MTB Geometry & Suspension Discussion

This is a new topic focused on discussing the differences in geometry between bikes that run the spectrum from XC (Cross country) to DH (Downhill) and everything between them.

It was spurred from some discussion in a separate thread wondering about the functional differences between seemingly similar bikes with close specs other than a few components.

This single comment is just a starting point and I may come back to add some rough summaries of the range of bikes related to geometry and suspension details. It may be necessary to couple geometry & suspension since they two are commonly intertwined, but they can diverge a bit at times as well.


@mcneese.chad I need help with the following…

Explain where an XC bike is to be ideally ridden and where that fine line is between a gravel and trail? Are these just smooth dirt path without trees that only have flats and climbs? I just want to know at what point do I need an XC, downcountry, and trail bike!

What idiot decided wider bars are better for trail bikes? Trails have trees, usually close together.

Can understanding suspension forks be any harder? Is a 32 better than a 34? What size tire can it fit? What are the differences between all these dampers? How heavy is it? Why does the really expensive fork sound like it has worse stuff than the middle fork? Why do they make the low speed compression adjuster hard to play with on some forks? The websites answer NONE of these questions.

XC bikes generally are light bikes with short travel suspension meant to be snappy for acceleration as well as handling. Steeper head tube angles and shorter wheelbase that lead to the ability to change direction quickly.

  • That “fast” handling is aimed at things like sharp cornering for stuff like switch backs and/or line selection on or around obstacles. These have a genesis of being “road race bikes for dirt” in many ways despite taking on more MTB specific features over the decades.

Trail bikes tend to me in the middle of the MTB spectrum (XC to DH) and are most often what people tend to have in mind with the overly generic term of “mountain bike” these days. Geometry that is slacker than XC race bikes and more suspension travel as well.

  • These are often aimed to be the Swiss Army Knife type of bike that can do a bit of everything and not really suck at any of it. These bikes are more “stable” with the slacker head tube angle and longer wheelbase. They still handle reasonably quickly for cornering, but lose some of the “twitchy/nervous” nature that comes with XC bikes mentioned above.
  • Without knowing exactly where someone rides or what they really want from a bike, this is one of the “safe” options to start with for people wanting a “mountain bike”.

Gravel bikes is really a separate thread here, but I understand why you are asking about them in this context and will cover them a bit. These tend to be drop style handle bar bikes (but more flat bars are emerging) with wider tire options than road or CX bikes. These have their own spectrum that runs from stuff like “race” bikes out to “touring” bikes which means large differences in geometry, suspension, tire and gearing options.

  • Broadly speaking, “gravel” seems aimed at “roads” that are not paved or concrete, rather they are less maintained dirt roads that may have a wide range of rock (gravel) embedded or atop the dirt surface. The makeup of gravel roads varies WIDELY even within a single county, and gets wider when you look at different states and countries.
  • Then consider that some people are pushing gravel bikes into “trails” that are most often associated with MTB and we get a bike that may well be a different version of a Swiss Army Knife in the bike world.
  • I will either find a related article that covers the range or make a quick summary for this topic.
  • A bit like differences in roads, MTB trails and such vary WIDELY between regions. Some have super narrow trails with trees, rocks and such that may dictate more narrow bars. But there are also regions with wide open ranges of trails where flopping the bike around is aided by wider bars (and associated shorter stem).
  • There is no singular solution that is right for everyone. But with bars, it’s easier to cut wide ones down than make people upgrade to wider ones after the initial purchase. So that is the “better” starting point and people can cut them down as they see fit
  • I have seen some guides on forks that can help with this and will link them in once I find them.

Suspension Fork Info

More to come…

Reference Appendix A on starting on page 31 for how Giant explains use cases. As use conditions go up, so does travel, durability and weight. Geometry gets adjusted to suit the different use cases.

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Types of Mountain Bikes

I mentioned it in the XC bike thread but I switched my fork from 100mm Judy Silver to 120mm SID Select on my hardtail XC and it surprisingly drastically altered riding characteristics.

Extra degree of HA, I don’t feel like I might go over the bars anymore. Really, I no longer think about it, so it’s working quite well.
Longer wheelbase (my handling skills have improved significantly, so I cannot comment how large an effect this played).
Raised my head tube… Front wheel was much more eager to pop off the ground during climbs… I rectified this by going from a 0* stem to -17*, same length. This was probably the most immediately noticeable change.

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All else being equal (travel, construction, etc.) the larger diameter fork tubes will mean a stronger and stiffer fork. This tends to be a benefit since the fork will be more stable and less “flexy” under hard loading. This leads to more consistent steering for line chose and less tendency for the bike to “wander” from the path you direct via the handlebars.

Hopefully, this info is stated in the “Specifications or Tech” sections of the related product page. You won’t likely find it on a complete bike page, so you may have to track down the specific fork product page from the manufacturer.

Like the tires spec above, the actual manufacturer site is a place to start. Hopefully they have some pros/cons or comparison descriptions between the tiers of options they offer. Or you can look for guides like I linked above, that cover those differences. You won’t usually get that depth of info on bike product pages either.

Maker product page, again.

I’d have to see a specific example to know what you are comparing and why you think that.

Again, depends on the forks in question. But tuning options are often more limited at lower price tiers.

  • Partly related, it’s sort of scary how many people don’t know about or bother to experiment with the adjustments they have on their forks.
  • Far too many don’t even have a proper air pressure set, not to mention considering rebound and compression settings. I’ve helped numerous people around here when we were on a ride and I see evidence of poor setups. It is partly black art but even some basic setup steps can help people get a better experience.

Per my mentions above, are you talking about bike product sites or actual fork maker sites?

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I was watching some of these videos the other day when trying to see how a DW link suspension system works. He’s got a few informative videos, though the models are all a bit long in the tooth now.

Also, XC is definitely not dirt paths. Well, for some people it is, but a lot of XC these days is still anything up to Grade Four (or even some Five depending on area) trails and decent technical riding.

Generally wider diameter and more adjustable forks are “better”: they have better damping, are stronger, more capable/robust. But they’re also heavier. Figuring out the best size really depends on the terrain you ride, but also your weight and riding style. For example, My ride weight is near 200 lbs and I like to go fast and power through rough trails; a fox 32 feels like a noodle underneath me.

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My addition: Chasing small bump compliance makes your bike ride worse.

  • Running too low of air pressure usually results in blowing through the travel. Any sort of medium sized impact or movement results in too much travel usage and running into the “wall of progression” at the end of the air spring. People often mistake this sort of “harshness” for poor small bump compliance.

  • Running too little compression damping results in increased chassis movement. Excessive chassis movement requires much more body control/movement to properly weight the bike during cornering and impacts. This can be the difference between rolling over an obstacle and going OTB on an obstacle.

  • Combination of too little pressure (excess sag) and too little rebound damping can result in excessive/fast down travel (ground pounding) when your fork returns from an impact/bump which can be perceived as poor small bump compliance.

I’ve seen people chase small bump compliance by continuing to lower pressure and reduce damping, which actually makes the bike feel/ride worse . Then they think the only solution is to lower pressure/damping more!


My opinion:
when you really simplify it, there are only two things that matter when figuring out which type of off-road bike to get:

  1. What kind of trails you’re going to ride (driven by where you live in the world)
  2. How strong your technical bike handling skills are.

There are many Forum contributors in different parts of the world, so if you post where you live and how good your bike handling skills are, you’ll probably get some good responses.

I live in Philadelphia, PA USA (east coast) - typically dirt singletrack with lots of tree roots, some logs, and a few rocks. Lots of short up and downs - not too many extended climbs or descents. There are a few bigger mountains and bike parks nearby, so there are people who ride more rock gardens, but I tend to focus on the trails I mentioned. My bike handling skills are better than my fitness.

I primarily ride an XC or downcountry bike - the Santa Cruz Blur TR. I often ride a gravel bike with big tires and a dropper seatpost for fun on some of the trails, too.


Yup, from a sales / selection perspective, the Trek training focus on “intended use” as the primary factor. Where you want to ride is key in narrowing down the selection within the entire world of bikes, and then within any given genre as you whittle down the options.

Rider experience and future goals play into that as the next points of interest. People might ask for a “race bike” but could be better off with something a bit less “racy” if they lack the ability to make the most of that from a handling perspective. Then budget has to figure into the equation and may point towards one option range or another.

As mentioned, if the goal from the OP that sparked this is to pick a bike, getting to know more specifics about you and your intentions is necessary vs diving into minutia of components at this stage.

Not really sure if we are in the “helping catch a fish” type of scenario or one where we are “teaching you how to fish” instead? Could be a bit of both too.

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Again, NONE of that info is on Fox or RS’s main website. LOL. Fox you need to look at a freaking CAD drawing with half the measurements you want.

OK, are you looking for help with any of it or pointing out the difficulty of finding the info?

  • If the former, I’m happy try and track down specs once I know the precise forks of interest.
  • If the latter, an email to either/both fork makers is the best I can offer.

I got it figured out. Just complaining.

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They’re simply going about it wrong. Air pressure is spring rate, and the right air pressure is what gives the correct sag.

Definitely agree that a lot of people don’t know much about tuning. Even paying attention and researching, it’s still hard, and I do a lot of set and forget riding.

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I would add a third item - do you intend to race XC/marathon, or enduro? 1 bike will not do both, and for that matter, a trail bike would suck equally at both.


So if I understand what y’all are saying, if I wanted to buy “dirt bikes” for my son and me, to ride around wherever, knowing that our MTB skills are nonexistent but we want to have fun with light, nimble, maneuverable bikes – ideally bikes that can handle jumps like I used to do 40 years ago – we’d want XC bikes? Or what?

Seems so complicated. When I was 12, everyone had the same kind of dirt bike. And we went EVERYWHERE and ANYWHERE on those bikes. We did wheelies, jumps, tricks, and rode for hours on trails. I just want a dirt bike that can do everything, and is flexible and nimble and has quick handling, even if obviously it won’t be “the best” at anything.

I’m happy to learn more – I love learning more about anything – but as a starting point, does it sound like there’s a an answer to my question?

  • We would need some context as to what this means, exactly.
  • My first instinct (without more specific info) is something in the Down Country (I know… I hate the term too) or “Light Trail” (my own term I like better than DC) end of the spectrum.

    • Specialized Epic Evo, Trek Top Fuel, Transition Spur, Scott Spark (I never remember the best one, but someone will know), Santa Cruz Blur TR and bikes like these come to mind.
    • A tad more travel than pure XC bikes with slightly slacker angles. But still very “playful” bikes in the sense that the geo is snappy vs really “long/low/slack” like many trail bikes.
  • Pure XC race bikes are steeper angles with less suspension usually, and less forgiving from a rider perspective. You can rally the piss out of them, but if you overstep a bit, they can bite you quickly. that is where the DC and Light Trail bikes gain some in the “forgiveness” department IMO.