For a easy machine, a motorcycle is definitely fairly complicated. The way it rides and the way straightforward it’s to function might be affected by issues resembling its groupset and gearing, wheels and tyres. That’s earlier than you take into account body supplies and building and a motorcycle’s aerodynamics.
However essentially the most vital impression on a motorcycle’s dealing with and journey traits is its body’s geometry. A geometry chart will function within the specs for nearly each bike and may be discovered on-line on bike makers’ and retailers’ product pages.
Normally accompanied by an annotated diagram of the bike’s body, the geometry chart is a sequence of linear and angular measurements which outline a motorcycle’s geometry and can differ by body dimension. However what do all of the figures imply and the way do they have an effect on the bike’s match and journey?
We’ll run by means of how a motorcycle body is measured, with all of the numbers that make up a motorcycle geometry chart, and their significance. It’s value noting that whereas linear measurements could also be quoted in millimetres or centimetres, angles are all the time in levels.
Seat tube size
Seat tube size is the space from the centre of the underside bracket shell to the highest of the seat tube.
It’s an vital measurement to find out bike match and is among the principal measurements used to indicate body dimension, alongside a extra generic S, M, L sizing. Some manufacturers have their very own sizing system although – Colnago is the traditional instance, though different manufacturers resembling Moots go their very own means too.
Seat tube angle
The seat tube angle is the angle that the seat tube makes with the horizontal. Typically, extra racy bikes are likely to have steeper seat tube angles and time trial and triathlon bikes a lot steeper seat tube angles than highway bikes.
The newest race content material, interviews, options, critiques and professional shopping for guides, direct to your inbox!
Some seat tubes are kinked or curved, so you may even see a digital seat tube angle on a geo chart. Suspension might also change the seat tube angle, as most programs have sag in them, the place the rear of the bike is compressed when a rider sits on it, altering the seat tube angle. In such circumstances, you may even see an efficient seat tube angle quoted in addition to a sag-free angle.
Head tube size
The pinnacle tube size is the space from the underside to the highest of the bike’s head tube. Typically, an extended head tube will improve a motorcycle’s stack (see beneath) and supply a extra upright journey place, so that you’re extra prone to see an extended head tube in an endurance bike.
There are exceptions although, because the body stack is a mix of the pinnacle tube and fork size. The pattern to suit wider (and subsequently taller) tyres signifies that fork size to the fork crown (the bit the place it meets the body) could also be longer than on a motorcycle designed for narrower tyres, offering further clearance, and programs resembling Specialised’s Future Shock additionally add top to the fork. In such circumstances, a shorter head tube size could not point out a decrease stack and racier driving place.
Head tube angle
As with the seat tube, the pinnacle tube will make an angle with the horizontal. That is often higher than 70 levels on efficiency highway race bikes. Gravel bikes could have a slacker head tube angle and the pattern is in the direction of decrease angles, in some circumstances significantly beneath 70 levels.
Prime tube size
Prime tube size was once a foremost measurement used to indicate bike sizing. However the creation of the sloping prime tube, which was first launched on the Large TCR in 1997, signifies that the precise prime tube size is very depending on the body’s design.
So now you’ll often see a horizontal prime tube/digital prime tube measurement quoted in bike geo charts, typically accompanied by an precise prime tube measurement. Digital prime tube is the space from the highest centre of the pinnacle tube horizontally to the place the measurement intersects the seat tube/seatpost and so is impartial of the particular prime tube slope.
Fork rake
The fork rake is the space that the fork dropouts are in entrance of a straight line persevering with on the similar angle as the pinnacle tube. A higher rake will put the entrance wheel additional in entrance of you.
It’s one of many explanation why traditional metal bikes have curvy fork blades, though it’s now extra standard for the fork blades to be straight, however angled on the fork crown.
Path
Path is the horizontal distance between a continuation of the fork rake to the bottom and a vertical line from the axle dropout to the bottom. The path impacts how edgy the steering feels, with an extended path including stability. Counterintuitively, a higher fork rake will lead to a shorter path.
Path can also be affected by wheel and tyre diameter, so in some circumstances bike manufacturers will quote path figures for various tyre sizes. It’s the explanation why some manufacturers, together with Cervélo and Ruut incorporate a ‘flip chip’ within the entrance dropout of their gravel bikes to change dealing with traits.
Backside bracket drop
The centre of the underside bracket is nearer to the bottom than the bike’s axles. How a lot nearer is measured by the underside bracket drop. A bigger backside bracket drop will have a tendency to boost a motorcycle’s stability, whereas much less drop will make for an edgier journey.
A decrease backside bracket can even put your pedals nearer to the bottom, and can lead to pedal strike in excessive circumstances in the event you’re pedalling whereas leaning over very arduous in a nook. In an effort to take care of driving off-camber slopes cyclocross bikes typically have fairly excessive backside brackets, or a really small backside bracket drop determine.
Backside bracket top
The counterparty to backside bracket drop is backside bracket top, the space from the bottom to the centre of the underside bracket axle. On the excessive, this can impression floor clearance, notably for a gravel bike which can be ridden over tree roots and uneven terrain. However extra pertinently, it’s one of many components that impacts how far you may lean a motorcycle right into a flip with out the chance of grounding a pedal.
Chainstay size
The chainstay size is the space between the centre of the underside bracket and the rear axle. It impacts tyre clearance, but additionally the bike’s journey really feel. Typically, bike designers will intention to maintain the chainstay size as brief as attainable, whereas nonetheless offering satisfactory clearance for tyres appropriate for the bike’s meant use. You’ll typically see bike manufacturers highlighting a efficiency bike’s brief chainstays and snappy dealing with.
Entrance centre
The entrance centre is the space between the centre of the bike’s backside bracket and the entrance wheel axle. It’s a product of a body’s different dimensions, principally its prime tube size, head tube angle, and fork rake.
The entrance centre impacts toe overlap with the entrance wheel. On bigger sized frames, that is usually not a problem, however for smaller frames, toe overlap may be an vital constraint affecting bike security. That’s why smaller frames typically have a decrease (slacker) head tube angle than bigger sized frames for a similar bike, because it locations the entrance wheel additional ahead and helps cut back toe overlap.
This, in flip, impacts the smaller bike’s dealing with, which is why some manufacturers match their smaller sized highway bikes with smaller diameter 650b wheels, to supply extra constant geometry, and therefore dealing with, to bigger frames.
Wheelbase
The mixture of the entrance centre and the chainstay size approximates to the bike’s wheelbase (it’s not fairly the sum of the 2, because of the backside bracket being decrease than the axles, in order that the wheelbase is the longest aspect of a triangle product of the opposite two measurements).
Typically, an extended wheelbase provides stability to a motorcycle’s dealing with, whereas a shorter wheelbase makes for a extra racy really feel.
Standover
Standover is a measurement of the space from the bottom to the highest of the highest tube at its lowest level. A body’s standover needs to be low sufficient which you can put each toes on the bottom when standing over the bike’s prime tube.
It’s one other gauge of whether or not a motorcycle is the right dimension for you, though with most bike frames having a sloping prime tube, it’s much less vital than it was once.
Stack
The variety of variables in a motorcycle body that may have an effect on match has led to the usage of stack and attain as extra dependable and constant measurements of how a motorcycle will match.
Stack is the distinction in vertical distance between the center of the underside bracket and the highest of the pinnacle tube. It’s unaffected by different components of a motorcycle’s geometry and a typical measurement quoted in nearly all bike geometry charts.
Attain
Attain is the complement of stack. It’s the horizontal distance between the underside bracket and the centre of the highest of the pinnacle tube and signifies how stretched out you’ll be as you journey. Usually talking, a race bike body might be designed with an extended attain and decrease stack, whereas an endurance bike can have a shorter attain and better stack.
Attain and stack aren’t measurements that may all the time enable direct comparisons between bikes although, as issues such because the handlebar and stem design and sizing can even affect the way it feels to journey a motorcycle.
Because of this, you may even see stack and attain to the handlebar tops quoted by some manufacturers. Canyon, for instance, quotes what it calls stack+ and attain+ for its bike, alongside stack and attain for the body, to take this into consideration. Wilier calls the identical mixture of measurements Accu-Match.
Now you already know what all of the measurements on a motorcycle geometry chart imply, you are in nice form to make use of them to assist select the finest highway bike or finest gravel bike – or certainly any sort of motorcycle – to fulfill your wants.
