Egerie Aston-A-Tron - PT4 Kinematics
I’ve ridden loads of different linkage bikes in various guises and enjoyed many of them, but there’s nothing wrong with a good single-pivot bike. Many people have preconceived ideas of how SP bikes don’t work that well because they rode a crap one in 1998, multi-pivot-marketing has gotten to them or they have spent too much time looking at graphs.
You can get most of the characteristics needed to make a great bike with one pivot and its excellent low friction and low maintenance layout, and modern tech like Ochain and well-tuned suspension can give you an excellent ride.
In its current setup, this bike has 200/200mm of travel with a 250/75mm shock. I requested extra shock mounts to use a shorter fork and shock to give 160mm travel. My idea here was that even though the bike is responsive in the long travel setting due to the linear ratio and great suspension, the main difference in response is the travel and wheels: swap out the heavy DH tyres and wheels for something lightweight and fast-rolling, swap to a shorter fork and shock, and you’ve got a sweet bike for UK-style trail centre type riding or ‘extreme downcountry.’
For where I live and ride, I will keep it in the long travel version and just get on with the job of pedalling it up, but receive the rewards on the downs. Anyway, nobody goes to the gym to pick up the lightest weights they can, do they?
Leverage Ratio
The leverage ratio in 200mm travel gives about 5.6% progression (left), and in shorter travel, 8.6% (right), which can be adjusted slightly with the extra shock mount holes. I love the feeling of a more linear bike as they offer all the mid-stroke support you need. The bike is much more predictable, the suspension is easier to tune and feel, and needing all that progression to avoid bottom-out is a fallacy if you have a correctly tuned shock and decent bottom-out bumper (steel tubes also take the sting nicely out of harsh bottom-out events). The majority of current shocks on the market don’t offer enough compression force as standard for a single pivot bike, so people think they need a progressive linkage or shock to prevent bottom-out.
When testing the Commencal Supreme with two different linkages, approximately 40% progression and 20% progression, using a data system with Rulezman, we found on a 2-metre drop the less progressive setup actually used 10mm less travel and pushed only 20% of the G-force into the rider than the more progressive linkage - the opposite of what is expected. This is because the bike absorbs more force during the initial phases of the travel and has less to deal with when it gets to the end-stroke.
I find in this setup (and on the other Egeries) that most of the time I’m using 90% of the travel while riding, and only on special occasions is there an impact hard enough to bottom out. In these cases, it’s easy to see it coming and brace, plus that kind of force would bottom out nearly any bike.
Anti-Rise
I don’t like bikes with lower anti-rise values, as I find they allow the bike to pitch forward too much under braking. Of course, this isn’t only due to the kinematic but the overall setup of the bike. I find that 100% at sag is ideal for most riding but still prefer a bit more for steep downhill. This bike is 118% at 60mm sag and the high-pivot Titanium version is 128%.
After about 60-70% travel the AR isn’t important as you can’t really be braking and having that size of impact simultaneously. Before the sag point, a higher value can help pull the back wheel down towards sag when feathering the brakes over rough terrain.
How do I know it’s working well for me? I ride down a road about 20kmh, brake as hard as I can with the rear brake only and watch the shock for any extension. If it stays in the same place in the sag, great, if not it needs something more.
Then progressing to the trail I should be able to brake as hard as I can with both brakes before locking the wheels, I’ll be in a braced and upright position and won’t need to move my bodyweight/hips backwards to keep balanced.
Anti-Squat
Anti-squat on a single-pivot bike is generally really good with the pivot close to the top of the chainring.
The AS on this frame is around 142% at 60mm sag and 130% on the Titanium high-pivot version. As you get deeper into the travel the AS is not important as you won’t be pedalling. In harder gears, you accelerate less so it can be lower without a problem, although on this design the AS increases in the harder gears too.
Again suspension setup comes into play and this bike has a nice custom-tuned shock for support, this bike pedals really well seated on roads and sprints with no bobbing. The only place it could be more supportive is on technical climbs through dips and up steps to support your bodyweight movements more - I’ve found the best solution for this issue on any bike is a Cane Creek DB or Kitsuma whose climb switch makes any bike awesome on the pedals.
One downside of anti-squat is pedal kickback, especially for flat pedal riders. On my other bikes, I have a high pivot + idler whereas here I designed this bike around the OChain. I found using the Ochain R version with the easy-to-switch degrees of float that 4º is fine for the majority of riding to just take the sting out of any chain tension. There are 20º of PK across the entire 200mm travel, but the majority of bumps are much smaller. For a few rougher bike park days I switched up to 12º.
High V Low Pivot?
I had tons of questions about this: why this bike isn’t a high pivot and which is better. This was something I wanted to double-check on very similar machines.
Axle path on the low pivot gives about 3mm rearward travel.
TheAxle path on the high-pivot gives about 11mm rearward travel.
This bike isn’t a high pivot as I wanted it to have a more sporty feel and also be switched in to trail/extreme downcountry mode with lighter wheels and shorter travel. It did this precisely, it’s better on the pedals thanks to increased AS and the direct connection between the chainring and cassette, but doesn’t quite eat up the bumps so well thanks to the less-rearward axle path.
The higher pivot bikes are better in the rough surely, but are more of a drag on the climbs. The titanium bike eats up the bumps better, but it also has the sprung/unsprung mass advantage of the Pinion compared to the heavy 12spd XT on the rear axle of this bike.
Verdict
I started this article saying that there is nothing wrong with a single pivot, and I really believe that to be true and the proof is in the pudding. If a bike pedals damn well, is balanced on the brakes, is supple with plenty of midstroke and bottom-out support then that’s nearly all of the battle won!
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