Month: October 2020

As sure as the day is long, post a picture of my bikes on Pinkbike and someone will say the seat stays are far too thin.

“I could snap them just by looking at them” they’ll say… or something similar.

Let’s be clear here. Steel IS STRONG. 

Think about the tiny screws holding up your shelves, they’re plenty strong enough to support a lot of weight. 

Think about the thin steel wires used to support telegraph poles and the like.  An 853-steel wire with an area of 1mm² could easily support your weight.

The UTS (Ultimate Tensile Strength) of a 7005 aluminium typically used to build a bike is around 350MPa.  The heat-treated steel used for my swingarms is around 900MPa.  

But static tensile strength is only half the issue. 

Aluminium fatigues, that is – multiple load cycles – cause cracks to initiate and ultimately grow.  Because of this behaviour, you need to add extra material to counteract this potential behaviour. 

Steel doesn’t exhibit fatigue behaviour to anywhere near the same level as aluminium and it isn’t a design driver. 

So the short story is that aluminium swingarms need to be a much bigger diameter and use much thicker tubes to stop them from failing when compared to steel tubes.  It’s just that people are familiar with aluminium tubes and believe that’s what all material should be like.

But what about carbon? 

Well, carbon used in a quasi-isotropic manner (i.e. strength is not biased towards any direction) actually has strength similar to aluminium. But it doesn’t suffer from fatigue issues, so it can potentially use less material. 

However, manufacturing constraints and the need for internal tooling mean making very small diameter carbon tubes is very difficult, larger diameter tubes are much easier.  So, again you need to use much more material than with steel resulting in bigger, thicker tubes.

To summarise. Steel IS STRONG!  My thin seat stays are plenty strong enough and the compliance they give results in the great ride feel and grip of my bikes.

The steel construction of my swingarms, and the single pivot design, makes them more flexible under the loads acting on them. 

“But that will knacker you shock shaft!”, the ill-informed keyboard warriors often cry…

Any engineer worth their salt will tell you that load tracks along the stiffest path. 

If you make the load path for lateral loads (sideways loads acting on the rear wheel) super stiff, the force will go that way.  Typically, the load is either taken by the chainstays or seat stays. 

A stiff mounting for the shock, such as a yoke or a swing link, means more load will track via the seat stays and then via the shock to the mainframe.  Make this load-path flexible and all the lateral load will track via the chainstays.  

There may be a lot of movement in the shock, but this does not equate to load.  As long as the shock can accommodate this movement, there will be no issues.  Spherical bearings do a good job of making the shock free from loading.  But the plastic top hat type shock bushes also do a pretty good job – the bushes do wear pretty quickly, but this is a £2 easily replaceable part, so no worries.

So some flexibility in the way the shock is fixed is a good thing.  This is counter-intuitive to many people, but it’s true. Speak to any suspension service centre and they will tell you about bikes with stiff yokes or linkages destroying shock shafts on a regular basis.

Starling Cycles bikes have quite low bottom brackets compared to many other manufacturers. 

A low BB drops the centre of gravity of the rider a little bit and helps with cornering.  I think there’s also a strong relationship with BB drop and stability of the bike, but I need to further understand the science behind that.

Low bottom brackets are good for stability and cornering, but there’s a common belief it results in pedal strikes and crashes  This is true to a tiny effect, but we’re only talking about 20mm lower than other brands in the most extreme cases. 

In most cases, pedal strikes comes from one of two other reasons…

The first reason is bad technique.  People who try to pedal through technical sections tend to whack their pedals.  Getting your timing right is hard and takes time to learn.  Using the ground contours to help generate speed without pedalling can also help. 

Also, freewheeling through technical sections with your cranks vertical is not a good idea, vertical cranks compared to horizontal gives, say 170mm difference in pedal strike height, way more than BB differences.

You may not necessarily ride with your cranks in the 12/6 o’clock position, but anything less than horizontal and you’re instantly dropping that lowest point way below your BB.

However, the main reason for pedal strikes is bad suspension set up.  A poorly set up shock that has too much sag or blows through the mid-stroke on the smallest hit will sit way lower in its travel.  Similarly, a bike that bobs as you pedal (not something you see on a Starling) will have a higher risk of pedal strikes. 

Again, these behaviours will have way more impact on pedal strike than static BB height differences.

So, if you are whacking your pedals, check your technique and suspension, before blaming BB height!

Starling Cycles started out making custom sized bicycles for a wide variety of customers. 

This gave us a wealth of knowledge about how bike sizing works best for mountain bikers. 

We’ve used this knowledge to ensure our standard sized bikes fit as well as they can.  We also take note of considerations such as modern dropper post requirements, stem lengths, chainstay lengths and seat tube angles.

Start With Wheel Size

When selecting which bike is best for you, we’d recommend starting with wheel size. Broadly speaking, the 29″ Starling Murmur is better for taller riders, the 27.5″ Starling Swoop for smaller riders. 

But bear in mind the discussion about each bike’s bias towards certain types of riding. The Murmur is better suited to flat out speed, ideal for racers. The Swoop leans towards slashing turns and playful riding. The Starling Twist, our mullet bike, might well offer riders an ideal midground.

But What About Reach?

Once you’ve made a decision about frame model, height is a pretty good starting point for sizing. People often tell me they have long bodies so need a longer bike, but longer arms often mean shorter legs for any given height, so these things tend to average out.  The bike pages give sizing guides based upon height.

Often people tell me they like a certain reach on their bikes.  Reach is useful but is perhaps more a measure of the stability (along with chainstay length) you will achieve with the bike.  But it’s not necessarily the best ergonomic measure for a bike to be ridden up, down and along.  

When stood up on the pedals we can easily accommodate big changes in reach.  But when seated, with our bum position fixed, it is harder to accommodate a big reach increase. So to counter this longer reach, seat tube angles have got steeper (be careful of some manufacturers who haven’t quite worked this out yet!).

What we end up in is a position where to size a bike for the rider, the reach becomes less important, and the old measurement of effective top tube reverts to being the best ergonomic measurement to use.

Effective top tube (horizontal length from top of head tube to seat post) isn’t perfect, it doesn’t take into account saddle height, bar height, stem length and bar roll. But it’s pretty good and with saddle and bar changes you can get near something that fits you.

Seat Tube Length

Seat Tube Length is another aspect of bike sizing we need to consider.  But luckily it’s an easy one. We just need to understand the pedalling height of your saddle, and the dropper you intend to use.  

Measure your existing pedalling seat position at full extension; from saddle rails to centre of BB. For example, my measurement is 740mm.  You then need to compare this measurement by the extension of your chosen dropper, for example a 160mm BikeYoke dropper has a minimum extension of 202mm, and a maximum extension of 334mm.

For a large sized frame, seat tube is 440mm. Considering my saddle height, 740mm, minus minimum extension, 202mm, we have 538mm, so 98mm of post showing, all good.

For 740mm, minus maximum extension, 334mm, we have 406mm, so with a 440mm seat tube, we will not exceed maximum insertion.

And If That Still Doesn’t Work…

But if you feel that the current range isn’t quite right; either you are between sizes, or outside the range of sizes, or are a full on ripper and want the bike a degree or two slacker, don’t worry too much. 

Starling Cycle can build you a bike to your exact fit, see our custom frames page.

Starling Cycles started making bikes when the “long, low, slack” geometry movement was in its infancy. 

With early prototypes, we experimented with a range of geometry configurations. We tried super low BBs, super slack head angles, long and short reach, slack and steep seat angles, long and short chainstays… And more.

We found that a longer and slacker bike with a reasonably low BB and longer stays worked well, albeit within reason. 

If you’re a strong, powerful rider capable of manhandling the bike around in rough terrain, or regularly ride very fast tracks, you might benefit from a slightly longer and slacker bike.  Similarly, if you only ride super tight tracks, there’s a slight benefit from a steeper bike… but not much!  

We also found a strong relationship between front-centre and chainstay length. The longer front-centre compliments the longer stays. But it’s not quite that simple, longer stays do make the bike more stable, but also harder to manual and generate pop.  So longer stays are better for a rider who takes the direct line and carves corners, the shorter stays for those who pop off every root and lip and like to smash corners.

There’s also the effect of wheel size and weight, and there’s a whole article on that elsewhere here in the Starling Tech Journal. Similar to chainstay length, bigger and heavier wheels are suited to bigger bikes and more stability.  Smaller, lighter wheels are more suited to pop and manoeuvrability. 

There’s also one other aspect to smaller wheels, that for smaller riders the smaller wheel helps reduce the chance of the rear wheel hitting your bum on steep drops.

The geometry used on Starling bikes is good for pretty much everyone… except for those who aren’t honest with themselves and their capability. 

The range of wheel sizes on different models and chainstay length means most people can find the right bike. It should be noted that a small size bike is not available with the longer stays and bigger wheels of the Murmur. The balance of reach to chainstay length is not correct. Similarly, there’s no XL option with the smaller wheeled Swoop, it just doesn’t work as well.

It’s quite interesting to see that nearly all of the major brands are now zeroing in on sizing that Starling Cycles first used 3-4 years ago.  

Popular bike marketing suggests increased bike stiffness is a positive thing. 

I believe this has historically come from the road bike world where stiffness is believed to be important for pedalling performance. 

Firstly, this isn’t true, a bike only needs to be stiff enough that you can generate full leg power, any stiffer doesn’t increase pedalling performance. 

Secondly, pedalling just isn’t as important for gravity fuelled mountain biking, and let’s be honest, that’s where the fun is and where we want our bikes to perform the best. Sure, you want your enduro bike to pedal well but it doesn’t need to win the Tour de France. Stiffness is less important than, say, on a road bike.

Lateral Flex and Your Mountain Bike Frame

Then we get the argument that you need a stiff bike for the suspension to work correctly.  Why let any movement happen in the frame rather than in the suspension?  I totally agree with this argument, but it’s overly simplistic and needs some expansion.  

When the bike is vertical and the bumps are acting in this direction, yes it makes sense that our very expensive suspension unit should be allowed to work as well as possible.  But in reality, all frames, even skinny tubed steel ones, are massively stiff in the vertical direction.  The overall depth of the frame, the distance between the tubes, dominates the stiffness equations.  They are plenty stiff enough!

But what happens, when you lean the bike over in a corner or an off-camber section?  In this case, the bike is no longer vertical to the ground and any bump forces will act to bend the frame sideways, let’s call this lateral flex. 

At 45° lean, only half the bump forces act in the plane of the suspension, the other half acting to bend the frame sideways.  If your frame is massively stiff in the direction then it will not be able to conform to the forces from the ground and you will be losing grip. 

A laterally compliant frame will be able to soak up these bumps giving grip and control.  In an ideal world, we would design suspension to work in this direction, but until then, steel offers a great opportunity to add this compliance in.

And Why Steel?

So why is steel better suited to a laterally compliant frame?

Firstly, let’s get it clear, steel is stiffer and stronger than both than both carbon and aluminium for a given amount of material, but it is denser. 

The less-dense and weaker materials lend themselves to large-diameter tubes with thicker walls for impact resistance.  To keep steel wall thicknesses acceptable for impact resistance, the diameter needs to be much less. 

The small diameter steel tubes are plenty strong enough, but tube bending and torsional stiffness is a function of tube diameter cubed.  So, these smaller tubes are more flexible.  So naturally carbon and aluminium lend themselves to large diameter stiff structures, steel to smaller compliant ones.

Starling Cycles founder Joe McEwan loves steel bikes, single pivot suspension and coil shocks.

Here’s his take on why he’s such a big fan of simple, steel bikes.

Take it away, Joe:

In my opinion, a simple and elegant design is always a better option than a more complex one. 

It’s easy to add complexity but much harder to keep a design solution simple and efficient. 

I believe a single-pivot suspension design is efficient and can work as well as the best multi-pivots, especially when combined with a steel swing arm. 

More isn’t Better

A single pivot design has fewer bearings, not only is it better for maintenance, but this means the suspension will be more free-moving, contributing to more sensitive suspension.  A badly maintained single-pivot will always work better than a badly maintained multi pivot!

Progressive vs Linear

A single pivot naturally has a leverage ratio that is more or less linear through the full travel.  Due to a hard marketing push of the word “progressive”, linear suspension is deemed to be a bad thing.  But a single pivot allows for a supple initial stroke with good mid-stroke support. 

The linear ratio is judged to be a negative trait in that it doesn’t ramp up to provide good bottom-out capability (where progressive is deemed to be good).  But, I suppose, first of all we need to be honest with ourselves and do we really huck to flat that often?  Even if you do, the natural ramp up in an air spring or the big rubber bumper (or hydraulic bottom out) in a coil shock, goes a long way to helping. 

But the biggest factor to a smooth bottom out capability in Starling frames is the compliance of the steel frames.  There’s enough ‘give’ in the frame to acts as an extra bit of top-end suspension.

Coil Shocks

Starling is also a great believer in using coil shocks.  Air shocks are not bad, but due to frame designers seeking out ‘progressiveness’ to keep their marketing colleagues happy, modern air shocks have become more linear to overcome this, good but not as good as a coil. 

Air shocks tend to suffer from initial stiction. You can reduce pressure to overcome this but it results in blowing through the mid-stroke.  I’m sure we have all suffered from trying to find the balance with an air shock, too much pressure and it’s harsh, too little and it has no support.  

Coil shocks, on the other hand, have smaller diameter shafts and looser tolerance seals (not required to keep air in). They don’t suffer from this initial stiction and are supple at the start of the stroke.  This suppleness means you can effectively run a firmer pressure than with an air shock, meaning much better mid-stroke. It’s the spring, after all, that supports the mid-stroke, not any damping. 

Then as discussed as above, rubber bumpers and steel swingarm compliance give a smooth bottom out.  One other thing we’veve noticed with coils is much longer service intervals, and being much less sensitive to set-up, all of which compliments the qualities of Starling frames.

To quote the EnduroMag review of the Starling Cycles Twist, “On paper, the linear suspension kinematic doesn’t look well suited to coil shocks, but whether down to the naturally compliant rear triangle or big bottom-out bumper, we never felt a harsh bottom out, despite our best efforts at finding bad lines.”

Keep it Simple, Stupid

Last, but not least, the final benefit of a linear leverage ratio is that it gives suspension tuners an easy job.  They can concentrate on getting the shock to suit the rider’s requirements, rather than making it work with a strange leverage ratio.

Want to check our our range of steel, single pivot mountain bike frames? Check them out right here.