Why Steel?

Cy Turner, the Founder of Cotic and Chief Designer, writes about why he decided to use steel for the Cotic droplink full suspension frames

One of the most notable features of the Cotic droplink bikes are their choice of material. Up to a point I've been expecting rolling of eyes and 'what of have those silly steel sniffers at Cotic gone and done now?' and 'why on earth would you use steel on an FS bike? It'll be flexy and heavy, surely?' type questions. I'll be honest with you, before I started this project I'd have been right there with you if someone else had built a steel FS bike. Although we love steel for our hardtail frames, our first full suspension bike, the Hemlock, was aluminium because...well...that's what you make full suspension frames out of, right?! I'd not challenged assumptions at all with that bike, I'd just done what everyone else did. And that was the plan when the original Rocket project kicked off. I was focusing on geometry and suspension feel and all the other improvements that I've talked about in the other essays I've written recently. But a couple of things made me challenge those assumptions.

Sad

Firstly, I'd come back from the trade shows in late 2009 quite disillusioned with the road bike market of all things. At Eurobike there was all the usual carbon loveliness and aluminium swoopiness, even a bit of ti, but anything steel and skinny tyred seemed to be trying incredibly hard to look like it'd been built in a shed in Italy in 1953. It made me sad, because I do love steel as a material for rigid frames. The reputation of our iconic Soul model and the rest of the hardtail ranger is built upon it.

Despite the fact that any frame made from steel would be heavier than the above materials I felt that no one building something modern and forward looking in steel on the road was doing the material, and its fans, a disservice. You could build a road bike with lovely feel and durability at a great price and I thought there was a gap in the market, so I designed a road frame to fulfil this brief. Although we've not moved that project much further forward, I'm really pleased to see that Condor have taken the batten and run with it with their Super Acciao. What this highlighted, when we were talking about the road project, was that what we appreciated about steel: its durability, its strength, its feel and the look. I guess you could say there was an element of dogma involved, but it wasn't that there were no advantages to using steel, it's just that weight wasn't one of them and we liked the other upsides.

Bluff Called

Secondly, as I was kicking around the specification of the new bike with some of the guys I ride with, and one of them asked why I didn't just start with a BFe front end and graft the suspension onto that. His point being that, with its 35mm seat tube and other large diameter tubes, it's incredibly tough and strong and not exactly a shrinking violet when it comes to stiffness. With my firmly held assumptions and 'received wisdom' I dismissed this out of hand, but when I mentioned it to Paul he reminded me of our conversations about road bikes and asked why I hadn't looked at it harder, so now my bluff had been called!

It was time to do some numbers and justify myself properly. One of the key things I wanted to improve on from the Hemlock was the stiffness of the connection between the front and rear ends, so I started with the seat tube as it's where all the suspension pivots would be hanging from. This would be critical. I made a comparison between the 35mm aluminium seat tube we used on the Hemlock and the 35mm seat tube from the BFe. Let's do a science bit now so you know where I'm coming from with this...

The Science Bit

Tubing stiffness comes from two elements; the material stiffness (the Young's Modulus, or E) and the mechanical stiffness (Second moment of area, or I). Combine the two (EI) and you get compare the overall stiffness of the part you're analysing when they aren't in the same material. Usually rigid steel frames exhibit less stiffness than aluminium ones because steel is so strong that you can use it in small diameter, very thin wall tubes, so despite steel being 3 times stiffer than aluminium as a material (E is around 77 for aluminium, around 210 for steel), the mechanical stiffness I is low because of the small diameter and thin wall. Because I is quartically related to diameter (d^4 is an element of the I calculation), increasing diameter from 35mm (usual steel down tube) to 50mm (usual aluminium down tube) makes the mechanical stiffness 4 times larger. And that's before you consider that aluminium needs thicker walls than the steel tube. So the lack of material stiffness in aluminium is overcome by using mechanical stiffness. The reason you can't build aluminium tubes as small and thin as steel ones is because aluminium is also very much weaker than steel (typically 300-400MPa Ultimate Tensile Stength vs 1300MPa for 853), so in simple terms the mechanical stiffness in aluminium tubes is a function of needing to use lots to stop it breaking.

So, that's the simple version of the basis of my comparisons across different materials. The key difference in this case is that the mechanical stiffness is similar. The seat tubes being compared are the same outside diameter - although the steel is much thinner wall - and aluminium can't play its 'big' hand here as you can't go larger on the seat tube without running into all sorts of compatibility problems with front mechs, tyres, seatposts and seatclamps. So where the mechanical stiffness is similar, you mutliply it by the material stiffness (steel is 3 times stiffer than aluminium remember) and what do you know? The steel seat tube is massively stiffer than the aluminium one. Not a little bit, but massively stiffer. Sure it's a little heavier too, but my main concern for this part of the frame is tying the suspension pivots to the seat tube as hard as possibly to give a solid ride feel. So, all of a sudden steel is in the game!

Analysis

From here, the next stage is a full weight analysis of a steel version of the frame. The seat tube was a little heavier than the aluminium one, so I needed to be sure that lot's of 'little bit' heavier didn't add to a whole lot heavier on the whole frame. The comparison was with the final 2011 spec Hemlock. Again, steel has the power to surprise. When you're looking at making a hard riding bike that needs a lot of durability and strength steel comes into its own as it's so strong and durable. Aluminium, conversely, needs to be used copiously in a frame of this type to make up for inherent low strength. That great big 50mm down tube on the Hemlock (our original aluminium suspension frame from 2008-10) weighed about the same as the 38mm steel down tube on the Rocket, but the Rocket down tube is stronger. Same with the top tube. In fact the only place on the frame where it didn't make sense to use steel was the swingarm. We have prototyped a lightweight, high performance steel swingarm, but it was extremely fiddly and expensive to make. Whilst the same could be said of the Reynolds 853 front triangle relative to an aluminium frame to some extent, there are definite performance advantages there and it's mainly the material cost that increases the price (Reynolds 853 is MUCH more expensive than alumnium). It's not especially hard to make in comparison. On the swingarm, aluminium was easily of a similar - and in some cases better - performance to the steel prototype, but as the large machined pieces required for the bearing housings and dropout sections were much easier and better value to make, it made sense to go this way for this part of the frame. So the swingarm is aluminium in nice big sections to tie the pivots and axle together properly. Play to the strengths of the material in the location they need to be used.

Competitive

What we ended up with was the original Rocket26 frame, which was weight competitive with the similar aluminium bikes, but had a level of durability and stiffness which is really high. I also have to come clean at this point and also admit that I love how it looks too. It looks like a Cotic. There, I said it.

Right Place, Right Time

The key thing here is that steel was right for this application, right for the Rocket, where high loads are going into the frame from the long forks and the type of riding a 150mm travel trail/enduro bike encourages. This meant that the high strength of steel made the weight of the frame competitive with other metal frames, but with a level of strength and durability we were really happy with.

Riding styles and expectations for all classes of bike have progressed a lot in the last 4 years, so with the introduction of the other droplink such as the shorter travel Flare model lines, we have taken some weight out of the frames compared to the Rocket, but kept the strength, stiffness and durability high because people are razzing these 120-130mm trail bikes around nearly as hard as a Rocket level enduro bike can be ridden. But with modern 1x drivetrains, tubeless wheelsets and other steps forward in kit technology, it's easy to build a hard riding, steel framed trail bike like the Flare down to a great weight.

So whilst the Rocket, RocketMAX, Flare and FlareMAX are a great use of steel material for trail and enduro applications, we won't be dogmatically using steel for all the other suspension projects we're working on. We're not about to build a 100mm XC race bike in steel, for instance!! Just as with the original Rocket project, I'll sit down and do the numbers and make an informed choice, only this time I won't need pushing into it by other people ;-)


Learn more about the Cotic droplink suspension frames here…

Sign up to the Cotic mailing list to hear more from Cy…

See Rocketman saving the day aboard his steel superbike…

Watch Rocketman's nemesis Straightline …

Order your Cotic droplink bike today…