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AASQ #93: How do manufacturers actually measure mountain bike geometry?

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We know, there’s no such thing as a stupid question. But there are some questions you might not want to ask your local shop or riding buddies. AASQ is our weekly series where we get to the bottom of your (and our) questions – serious or otherwise. This time it’s all about measuring full suspension mountain bike geometry! Hit the link at the bottom of the post to submit your own question.

When you’re about to shell out several thousands of pounds on a new full suspension mountain bike, you might spend an unfathomable amount of time poring over bike geometry charts. Is the seat tube angle steep enough, how big a dropper post can I fit to it, is the head angle slack enough, are the chainstays long enough and will they give me enough tire clearance for a 2.5” WT tire caked in slop?

All great questions to be asking, of course. But, how do you know those figures are comparable between manufacturers, or even bikes from the same manufacturer? Some brands will add information to their mountain bike geometry charts detailing how they measured the angles or dimensions.
Most will publish static geometry, or geometry measured while the bike is in an unloaded state. Others (not naming any names) will publish geometry figures measured when the fork is under SAG, but the rear end is unloaded. Others won’t give you any information about how they measure bike geometry, which begs the question, is there an industry standard method and if so, what is it?
To get some clarity on the subject, we asked the bike designers at Ibis, Spot and Cotic exactly how they measure mountain bike geometry in house. How do they come up with those figures we pore over so carefully?

Ibis: There is no industry standard for measuring geometry, only conventions. Some aspects of geometry are well defined and straight forward, like stack and reach, and others can be a little different between companies.

ibis ripley geometry drawing

Mountain bike geometry is generally measured at top out for consistency. We do state which tires are used for calculating BB height but calculate it with the tires uncompressed, again for consistency.

The measurement that seems to have the most variation these days is effective seat angle. Some companies, when offsetting a seat tube for clearance, pivot the seat tube at the stack height. Their measurement doesn’t take saddle height into account. That’s how you end up with frames where tall people feel like the frame has a much slacker effective seat angle than claimed.  

Ibis publish a 76° seat tube angle for the Ripley 29er trail bike. Photo by Rupert Fowler – Winter Bike Connection
Other companies, including Ibis, pivot the seat tube from the average seat height for that size frame. That way, if the rider is on the correct size bike, their seat height will put them close to the stated effective seat angle. The one problem with this method is that the assumed average seat height is not always listed. 
If customers ask, we’re happy to share all the geeky numbers. We’re also working on incorporating this information on our website. In general, we try not to offset the seat angle more than 5 degrees, so the effective SA doesn’t really change much as you raise and lower the saddle.

Spot: The primary function of the mountain bike geometry chart is to serve as a comparison tool. In order to add as much value to the comparison as we can, we present direct, raw numbers as though you measured the bike in your workshop without someone sitting on it.

150mm spot mayhem mtb 29er
Spot Brand’s 150mm travel Mayhem 29er trail bike runs a leaf spring suspension platform

Bike manufacturers don’t subscribe to a “standard” by which geometry numbers are published, or even how they are measured for that matter. But the convention is to report geometry numbers unsagged – as we do. The reason for this is to reduce the number of variables a potential customer has to juggle when deciding on their next new bike.

Sag is a variable which describes a range. If we recommend 25-30% rear suspension sag and choose to refer to a sagged bike in our geometry charts, now we have to report a range for every angle and dimension.

spot bikes release new mayhem 130 150 trail mountain bikes from colorado
The Spot Mayhem 130 also runs a leaf spring suspension platform

But wait, the fork sags too. How much depends on how the rider tunes their fork to ride, and where their personal center of gravity combined with the bike’s center of gravity act to compress it.

So now we have compounding ranges, and the geometry chart becomes multi-dimensional. It’s a good thing no one takes it this far – it would be extremely tedious to make meaningful comparisons. Deciphering a conventional geometry chart, then painting an accurate mental picture of how that geometry will ride is a tall order even for an advanced bike nerd.

spot living link leaf spring
Spot’s “Living Link” leaf spring

We specify our geometry considering the bike built to the stock spec, tires included. When there are multiple tire size options or fork travel options, we publish those.

One subtle difference in geometry that has been coming to light in recent years is the comparison between effective seat tube angle and actual seat tube angle. On suspension frames, and hardtails where the axis of the seat tube does not intersect the axis of the bottom bracket, there are two angles to consider.

2016 Spot Rollik full suspension mountain bike with Living Link design by Wayne Lumpkin 76 degree effective seat tube angle
The Spot Rollik of 2016

Back in 2016, we launched our first suspension bike, the Rollik 557, with a 76° effective seat tube angle. Considered radical at the time by many reviewers, we were the first to reach 76° on a production bike. But, the effective angle doesn’t tell the whole story as to why this felt so different back then.

In order to make room for the rear tire to swing through its travel, typical suspension bikes must offset the seat tube axis forward of the bottom bracket axis. This is most extreme in long travel 29ers with short chainstays, and results in a significant deviation between effective seat tube angle (virtual) and actual seat tube angle (real).

new-spot-mayhem-130-trail-bike-leaf-spring-suspension-2021-effective seat tube angle
The Mayhem 150 has an actual seat tube angle of 74.5° and an effective seat tube angle of 77.5°

It is high priority in our design process to keep the actual seat tube angle as close to the effective seat tube angle as possible. When a steep effective seat tube angle is paired with a slack actual seat tube angle, a tall, or long-legged rider will feel the drawbacks of that slack actual seat tube angle. We want to eliminate the “behind the bike” feel on the climbs for all riders, not just the middle of the bell curve.

It’s important to know whether the 78° effective seat tube angle frame will sit you farther back than the 76.5° effective seat tube angle frame. There are plenty of cases out there where something like this could be true.

As of now, there is no accepted method to normalize these numbers between frames. Most suggestions simply amount to measuring the effective seat tube angle at a point higher than the one we use now, the top center of the head tube.

Conventional geometry charts put the burden of this determination on the customer, or even worse, some don’t even publish the actual seat tube angle. We always publish this number, as we work pretty hard to make it favorable!

Modern mountain bike geometry has evolved to favor a binary winch up, bomb down riding style. Steep seat tube angles put the rider in a comfortable, powerful position for gaining vert and dispatching technical challenges.

new spot mayhem 150 2021
The Spot Mayhem 150 has a 64.5° head angle in low mode, steepening up to 65° in high mode

On the flip side, slacker head tube angles increase fun and confidence on descents. In the middle, mountain bikes are getting longer. The trend of ever increasing reach on trail bikes has the effect of sacrificing agility, in every climbing and descending situation, for improved stability at high speeds.

Long bikes benefit some riders in some riding locales, but we don’t see it as a panacea. A moderate length reach, combined with excellent suspension kinematics and well tuned frame and wheel flex/damping response will yield plenty of stability, without sacrificing your chance to correct your line for that surprise rock in a blind corner. A number of top EWS pro riders use shorter bikes because they deliver faster times!

Cotic: We get asked this a fair bit, because we list our droplink suspension bikes static, but our hardtails at ride height (i.e. with sag on the fork) in our geometry tables. There’s a couple of reasons for this.

Many of Cotic’s front triangles are now fabricated by Five Land Bikes in Edinburgh, Scotland

Firstly, because we’re a direct to consumer brand, our geometry charts carry more weight than perhaps a shop bought bike might. This is because, particularly at the moment, we can’t offer a demo. We have to balance what we think will be most representative of the bike to the consumer, versus how easy it is to compare across other brands.

We choose to list the droplink bikes static for three reasons: 1) because all other brands do, so it’s a little bit easier to compare across brands, and 2) because they sag front and rear (albeit to slightly different amounts), the static geometry does bear some resemblance to the dynamic geometry. 3) Dynamic geometry is changeable due to rider preference much more than on a hardtail.

The RocketMax’s 160mm rear wheel travel is serviced by their DropLink suspension platform

Setting up a shock with 15mm sag and not 17mm sag can make nearly 1 deg difference in angles and several millimetres of BB height. Fork setup varies a lot too. This means that the ‘dynamic’ geometry is kind of a myth. Mine will be different to yours will be different to the next rider’s. Stating static geo limits the variables and helps comparison.

We choose to list the hardtails at ride height because with only the fork to sag, the static geometry tells a consumer pretty much nothing about how that bike will ride. It’s completely useless as a decision making tool.

By using ride height and stating how much sag we are assuming, a Cotic customer can see that the angles listed are what they will actually experience (give or take a few 10ths of a degree), and if they have some experience with other bikes they will have at least some level of feel for what to expect in terms of handling.

The Cotic BFe Max

For instance, the BFeMAX runs at a 65° head angle with a sagged fork, but while static the head angle is 63.5°, and the BB seems ridiculously high at only 48mm drop. The static angles tell you absolutely nothing about how the bike will feel if you have any level of intuitive feel for other bikes.

The only way stating dynamic geometry would be useful to riders is if there was some kind of industry standard way of doing it, or at the very least a commitment from a majority of brands to change the way they do it by a certain date in order that potential customers are always comparing like with like as far as is practicable.

cotic rocketmax enduro 29er steel framebike
The Gen3 Cotic RocketMax is a 160mm travel steel 29er enduro bike

Ultimately, how we state mountain bike geometry is just one tool – albeit a relatively powerful one – to explain to our customers what our bikes are all about. Would a RocketMAX be more attractive to certain riders because the dynamic head angle is 63°, when the static is 63.5°? I’m not sure it would, especially if the other brands under consideration are stating static geometry.

So long as the whole industry is doing this fairly consistently, how we measure it is almost a moot point. The key is that we’re presenting similar data across brands so people can make as informed a decision as possible.

Regarding the other variable, we always assume tyres are the same diameter, and our experience with WTB – our main tyres supplier – is that this is correct within a couple of mm.

Thank you to Andy Emanuel (Engineer) and Andrew Lumpkin (CEO) of Spot, Colin Hughes (Engineering Manager) of Ibis Cycles, and Cy, the founder of Cotic for contributing to this week’s Ask A Stupid Question.

Got a question of your own? Click here to use the AASQ form to submit questions on any cycling-related topic of your choice, and we’ll get the experts to answer them for you!

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3 years ago

I think seat tube angle should be two measurements: post angle and offset.

3 years ago

Another GREAT article bike rumor!

Eggs Benedict
Eggs Benedict
3 years ago

The geometry charts that the manufacturers provide should have an Effective Seat Tube Angle @ Height column. Take the measurement from the BB center line (as usual) then to the fore-aft center line of the seat clamp (on a zero setback post).

Cy Turner
3 years ago
Reply to  Eggs Benedict

That’s exactly what we started doing at Cotic. Any bikes with an offset seat tube are stated with actual seat angle at 815mm saddle height and 720mm – which happens to be my saddle height at 6ft 3in and Paul’s at 5ft 8in. We are happy to work it out exactly for customers, but we have only 0.3deg variation across those heights so it’s not much.

3 years ago

Poorly and inconsistently. There is no “industry”. Industries have standards that mean something.

No need for 10k words.

3 years ago

I’ve been on a large Trek Fuel Ex and Spez Levo SL. According to the charts the Trek has a longer ETT and Reach, yet the Spez feels bigger. Could also be due to stem length and handlebar width.

Ol' Shel'
Ol' Shel'
3 years ago

“BB Drop” is confusing, especially when dealing with multiple wheel sizes and fairly large variations between standard and ‘plus’ tire sizes. Just tell consumers the ‘set up’ used for the geometry table, and let us know how far off the ground our cranks are going to be.

It’s nice when they list both Effective and Actual seat angles, but U suspect they typically only supply one because of the confusion caused when folks don;t quite understand the difference between the two, and what it means for them. Perhaps a basic and advanced geo chart option would help.

Cy Turner
3 years ago
Reply to  Ol' Shel'

I would respectfully disagree here, for exactly the reasons you mention. Tyre radius varies an awful lot between specifications and manufacturers which to me – as someone trying trying to state dimensions with at least some level of consistency and accuracy – BB drop gives me a sibgle unmoving dimension to state. For instance, the difference in rolling radius between a WTB Judge 2.4 and Trail Boss 2.6 – both of which fit the 27.5 wheel Cotic Rocket – is 6mm on radius. 6mm difference in BB height just by choosing one tyre combo over another, and that’s just within one manufacturer. At least with BB drop it is consistent with itself within our range and with other manufacturers for comparison. I recognise wheel size complicates this, but a good rule of thumb is around 17mm more BB drop required on 29er vs 27.5 for a like for like ryre design. So for example, the Rocket has 10mm bb drop, the RocketMAX has 27mm. Hope that helps.

3 years ago

Hopefully in two years time all manufacturers will give us the geometry at the recommended Sag for that bike. Like Cy from Cotic said, with hardtails they already do this and all the reasons he is stating for doing so also apply perfectly to full suspension bikes. And the current static geometry of fullies renders the comparison between hardtails and fullies completely useless.
You never ride a Full suspension at static geometry and there are more variables that will change the geometry under sag. E.g. the wheel path of the rearwheel ánd the headtube angle will heavily influence the wheel base. The same goes for BB height (no BB drop is not a single unmovable dimension, because again, influenced by wheel path and headtube angle) And then they all refer to effective seat tube as being inconsistent and problematic, come on, at least that one you can easily fix with your seatpost or saddle. But wheel base and BB height not so much.

Pro tip: for the next two years just show two geometry charts; one bs static and one useful at the recommended sag like hardtails do (no you do not need a range of sag, Mfr decided the geometry, Mfr decides the optimum sag). Then, when every Mfr is converted, in two years we consumers can finally chose a bike that does not suffer from pedal strikes at every small stone and root.

Helas, small chance it’ll happen; they are stuck. Like the USA will never go Metric even though it would make sense.

3 years ago
Reply to  Peter

R E C O M E N D E D S A G I S N O T F I N I T E.

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