Why Steel? For Performance!

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. We get asked why we have done this. Most people expect us to say it's because we make our hardtails out of it, but it's much more than that. "It'll be flexy and heavy, surely?" they say. I'll be honest with you, 10 years ago before I started the original Rocket 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. 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 didn't move that project any further forward, I'm really pleased to see that Condor have taken the batten and run with it with their Super Acciao. More recently, our friends are Mason Progressive Cycles have knocked it out of the park with their Resolution disc road machine. 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 at all, there are a lot of performance benefits.

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...

Some Maths....

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 and RocketMAX, but the steel 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 aluminium). 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.

Breeeaaathe

For all this talk of maintaining stiffness in the frames, this was - and still is - mainly concerned with the seat tube area of the bike. Making sure the front is tied to the rear solidly. Despite the strength and durability advantages of our Reynolds 853 top and down tubes compared to other materials, they aren't as stiff. However, the more I've ridden our bikes, and the more feedback I've got from testers, owners and journalists, the more I've come to realise that the famous 'steel feel' is just as obvious and advantageous in suspension bikes as it is on hardtails.

What the top and down tubes allow the bike to do is twist a little along its length. Top tubes in particular are very much the defining feature of the ride character of a bike, and bike to flex just a little bit along its length as you cover rough terrain is a huge advantage in terms of traction and confidence. It's what makes good steel hardtails feel so great, and I love the phrase 'breathing with the trail'. Instead of "Stiffer is better. MORE STIFF", let the bike give a little across those bumps. This is especially true on cambers where the hits are no longer in line with the suspension movement. The ability of the bike to mould itself to the terrain instead of being pinged off the line is a great trait of a steel bike - whether it has springs or not!

A great analogy comes from MotoGP. These guys get over to crazy lean angles, and at those angles, if you hit a bump the suspension won't absorb it - the force is going at 40 degrees to the suspension movement plane. A few years ago, the 'stiffer is better' mentality got into the bikes, and Ducati in particular built a carbon chassis bike with the engine as a stressed member, with incredible stiffness. And it was borderline unridable, because it had hardly any grip at all when angled over in the corners, and they couldn't change the engine block to tune the stiffness. They eventually went back to a fabricated metal frame where they could tune the stiffness/flex more easily and become considerably more competitive. It's the same on a smaller scale with mountain bikes: We corner with some fairly large angles on the bike, across rough ground with bump forces going nowhere near the plane of the suspension movement.

Now - obviously - too much flex is a bad thing. You want the wheels to follow eachother, and I've had bikes that weren't stiff enough so I know where I want to pitch the frame stiffness. But there is definitely some performance advantage to be had by having a little give in your frame and wheels, and letting it breathe.

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. What's also happened since the original Rocket26 has launched is that aluminium frames - enduro frame in particular - have got much heavier in order to maintain the level of strength and durability they require. It's not at all unusual to see aluminium trail and enduro frames in the 7-8lb weight range, which means unless you go carbon there isn't even a weight downside to using steel anymore. I also have to come clean at this point and also admit that I love how it looks too. droplink bikes look like Cotics.

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 and FlareMAX 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 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. 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!