Vertical Compliance?

At the recent Bespoked pop-up at Ard Rock I was asked a lot about the move from the previous skinny seat stays to the new chunkier ones on the Eponym and Rallye frames…

“Doesn’t it make for a harsher ride though?”

Absolutely not. If it did I wouldn’t have made the change, I replied.

Because once you fit the seat stays to the rear triangle there’s almost zero vertical compliance. In fact there’s very little even before the seat stays are fitted. Any perceived comfort or compliance is coming from somewhere else…

I’ve said elsewhere - that the first part of our suspension is our tyres and that, for a hardtail, is our only suspension! My hardtails have always been designed around bigger tyres and those, plus the wheel itself, will move A LOT more than the rear triangle ever does - in any direction.

That’s why I recommend 2.6” Schwalbe rubber and a good insert (not because I’m paid to, I’m really, really not). It creates a great system for my frames which complements the ride, improves comfort, compliance and grip without feeling too bouncy or too dead. My hardtail frames are designed with the 2.6” tyre/ wheel system in mind from the ground up, to work as a whole. Because that’s the best way to create a feeling of vertical compliance and ‘cushion’ - even float - in a hardtail.

“But steel is real! It flexes right?”

Yeah, it definitely does. Steel tubes for bicycles can be drawn thinner than other materials because steel is stronger. And those thinner-walled tubes can flex and move more than a thicker-walled alu tube (for example). That’s what gives steel frames their lovely ride character. If we then take that a step further and use a very strong, heat treated steel alloy such as Reynolds 853 we can use even thinner tube than the equivalent frame made from 4130 which again is why frames made from the famous Reynolds tubing are said to be comfier, ‘zingier’, more compliant, etc, etc than their 4130 counterpart. It’s truth based in fact, not magic or myth and we feel it in the ride.

“So…why doesn’t that give us flex in the rear end?”

Well, it does…kinda. A well made steel frame will almost always feel nicer to ride than an equivalent frame in another material because of the aforementioned thin walled tube and greater potential for flex. But because of the short, triangulated tubes in the rear triangle they just can’t move that much, regardless of the material. Certainly not enough to provide any meaningful ‘vertical compliance’. Add to that the geometry and shaping of the chainstays required to accommodate a proper mtb tyre, short rear centre and our chainring - you don’t need to be an engineer to imagine how little those bad boys will budge in a vertical plane no matter what type of seat stays complete the rear triangle.

Not buying it? Consider this: if there was some meaningful amount of ‘vertical compliance’ in the rear end tubing structure it would have to be exhibited by both sides of the rear triangle equally and simultaneously on every hit or the wheel would be sloshing about all over the place and our tyre crashing into the stays. How many bumps do we encounter on a mountain bike that are perfectly aligned with the plane of the rear wheel? Without a proper (*suspension*) system in place it would be very difficult to create real and meaningful vertical compliance that also held the wheel in plane.

So no, I don’t believe we can use the rear triangle of a hardtail to create vertical compliance. I don’t think we should even try. The geometry of the rear triangle assembly, for a mountain bike, just isn’t going to do that without some serious compromises.

But…there are some things that we can and should consider to create not only a comfortable ride but all round improved performance in a hardtail mtb.

I already mentioned the wheel/ tyre system. The tyre has a potential ‘travel’ of what?  Up to 50mm? I won’t pretend I know how much a typical 29er mtb wheel can move before it fails because I haven’t spent the time studying it (yet) but I’d wager it’s a lot more than we may think upon first glance. The rear triangle is never even coming close to the amount that the rear wheel system can move regardless of what crazy shit you try with tube shapes, geometry or even ‘flex zones’.

So that’s #1 - a good, aggressive hardtail has to be capable of taking a good size tyre. Simple as that.

I’ve also already touched on #2 - high quality front triangle tubing. It’s known, thinner walled tubing is more compliant, more comfortable and easier to ride. Take two frames with an identical rear triangle, one with a Reynolds 853 front end and one with a thicker-walled 4130 front end and surprise, surprise the Reynolds frame will feel nicer in the rough because the frame works as a whole. Vibration inputs at the axle can be diminished (but not removed) by the front triangle and the rest of the frame.

This brings me to the last but absolutely not the least important way that I believe we can improve the ride of a hardtail. And it’s bound to be controversial, especially among the mtb crowd of 2022:

• weight

Heavier frames feel harsh. You know it’s true! In fact it’s one of the reasons that those mass produced 4130 frames are less comfortable. Sure the flex is a key factor but those heavier-walled tubes are also…heavier! The frames ride ‘dead’ and not in the nice, damped way we’d like (unless you’re capable of warp speed). In the case of a hardtail heavy frames fall into holes and are less keen to come back out. Rather than stay with us on the trail they want to stick to the ground and ‘float’ becomes more difficult to achieve. Last minute hops and un-weighting are harder. We tire quicker and ride slower.

I’m here to say that weight matters, damn it! Especially for hardtails. For a hardtail to ride really well it has to become a true extension of ourselves, and no matter what we do with the geometry or the setup that’s a tall order if the thing is just too flippin’ heavy from the start.

Steel frames may often look alike. But the differences are in the nuance and detail sweated by the designer. I use the highest quality tubes and components alongside geometry that not only produces a great ride but that also requires no more material than absolutely necessary. Indeed this is one of the reasons I switched to the new seat stays. Weight is very much at the forefront of my mind and key in my designs. And simplicity can often be quite a complex puzzle to solve.

So there you have it. This is the conversation I had with those patient enough to stick around and listen to me waffle at the show. It’s not my intention to dispel the ‘myth’ of vertical compliance, more to explain what I believe it is and where it comes from. And that’s not the seat stays.