Behind the Numbers is back!
If you’d like to know more about the Behind the Numbers series, aren’t familiar with the terms being used or want to know why the hell we’re doing, this then head back to out our introduction article
We’re also trying to take certain terms and set about explaining them, how they might be analysed and add some food for thought by re-inserting the violently chaotic real world in which we ride back into the analysis. Recently we touched on anti-squat
, but look out for more of the Enginerding articles in the future.
But, we’re back for another round of bikes and this time we’re looking at the vast category of trail bikes. There’s quite some discussion as to what a trail bike is. Our recent podcast
did just that with differing opinions on the boundaries of trail bike in terms of travel, geometry and bike intent. We’ve tried to take a good sampling of bikes that look a few different takes on that trail bike category with different suspension systems from some of the big names in the industry, different wheel sizes and bikes with hugely varying price tags all up for analysis.
We’ll keep our analysis mainly focussed on the suspension, looking at the layout, leverage ratio, anti-squat, pedal kickback, anti-rise and axle path. In the case of all the bikes in the trail category, they’re all bikes that have been ridden by one of the editors here at Pinkbike so, we can start to look at how riding feedback relates to what we see in the graphs.
We’ll have a new bike analysis out every week followed by a roundup of the five bikes we’ve analysed. If you have any questions feel free to comment, and, similarly, if you have any requests for ways in which you’d find it more interesting to connect the graphs and numbers to real life. We appreciate that this is a very technical topic and the more information we can get across then the better informed you can be on your next bike purchase or on the bike you have right now.
The final mesh file of the Commencal Meta TR 29 made up of more than 18 million triangles and with an accuracy of 0.1mm across the length of the bike.Scanning
We’ve continued to work together with 3D measurement technology company, Creaform, to 3D scan all the bikes and capture the real-world data as accurately as possible. Four of the bikes were captured in one fell swoop at the Creaform offices in Grenoble, France. But tracking down bikes can be tricky and that’s where Simon Côté and his team of 3D scanning experts and bike enthusiasts further helped out for our first bike of the trail category, the Commencal Meta TR 29.
Firstly, a big thank you goes out to Bastien and Thibaut from The Factory
bike shop in Fribourg, Switzerland, for letting us commandeer four of their most prized bikes for a day of scanning. The fifth bike we were after, though, the Commencal Meta TR 29, was tracked down through VLO Saint-Basile
in Quebec, and a thank you to Sam St-Pierre for that.
And to throw more spanners into the works, the current situation with the coronavirus made accessing the bike to scan just that bit harder.
Heading to the home of the Commencal rep to scan the bike wasn't an issue. Setup was quick and only required a background with additional reflective targets to allow the scanner to know where it was in space.
Nevertheless, the Creaform team were able to set up a scan in the home of the Commencal rep, Tobie Boucher. For this situation, the HandySCAN 3D was the ideal tool with its small size, easy portability and quick setup time. This is the same hardware that was used to scan the Santa Cruz Megatower in our previous enduro bike category when that bike was scanned out at the local bike shop.
The portability of the HandySCAN 3D made it easy to transport and set up in the home of the rep while still returning fantastically accurate data. For the size of objects that were being scanned it was accurate to 0.1mm. The speed of data collection in this situation was impressive too, with the whole setup and scan never encroaching on an outstayed welcome in the rep’s home.
Setup required additional small reflective targets to provide a reference frame for the scanner to position itself in space. These were attached to the bike and components as well as a flat piece of cardboard behind the bike.
There aren’t many tools available out there to allow this level of portability and ability to scan with such accuracy
in such a short time frame with minimal preparation. If you’d like to know more about Creaform, their products and how they can be implemented into vastly different scenarios and industries then visit their website
The Commencal Meta TR 29 is, as the name suggests, a 29” wheeled bike with 130mm rear travel and a 150mm travel fork. It was recently reviewed by both the Mikes during the Sedona Field Trip
It uses a single pivot suspension system to define the axle path and acceleration responses of the bike, and the seat stays, link and shock extender to compress the shock.
Most of Commencal’s less gravity orientated bikes use this same system, with only the Clash and Supreme using other linkage arrangements. One thing that is true of all the Commencal bikes is that they’re single pivot.
Commencal Meta TR 29 Analysis Details
Travel Rear: 129.7mm
Travel Front: 150mm
Wheel Size: 29"
Frame Size: M
CoG Height: 1100mm
Chainring Size: 32T
Cassette Cog Sizes: 50T, 24T and 10T
With the relatively long link lengths between pivots we generally see some smooth curves being generated. The Meta TR 29 uses a 50mm stroke shock to get its travel, with all the bikes in the range using air shocks, although varying between Rock Shox and Fox units. All the shocks see standard eyelets with no trunnion mount being used. With the Commencal design already using a shock extender the shorter eye to eye from a trunnion shock would only make the shock extender longer and up the non-axial loading transferred into the shock.
As opposed to many of the bikes we looked at in the endure category, the Meta TR 29 has no adjustability. In fact, none of the bikes in the trail category have adjustability. While this made my life a little easier for the analysis, and your lives a little easier with less curves on graphs, it also might say quite a bit about the trail bike’s intention to just be grabbed from the garage and ridden, rather than tweaked and tinkered like their longer travel variants.
Commencal also represents a direct to consumer brand, which often sees large differences in price to similarly specced bikes from more dealer-based brands. While there are downsides to this business model, it’s interesting to note that the price reduction shouldn’t necessarily yield a reduction in suspension performance. For the most part, pivot point position, which leads to the kinematic curves we see below, isn’t solely tied to price. Some of the bikes we will look at in the coming weeks have unique suspension parts, which would add cost to the bike. But there’s no substitute to getting the pivot points in the right place and paying $2,600 for a bike, like the Meta TR 29 29 Ride, shouldn’t mean your suspension performs terribly.
The Meta TR 29 has 14.63% progression with a starting leverage ratio of 2.87 and finishing at 2.45. For the whole leverage ratio curve there’s an average ratio of 2.59.
For our five trail bikes the Commencal comes in close to the least progressive bike. But in comparison to other single pivot bikes, where progression percentages can be under 1%, it does have a decent dollop of progression.
From the Field trip test, Mike Levy found options in setting the sag at 25%, 30% and 35% shock stroke, resulting in 26.75%, 31.91% and 37.01% of rear wheel travel sag respectively. The Meta doesn’t have a crazy high progressive leverage ratio curve and sees its shock sag followed within 2% by the rear wheel sag it produces. Some highly progressive bikes would see that rear wheel sag percentage vary by 4%, meaning the bike would be sat further in the rear wheel travel with slightly less available positive travel.
The leverage ratio curve is parabolic. It starts with a steeper gradient curve and flattens off towards the end of travel. If we differentiate the leverage ratio curve then we can see a straight, but rising curve that indicates the presence of a parabolic, x^2, shaped curve in the leverage ratio.
While I’m a fan of leverage ratios with as straight a line as possible (progressive but no roller coaster business in between), this curve would work well with the air shocks that the bike is specced with throughout the range and their inherent ramp up at the end of travel.
Despite the general parabolic shape, there aren’t any wild humps or rapid changes. This is a common curve shape, often seen in some rocker pivot Horst link bikes, and seems to play well with predictability and shock setup as the main portion of travel is smoothly chaining and only the end portion could need adjusting with volume spacers, which are an easy thing for everyone to play with.
The leverage ratios are on the lower side of things when compared to many other highly leveraged trail bikes of today. The lower leverage ratios would reduce the amount of air pressure needed to achieve a certain sag, reduce the overall amount of force being transmitted to the shock, pivots and frame members and also generate more damping force in the shock due to moving it faster than a highly leveraged bike. The Meta TR 29 should sit in a nice window of balance between spring rate forces and damping forces.
Supportive, predictable and composed were used to describe the Meta TR 29s descending capabilities during the Field Trip. That is in part due to that lower and smoother leverage ratio giving ample support from the balance of spring and damping and no nasty surprises thrown up as the bike compresses and rebounds through its travel.
The Meta TR 29 is specced with a 32-tooth chainring and SRAM Eagle gearing.
All gears produce high values of anti-squat. The single pivot layout means the transitions from start to end of travel are smooth without huge drop offs in percentage. There’s even a convergence point of all the gears close to the end of travel.
Taking a sag of 30% shock travel, the lightest climbing gear sits at 97%, with a drop down in the percentages if you were to encounter bumps while pedalling or have your body weight push the bike into its travel.
This goes a long way to combatting suspension squat from load transfer. Other movements and inputs during pedalling, from your upper body and legs, could have some suspension compression, but the combination with the lower leverage ratios and progression should provide a supportive response when you’re out and pedalling around.
As the gears get harder the anti-squat rises. Mid cassette range there’s around a 6% rise in anti-squat at sag to 103%. But the hardest gear sees more of a jump in the anti-squat, to around 116% at sag.
As both Mikes commented from their Field Trip, the Meta TR 29 climbs extremely well and felt supportive due to that combination of high anti-squat, lower leverage ratios and good seat position geometry. They even reported that there was no need for the climb switch on the shock due to how well it climbed.
Cycling the bike through its travel we can see how the anti-squat decreases with travel. This is with a 24-tooth cassette gear. The red dot shows the 100% anti-squat point and the yellow dot shows the instant center of anti-squat.
Higher anti-squat percentages with the single pivot design mean that there is more pedal kickback than if that anti-squat percentage we’re to be achieved with a design that has a virtual instant center. The single pivot design has a fixed IC at the main pivot, with little room to move fore and aft in comparison to a virtual IC from a four bar or short link bike.
The maximum amount of pedal kickback in the lightest 50-tooth gear is 26.9°. This then drops to 13.4° in a 24-tooth cog and down to 6.2° in the 10-tooth.
In an effort to put these pedal kickback figures into a more real-world scenario, rather than just on a graph, we can set up a real-life obstacle and see how fast the bikes we analyse would need to be travelling forwards to avoid any pedal kickback whatsoever. We can set a few parameters and compare the bikes over the coming weeks.
If we were riding along in the 24-tooth gear on the cassette and then rode off a 1m high drop that used 75% of our travel, how fast would we have to be going for the impact to not cause any pedal kickback? We don’t need to look at points of engagement in the hub, as we’re only interested in at above what bike speed the impact would never cause the freehub to catch up to the hub body.
In the case of the Commencal, we’d have to be going 35km/h, or 21.9mph, for the pedal kickback to never be a problem.
If we were in a harder gear, this limit speed would decrease and if we were in an easier gear then it would increase. The change in speed would also reflect the change in pedal kickback from that gear combination.
We can also see how the pedal kickback with a fixed rear wheel and a 24-tooth cassette gear works. The green line that remains is the 0 degree mark.
The single pivot design with its relatively high anti-squat also means that the anti-rise figures are pushed up too. At that same 30% sag the anti-rise is bang on 100% and the curve shape follows the same trend as the anti-squat with close to a straight line and a small drop off as you go through the travel, finishing at 85%.
With that 100% anti-rise at sag there should be no rising of the suspension due to the load transfer from braking with the rear brake only, and as the bike goes deeper into its travel there will be some rising of the suspension due to this load transfer as the percentages drop under 100%.
These higher percentages will usually translate to the bike staying closer to its optimal geometry rather than extending and potentially upsetting the balance of the bike and rider. But remember, anti-rise is only one fragment of explaining how a bike will perform under braking.
Cycling the bike through its travel we can also get a better idea for the anti-rise. The red dot shows the 100% anti-rise point and the yellow dot shows the instant center of the bike, or in this case, the main pivot.
Looking at the main pivot height of the Commencal, it’s pretty much bang on chain line with the 32-tooth chainring, and so contributes to the relatively high levels of anti-squat and anti-rise with this greater distance from the bottom bracket. In comparison to a true high pivot bike the main pivot is fairly low and this can be seen in the axle path.
There is a small amount of rearwards movement to just before sag, just over 1mm. But from then on, it’s a forward’s trajectory for 11.8mm until bottom out.
The Meta TR 29 is designed to be a trail bike, and to do that they have had to really balance all the qualities of the bike. Some of the high pivot bikes out there are a bit more aimed at the downwards side of the hill, and their drastically more rearward axle paths show this. But the Commencal has struck a good balance all through the bike with the geometry, suspension characteristics and even spec. So, the axle path would be more of a result of balancing all these other factors rather than the clear driver for the bike’s direction on, for example, the Forbidden Druid and Deviate Highlander.
Assumptions in Analysis
For all the trail bikes we took size M, and so we adjust our Center of Gravity (CoG) height to 1100mm above the ground.
It’s good to remember that the analyses for anti-squat and anti-rise always assume a static CoG. In the real world this is rarely the case, but needs to be done for analysis’ sake to allow it to be easily calculated and then compared to other designs and bikes. Once we have our analysis it’s then easy to add back in the real-world elements that are relevant to each of us and where we ride our bikes. For more chin scratching about that check out the Enginerding
article on anti-squat.
There’s no industry standard for the fork in anti-squat and anti-rise analysis. We can either fix the fork travel to generate a single curve or we can adjust the fork travel as we go through the rear travel to create a window. For these analyses we leave the fork at fixed at full travel. Again, as long as these assumptions about AS and AR are known and understood, it’s easier to analyse and compare bikes.