Ovalform
Founder and Cotic frame designer Cy Turner writes....
When developing the second generation Soul we were faced with a problem - how to increase the strength of the frame for 140mm forks and the new CEN14766 testing regulations (update 2015 - now ISO4210) without ruining the ride feel or adding weight.
One the main driving factors for ride feel and compliance on a frame isn't - as you might expect - the seatstays. It's the top tube. As your weight bears down on the middle of the bike through the pedals, via the bottom bracket and into the frame, the wheels splay apart and one of the main facilitators of this flex in the frame is the top tube. So, ride comfort is intrinsically linked with the top tube that is relatively soft in bending when the bike is viewed from the side.
Now, if your top tube is round - like the original Soul, BFe and Soda - the stiffness in bending is the same in all directions, so your nice compliant top tube is also relatively flexible in the plan view - i.e. as if you are looking down at it when in the saddle. This bending mode is important for precision in the handling. When you yank on the bars, a lot of the load is trying to bend the top tube sideways as you look down on it. If this isn't stiff enough you get a vague ride feel and slightly weird, disconnected feel between the front and rear of the bike. If it's too stiff it's one of the factors that creates a jarring ride where the bike pings off trail chatter, instead of moulding around them a little (see also aluminium frames). The fact that steel - and in particular the Reynolds 853 we use on our flagship frames - allows this tube to be relatively flexible whilst being incredibly strong means we get that fantastic ride feel with plenty of strength and durability.
However, for the new Soul and BFe, several factors were putting pressure on the poor old top tube. Firstly the combined effects of the new regulations and wanting to be able to put longer 140mm forks on the Soul meant the new bike had to be quite a bit stronger than the original frame. Secondly, bars are getting progressively wider as time goes by, which means that leverage and steering input that the rider exerts is getting higher too. Finally, I set myself the targets of not increasing the weight of the Soul for the new regs, and obviously I didn't want to ruin the legendarily good ride feel of the Soul with a massive stiff tube.
To get the ride feel I wanted, I need some more stiffness laterally (looking down from the saddle), but I didn't want to add vertical stiffness (looking from the side of the bike). Also, adding vertical stiffness made meeting the test regulations harder in some ways, because when you make something stiffer it flexes less, which increases the stresses. Up to a point you need to balance letting the frame flex a little in order for the stresses to spread out evenly around the frame.The solution presented itself in the form - literally - of an oval tube. Our original Soul top tube was an old school road bike down tube specification originally. That being the case, there was an oval option from the days of 'aero' steel time trial bikes. These used to orientated vertically to keep the bike as narrow as possible. What I needed was the opposite; wide a possible laterally, with a reduced section vertically for compliance. It was simply a shaped version of the tube we used before, so there was no weight penalty at all, but there was an increase in lateral stiffness of 14%, and a reduction in vertical stiffness of 19%. So, improved compliance AND increased precision. Winner!
It works brilliantly across all platforms of bike, so our signature Ovalform top tubes are now a feature of all our bikes.