Frame Geometry Explained
Spring/summer 2017 was The first time I rode Cypress Mountain. This place has a lot of steep, technical, freeride trails.
My buddy, Andrew, rode Cypress many times and was the tour guide for the group—all of whom hadn’t been to Cypress yet.
Andrew warned that we’d be doing some steep, gnarly trails. He recommended that everyone bring their downhill bike “just in case”.
Not wanting to risk showing up with a knife to a gun fight I brought my trusty 2014 Giant Glory.
That day was hot and dry. The trails were pretty dusty. Just as Andrew had said, the trails were steep. How steep? One lap I was riding behind my buddy, Joeben, and thought, “Is something burning?”
Almost. Joeben’s brakes were smoking.
But that’s a story for another article. Let’s just say the trails were steep. And dusty.
At some points my front wheel couldn’t track the turn and slowly slid out. I wish I could say I was drifting but it was unintentional.
Like this, but steeper. And both feet on the pedals. And not sponsored by Trek.
I didn’t lose control of the bike, but afterward I thought, “I think my Range could’ve carved that.”
Why? Because it has a steeper head angle than my DH bike. Not saying that one bike is a better choice than the other.
This article will go over different aspects of bike geometry, so you can understand what does what. Then, you can pick the right tool for when you want to rail corners, or drift ‘em.
Impact on Handling
I won’t go over all measurements of mountain bike geometry, just the ones that have the biggest impacts on a bikes handling:
Head Tube Angle
HA - The angle of a bikes forks relative to perpendicular. More simply, “head angle”.
This measurement gets a lot of attention these days. The current industry trend is to make head angles as “slack” (low angled) as possible.
This makes a bike more stable at some key times where the easier for beginners, the better.
Slack Head Angle
- At high speed.
- When going down steep descents in a straight line.
- When plowing over rocks or roots.
- When landing jumps or drops.
Helps compensate (to a degree) for mistakes when landing jumps or drops, such as:
- Over rotating - landing front wheel first.
- Under rotating - landing rear wheel first.
- Not having the front wheel straight.
HOWEVER, there are, of course, drawbacks to having a bike that is too slack.
Life is all about balance, man.
- Front wheel has less bite into the ground; higher tendency to wash out the front wheel.
- Larger turning circle.
- Less responsive and maneuverable.
- Longer, heavier front end.
- More difficult to use on steep technical climbs.
Steep Head Angle
- Front wheel bites into the ground better under braking and/or turning; less tendency to wash out.
- Smaller turning circle.
- Quicker steering.
- More agile, responsive, and maneuverable.
- Better for steep technical climbs.
- "Twitchy"—less stable at speed and on descents.
Keep in mind, washing out a front wheel can happen regardless of how steep or slack a bikes head angle is.
Whether or not having a slack bike is good is also largely subjective. Lower responsiveness is often seen a plus for beginner riders as it provides a larger margin of error. Advanced riders, however, may appreciate more responsive steering. Or, they may just stick with whatever they started out with and still be good.
Bike makers seem to have settled on 63° being the lowest they’ll go to have as much benefit and as little drawback of a slack head angle as possible.
In this article, I go over different mtb types which will include their different head angles: Modern Mountain Bike Types
Chain Stay Length
RC (rear center) - The distance from the rear axle to the bottom bracket.
Chain stay length is usually constant between different sizes of the same bike frame.
Lately, the industry heavily leans towards making bikes with chain stays as short as possible. This makes it easier for a rider to shift his/her weight past the back wheel for intermediate techniques such as bunny hops or manuals. In general, it just makes it easier to pop the front wheel up when pulling on the bars.
Of course, this increase in playfulness comes with a trade off: Ease of high-speed cornering.
With a shorter rear end, it takes more effort to properly weight the bike in a turn by shifting your body forward.
Higher speed at anything generally requires more skill, which seems to have been de-prioritized in favor of beginner friendliness. This seems to be part of a grander, unspoken of, industry-wide ploy to make mountain biking easier for beginners to get into.
Bikes are becoming better but with more focus on playfulness, while trails are being made easier to ride. Trail features are being dumbed down. More flow trails are being built. More people that get into mountain biking—which means more beginners—equals more $$$.
Note: No tin foil hat here; just an observation. I think more people riding bicycles is a good thing.
Short Chain Stay
- Less effort to bunny-hop, manual, or wheelie.
- More effort for high speed turns.
Longer Chain Stay
- More balanced.
- Less agile.
WB - The distance from front axle to rear axle on a bike. This changes between the different sizes a bike frame may come in.
- Smoother and more stable at speed.
- Bigger turning circle, less agile.
- More agile.
- Twitchier handling, less stable at speed.
REACH - The distance from the bottom bracket to the head tube.
To make a larger size frame, a manufacturer usually makes the reach longer. This is the “long”, in the cliché mtb term “long, low, and slack”, you may have heard.
While chain stays have gotten shorter, reach has gotten longer to preserve stability at speed.
Longer or Shorter Reach has similar general effects as longer/shorter wheelbase.
Bottom Bracket Height
BBH - The distance from the center of the bottom bracket to the ground.
Bottom bracket height has the largest impact on a bikes center of gravity, which has a large impact on a bikes handling.
Something that amplifies this even more is bottom bracket drop (BB+/-)—the amount lower that the bottom bracket sits relative the axle height.
Have a look at the following comparison:
The cyan line represents the center of the axles.
The image may be small but you can see that the yellow bike has a bottom bracket that is further from the line of the axles than the red bike.
When the bottom bracket sits below the axles this creates a pendulum effect, which lowers the center of gravity of the bike.
When the bottom bracket sits above the axles this creates a sort of tower, which equates to a higher center of gravity.
Imagine which one would be more stable when leaned into a turn, going over rough stuff, or landing jumps. But of course, there are tradeoffs.
High Bottom Bracket
- More ground clearance for clearing gnarly stuff.
- Better rear wheel grip when climbing steep tech.
- Easier to lose your balance or go over the bars when going over gnarly stuff.
- Not as stable when cornering.
Low Bottom Bracket
- More stable overall.
- Less ground clearance.
- Less rear wheel traction when climbing steep tech.
Generally, the pros of having a lower bottom bracket seem to outweigh the cons, which is why modern bikes seem to favor them.
Seat Tube Angle
ESA (effective seat tube angle) - The angle of the seat tube relative to perpendicular.
Seat tube angle mostly affects climbing capability.
Steep Seat Tube Angle
A steep seat tube moves the riders weight forward and closer to being directly above the bottom bracket, when the rider is seated.
- Puts the rider in a more powerful, efficient position for pedalling.
- Helps keep the front end down when doing very steep climbs.
Slack Seat Tube Angle
With a slack seat tube angle, when raising your seat height, it will travel further backwards away from your bottom bracket than it would with a steep seat tube.
I have no idea why this would be beneficial.
Whoa nelly! That's a slack seat tube.
So what does this all mean?
If you’re looking for a downhill bike, buy a downhill bike. If you’re looking for a road bike, you have no business on this site (kidding․․․go buy a road bike).
If you can’t decide between an enduro or a trail bike, hope this article helps you pick based on what kind of riding you like to do.
Also read my article on Modern Mountain Bike Types.
- @Norco, if you don’t want me using your image, let me know and I’ll give the free plug to another bike company ;) [return]
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I'm an avid mountain biker and web developer. When I'm not riding I'm daydreaming of riding. Being a technically minded person I extend my analyses of MTB related things to you through writing.
Riding Styles: Enduro, Downhill
Preferred Terrain: Steep tech
Current Steed: 2017 Devinci Troy
Location: Beautiful Vancouver, BC 🇨🇦