Sixty One Degree – Why The Belter’s “Extreme” Head Angle Is Actually Normal

A hardtail with a sixty one degree head angle??  Thats crazy!

This is something we hear fairly often about the Belter‘s geometry, but it isn’t really crazy, and here’s why:

It’s standard fare nowadays for full suspension downhill bikes to have 63° head angles- they’ve settled on this number to achieve a balance of neutral weight distribution and steering stability.  These same full suspension downhill bikes are also designed for the suspension to sag fairly evenly when your weight is evenly distributed between the front and rear wheels; this means that on average the bike sags without any rotation of the frame, so there is no change in head angle between full extension and sag position.

However, on a hardtail there is no rear suspension movement, so any sag or suspension movement will result in the whole frame rotating forwards around the rear axle.  This rotation causes an increase in head angle, which has a knock-on effect to the steering geometry, and most importantly your weight distribution on the bike.

So, how big is the increase in head angle then?

Well that depends on a number of factors; initial head angle, wheelbase and the amount of sag/travel used. Lets ignore the wheelbase for the time being- I’ll look into the effects of that in just a minute.

The Belter with its 61° static head angle is designed around a 150mm travel fork with a 42mm offset.  30% is a decent starting point for sag- some riders may like to run more and use compression damping to prevent the forks from bottoming, others prefer to run less sag and rely on spring rate to keep the front end up.  So, 30% of 150mm is 45mm of travel.  Sagging the fork by this amount results in a medium Belter frame pivoting forward by a fairly convenient 1.93°, taking the head angle up to 62.93degrees at 30% sag…lets just call it 63° shall we?

Well, so what?  Who cares if its 63° at sag?- its still 61° static!

I have to admit that technically I’ve been lying a bit here; every time I said ‘sag’ I should really have said ‘dynamic sag’.  This ‘dynamic sag’ is the term given to the average travel position of the suspension during use, so while you’re out riding this is the point in the travel that your suspension will spend most of its time.  That makes it fairly obvious why we would want to optimise the geometry around this point in suspension travel; it’s where it’ll be most of the time.

Its 61° static, 63° at 30% sag…what happens when the fork bottoms out??

Scary stuff generally.  Going back to our medium Belter, when its 150mm fork bottoms out, the frame has pitched forwards by 6.69°- thats a 67.69° head angle. Pretty wild, especially when you consider the 61° starting point!

If the Belter is at 67.7° when its bottomed out, what does a ‘normal’ hardtail do?

68° seems to be a fairly normal head angle for a hardtail, and assuming the same 30% sag and 150mm full travel the frame will rotate forwards by 2.16° (to 70.16°) and 7.44° (to a gnarly 75.44°) respectively.  Notice that the frame rotates further forwards, compounding the effects of the steeper static head angle (2.16° > 1.93°, and 7.44° > 6.69°).

Anyone still awake??  If you’ve read this far I’m guessing you’re either a hardtail geek or you’re seriously patient, or both.  Whichever way, kudos.

I said I’d get onto wheelbase and its effect on the whole head angle thing, so here goes…

On a medium Belter with its 1191.2mm wheelbase the total head angle variation from full extension to full compression is 6.69°, on a large with its 1211.2mm wheelbase the variation is 6.57°, and a small with its 1171.2mm wheelbase the variation is 6.81°.  Its not a massive factor at only 0.24° difference across all 3 sizes, but it’s there.

You’ll be pleased to know, that’s all for this time!  Got any questions about this blog post or anything else about bikes, hardtail, downhill or otherwise; fire them over to [email protected] and I’ll look into them.

Cheers,

Tam