Size Specific Geometry….

Written by Admin - 5th March 2020

Chainstays, seat angles, head tube lengths, etc…


I just read a tech blog from a company known for their ‘progressive’ geometry arguing that although their hard tails get size dependant swingarm (Chain-stay) lengths, it’s not necessary for a full suspension bike.

Well, of course it’s not ‘necessary’ but it may be desirable!…

To talk, ‘necessary’, then suspension, is not necessary, decent angles are not necessary, decent brakes are not necessary… I used to ride a tiny late 1980s GT Avalanche and always used to have a huge amount of fun! I rode it down steep slopes and crashed it through the rocks just as I do my G1 now… I was not as fast or safe and I couldn’t climb such steep technical trails but it WAS fun. In the past, WAS…


But I don’t want to ever have to ride a bike like that again though! My modern G1 with better suspension, angles, brakes and other components is a joy to ride and there’s no way to go back!


So, I’m 185cm tall and what I ride (at the moment, angles, geometry, wheel size and components change often as I try things!) is an XL G1 with 450 Chain-stay, 335 BB height, 61.5° head angle, 175 travel front and rear, 41.5mm fork offset, 27.5mm stem length, 110 bar height. I have the saddle tilted down and the seat rails slammed right forward. I have 170 cranks, Mallet DHs, 32T chainring and 10-42T cassette out back… I run a 29er front with Magic Mary super soft Super gravity and I run a 27.5 rear with Maxxis DD Aggressor… DT Swiss EX 471 rims at 25mm internal width… There are some more pertinent measurements but for illustrative purposes that’s enough!!!


That’s how mine is configured… Not one single bike has left us for a customer in that configuration! We have different mutators for Chain-stay length and for seat stay lengths so we can adjust BB heights and angles to cater for different types of people with different types of riding styles, different wheel sizes (or combinations of) and different trail preferences.


And that’s the point… We at GeoMetron tend to ride with customers and test with customers as much as humanly possible. Although technically we are ‘direct sales’, that means direct to retail customers face to face and not just ‘internet sales’…. If we are to change our sales model and put in a buy button then we would simply reduce options because it’s way easier. It wouldn’t matter because we would never have to ride behind our customers wondering why ‘The Guy’ on the XXL is struggling to keep the nose of the bike down on climbs when our test ride chaperone on a medium bike isn’t! Well, if the medium rider is climbing comfortably with his relationship between saddle position and rear axle then ‘The Guy’ on the XXL will need a longer Chain-stay or a steeper seat angle (or a bit of both) to achieve that. Because we do ride with the customer (and care) we have the ability to configure the G1 with different Chain-stay lengths or different wheel sizes, different BB heights, different fork offsets, different head tube heights, different head angles, etc… We do this to make the bike work at its best for the customer and every customer is different…


So whilst ‘The Guy’ on the XXL who likes flat pedals and likes to stand up to climb steep stuff might like the shortest Chain-stay length, ‘The Next Guy’ with a tendency to sit climbing and with clip in shoes might like the longest option. They may also like different sized wheels for different reasons, and different geometries, etc… If we didn’t spend so much time with customers we wouldn’t find that out and we’d maybe even produce some ‘marketing engineering’ PR to explain it!


Whilst the ‘loading’ of the bike has some bearing on the relationship between Chain-stay length and front centre length it is as relevant in 3D cornering as the 2D side-on line drawing used for geometry sheets… The front tyre gets loaded not just became of the rider’s fore/aft position but also because the bike and rider are a mass in motion which wants to carry on (as per Newtonian physics) in a straight line. At the first part of the turn, the front tyre is loaded more… As the turn progresses towards the middle, the front and rear wheels will be loaded equally (the imaginary ‘intended direction’ of the bike and rider mass now angled a bit further round the turn and some of that ‘forward’ energy has been used to change direction) and as the turn finishes the rear wheel will be loaded more and the direction of the bike rider mass will keep coming around until the bike is stood up completely and the bike/rider combo is now moving in a different direction!


But… As above. All of this depends on the rider of the bike. Some riders are more dynamic and add load using their own strength as multipliers to change direction, change weight distribution or clear or use trail features. Some are simply less dynamic… A ‘click to buy’ button can’t see the rider and can’t change the chain-stay length or any other adjustment to suit the rider… WE CAN!


But all of that said! We still believe that there are some basic, ‘size dependant’ measurements and the chain-stay is certainly one of these!