Stirling Revolution- Bike geometries part 2
1 month ago
In a follow-up article, regular contributor Stirling Kotze JNR delves even deeper into the tricky world of mountain bike geometries.
Along with the frames Head Angle, Seat Angle, Reach and Chainstay length, the last few important geometry figures are the three heights we measure on a bicycle; Bottom Bracket (BB) height, Stack, and Seat Tube Length.
The BB height of a bike is influenced by the total travel the bike has, and should be similar across wheel sizes. However, it will drop about 5mm if you fit a 650 Plus wheelset into a 29er frame (such as the 2017 Santa Cruz High Tower or 2017 Specialized Stumpjumper). A lower BB will improve the cornering by lowering the centre of gravity and modern bike designs have seen their BB heights reduced considerably, but this can make ground clearance an issue with pedal strikes common place and some skilled cadence timing needed on clambering climbs. The Stack is the height of the headtube with the headset inserted. Bigger wheels and longer travel have resulted in a higher handlebar position in relation to the ground. This has pushed bike designers to make the stack as short as possible while still retaining frame strength and stiffness. Tapered headtubes have helped in this regard, as have negative stem angles and flat handlebars. Finally, shorter seat tube lengths have become important on trail and enduro bikes because we are trying to fit longer dropper posts into the frames, plus having more room for good stand over height clearance.
The various measurements and angles of bikes greatly affect the way they ride, their intended purpose and the characteristics of the brand’s design philosophy, and though I do not believe that good geometry numbers mean it’s a good bike, I do believe that more often than not, bad numbers are usually indicative of a bad bike. However, there is one huge glaring issue with the current geometry numbers given by manufacturers and I cannot believe that no one has called out the brands for this seemingly obvious issue … Currently the measurements are given when the bike is unloaded, just standing there with no weight on it, but this is not how the bike is ridden. We are on top of the machine squishing down the suspension and effecting many of the numbers. What I am saying is that the geometry numbers should ideally be given at the manufacturers recommended sag. Manufacturers, please start doing this!
Last month I mentioned that I was going to let you in on the big secrets of modern frame design and they all have to do with the way the rear suspension is penned … What are the “real numbers”? we should all be asking the frame designers about what seldom seems to be published anywhere.
First up, and the easiest to understand is the suspension’s Leverage Ratio. This number changes through the suspension travel but can be simplified as the amount of rear wheel travel divided by the amount of shock stroke. For example, the Pyga OneTen29 has 110mm of rear travel, and uses a 184 x 44mm shock (i.e. 44mm of stroke). That means that the Pyga OneTen29 has an average leverage ratio of 110/44 = 2.5. Most manufactures have in recent times reduced their leverage ratios allowing for suppler suspension and a greater degree of mid stroke tuning. Leverage ratios around 2.5 are today’s norm, with some hardcore XC frames having slightly higher leverage ratios to aid mid stroke firmness and efficiency.
More important than the leverage ratio is the Suspension Curve and this indicates how the force required to depress the suspension increases as the rear wheel moves through its travel. Different linkage designs have different suspension curves and the philosophies of the bike brand (and the patents they own) will influence which type of suspension linkage they employ. You want your suspension to get firmer as you use up your travel, and how hard and how fast it gets hard are the key measures. Suspension that firms up at a regular rate is considered to be linear, whereas suspension that gets harder at a growing (exponential) rate is considered to be progressive. Rear suspension progressivity is influenced by both the design of the frame’s suspension linkage and the rear shock (and its tune) that is being used, and frame designers work closely with the suspension companies to get the characteristics of the frame and the shock to work together and give the suspension feeling that the bike brand believes in. Most shocks in use today are air shocks which are progressive by nature and they get more progressive the harder you pump them, or the smaller you make the positive air chamber (e.g. adding tokens or bands). Most riders want a suspension curve that provides good small bump sensitivity in the initial part of the curve, a supportive middle stroke to handle multiple hits and a rear suspension that ramps up strongly near the end of the stroke to eliminate that horrible bottom-out surprise. Finally, the addition of pedal platforms and climb switches to rear shocks has further aided the frame designers, allowing them to build bikes that handle better when pointed down something technical, and climb more efficiently.
The last and arguably the most important measure is Anti-Squat. Basically, when you put your weight on a bicycle, and when you accelerate forwards, the forces on your bicycle make the suspension squat (downward and backward forces). Added to that, the force of your legs driving down on the pedals also results in some added squat. Thankfully, your chain pulling on your rear cassette forces your wheel back down again resulting in a force called Anti-Squat. The amount of Anti-Squat is determined by the positioning of the rear suspension pivots and is measured as a percentage of the amount of the squat forces it is counteracting. Good Anti-Squat means an efficient suspension when climbing, less bob and more drive of the back wheel into the ground for more traction. Designers aim to have the Anti-Squat percentage starting higher and slowly drop away as the suspension moves through its travel, with an optimal figure of between 100% and 120% Anti-Squat at the manufacturers recommended sag percentage (where you spend most of your time pedaling). Having the Anti-Squat (at sag) slightly above 100% allows the bike to eliminate the Squat forces making it more efficient while climbing, plus the extra bit of Anti-Squat (above 100%) gives more traction to the back wheel and helps eliminate the bobbing effect of your legs driving down. These numbers and the shape of the Anti-Squat curve are hugely effected by the type of suspension employed by the manufacture. Single pivot suspension has a more linear (flatter) curve, whereas Horst-Link can have Anti-Squat that drops away quickly. DW-Link and Virtual pivot can have various Anti-Squat curves with tiny movements in the pivot placements having a huge effect on the curve. For these reasons, frame designers spend a huge amount of time fiddling with their designs to optimize their frames Anti-Squat curves using advanced frame design software.
Now that these critical numbers have been put on your radar, go online to check out some of the technical articles and brilliant YouTube tutorials giving you more insight into what these numbers mean, how they are changing and how they affect you.