this, too, as I keep coming back to the Seigla, which has the shallowest BB drop of the bikes I’m considering.

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Yup, BB Drop (and effectively BB Height) definitely need to consider suspension when it’s present (or optional). Keeping pedal & crank to ground clearance is a real deal, especially for these bikes with suspension forks since it’s expected to use them on more aggressive roads to real MTB trails.

As MTB’s have progressed on the Long, Low (BB), Slack trends… they have often started including shorter crank lengths. 175mm used to be the common length for mid range sizes, and now 170mm lives there with shorter options in some cases (enduro focused builds).

That in mind, and presuming these similar mid range size gravel bikes may be getting 172.5mm cranks and then use a slightly higher BB (smaller drop) to keep pedals & cranks off the ground. As I mention in my BB Drop discussion above, I’ve ridden bikes with BB Drop 70mm or lower, as well as trials bikes with 85mm Rise and just about everything in between. I think most people would struggle to feel deltas less than 3mm or so, but 5mm and more get to a point you can tell more readily.

I have not dug into your specific bikes to look at the comparisons, so I need to do that to offer anything more useful than generalities.

ETA: I have not followed the new gravel suspension forks closely, but keep in mind the functional BB height if/when you have sag for the suspension. Most those forks seem to be around 40mm total travel. If you run 5-10mm sag (guessing here), you can expect the BB to drop a bit less than half that sag distance. This also relates to the stuff like Head Tube Angle and Trail, as these will both change from sag as well.

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All great points. The Seigla is the only bike I’m considering that has a suspension fork. It has 30mm of travel, and the BB is 7mm higher than the Empire - so probably noticeable but I doubt a deal breaker for my expected riding. If I get a bike without a suspension fork I’ll almost certainly get a suspension stem from either RedShift or CaneCreek.

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Gravel bikes are better for big jumps.

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Than road bikes… yah. :+1:

Than MTB bikes… nah. :-1:

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Another factor is where the rider sits relative to the BB, which is affected by the effective seat tube angle (and the BB drop). It’s key to the newer MTB geometry that they put the weight further forward so they climb well despite the long front, but then the slacker head tube and dropper post help keep the weight back descending.

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@mcneese.chad when you get a chance, can you please help me understand why the Canyon Grizl (small) has such a high trail. BikeInsights.com calculated it as higher than the Fezzari Schafer, which makes no sense to me given that the Grizl’s head angle is 2 degrees steeper. I’m wondering if Rake or some other value is wrong. I cross checked with Canyon.com and GeometryGeeks.com and those sites didn’t list Rake or Trail for the Grizl.

Compare: 2022 Canyon Bicycles Grizl CF SL 7 S vs 2022 Fezzari Bicycles Shafer Base M - Bike Insights,

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I think it has to do with Canyon not sharing the Fork Offset (Rake) value. Per the Bike Insights info, they have it shown as 36.3mm, but most importantly as calculated (per the underline). This seems like a very unlikely value to me, but it’s not possible to know without checking on a live fork or getting that from Canyon.

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off-topic but i find so cringy that you need to have permission from spouse to buy a bike or do anything for that matter.
is not like you are a fully grown adult who earns its own money.

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Off topic indeed, and borderline inappropriate IMO.

Doesn’t seem correct to judge in the first place, not to mention that we likely have less than a full story to even consider it, really.

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Wow…that seems unnecessary.

No one here has any knowledge of anyone else’s relationship. Not certain commenting on other’s it the best approach.

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If you chose to have a partner, than you chose to involve that partner in making decisions. It isn’t “permission” and more about communication and respect.

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@mcneese.chad thanks! Just looking at pictures of the bike I would think the Grizl’s trail should be closer to the Revolt. I appreciate your quick responses.

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This is a rather old podcast but one worth a listen IMO. I suspect many would disagree with Carl’s views on geo here since he is basically focused on “road bike like” handling for gravel & all-road. I happen to agree a lot with him and others that talk about things like weight distribution WRT overall handling with (front end in particular). Head tube angle, front center and the like all parallel my personal experience.

Much depends on a person’s preferences and priorities where something very different from Carl’s ideal would make more sense for plenty of riders. But it’s thought provoking to me and contrary to the notable push towards MTB like geo’s that seem to be dominating these days. As ever, there are benefits & drawbacks to any design decision, so understanding them better should lead to more appropriate choices via geometry evaluation.

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I find it cringy that this has essentially become a meme. Making financial decision that impact your household finances should absolutely be discussed with your spouse. Making jokes about spending 10k on a bike and not „telling the wife“ is irresponsible to say the least and pretending that it’s some kind if joke is not funny at all. Just childish.

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Let’s get back to bike geo discussion, please.

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