Getting my head round reach & stack after years of effective top tube.

nemesis – Member

The pivot is at the ground, not the axle.

Is it so simple? Bikes, when they turn, don’t rotate on the contact point as a fulcrum- if you turn left, the contact point doesn’t stay still while the bike leans left, as if toppling, the contact point moves right. So the bike’s rotating around a point somewhere higher up, but also that point moves downwards as the bike rotates and leans…

Yeah, I know it’s not that simple but I’m trying to make the point that in this instance at least, the BB drop doesn’t affect what he says it does. Trail and so on are of course affected by wheel size and they will also vary on steering angle, etc so it’s a complex situation once you start cornering.

So the bike’s rotating around a point somewhere higher up, but also that point moves downwards as the bike rotates and leans…

Northwind is on the money, the bike is rotating about the radius described between where the centre-lines of the axles meet the ground and the contact patch of the tyres

Is it so simple? Bikes, when they turn, don’t rotate on the contact point as a fulcrum- if you turn left, the contact point doesn’t stay still while the bike leans left, as if toppling, the contact point moves right. So the bike’s rotating around a point somewhere higher up, but also that point moves downwards as the bike rotates and leans…

It’s really complex. When the bike is leant it doesn’t do so around the centreline of the tyre, the pivot point moves outwards towards the side knobs. But when a bike is moving and you lean it the front wheel countersteers slightly to pull the contact patches outwards so that the mass of the bike+rider falls inwards.

Something I’ve noticed when swapping between bikes (16″ Brompton, 20″ BMX, 26″ & 27.5″ MTBs) is that the turn is much quicker to initiate on the smaller wheels. Does that mean the moment of countersteer is briefer? Does the bike just need to countersteer laterally by an amount which is proportional to the wheel radius?

Other things to think about – forks have offset so when you turn the bars the centre of the wheels are no longer in a straight line which passes through the BB. Does the BB drop related stability interact with the gyroscopic wheel force? That’s something where the axle height definitely matters.

No it’s not or at least not as you’ve described it. You could lean your bike as you’ve drawn and still ride in a straight line (well you’d actually have to countersteer slightly I reckon) or you could be riding various different arcs depending on other variables (speed, weight, geometry).

And what does that have to do with reach?

Anyhoo, back to reach and stack…

In a similar manner to BB drop, whereby a lower COG between the axles improves stability, the same will apply in terms of reach and stack, though ergonomics will come into the equation for comfort and manoeuvrability.

It’s effectively the whole ‘on the bike’ vs ‘in the bike’ thing

You still haven’t defined “stability”…

And you didn’t show that a bigger BB drop improves anything.

So it’s down to ‘on’ vs ‘in’. Great 😉

I’m sure others can comprehend what I’ve already conveyed…

I get what you’re trying to say but they’re really points about the ability to hit obstacles with different size wheels which I think are well established fact.

You haven’t shown anything about stability or even explained what it is yet…

As in the fore/aft example, cornering stability basically means the bike will hold a line better.

In many respects it won’t just be down to Reach… for a given geometry (i.e. head angle/seat angle/chainstay length) a change in reach is automatically going to change the wheelbase.

Stack is another matter…

Did someone say “different size wheels”?

Burn the heretic! 😉

It’s fairly simple that the lower the stack and the longer the reach, the more stable a bike will be under deceleration, due to the reduced moment around the front contact patch applied via the bars. However, there’s a lot more to MTBing than braking – in fact a bike with shorter reach and higher stack will be easier to lift the front and therefore jump (look at DJ bike geometry!)

Yep, that’s very true, though it’s not just braking that improves~ it’s also passing over obstacles (taking wheelsize out of equation) and high speed cornering

This piece on when GT brought out the new Fury probably explains it better than I can

(look at DJ bike geometry!)

And, as a greater extreme, BMX geometry.

Steep angles, small wheels, short reach and a high stack (including the bars)

Guys
my 2c, I don’t THINK this point was made above, apologies if it was.
1: IMO top tube is not a relevant measurement because 2 bikes with same “effective top tube” length but different seat tube angles differ in the vital “BB to front axle” measurement. Reach takes this out of the equation. You can’t change the most important measurement on a bike, which is the down-tube length IMO. Sure, you can bugger up the handling by fitting a longer stem but that’s not really an option. Given similar fork length, head tube length and head angle the down-tube measurement is a good way to compare different maker’s bikes IMO. [as is reach/stack]
[down-tube means Centre of BB to Centre of lower headset race].
2: some interesting points about when a bike turns from Liteville/Syntace / Pinkbike – http://www.pinkbike.com/news/liteville-601-mk-2-review-2015.html
The argument for a 26″ back wheel with a 650 front is pretty interesting. nice theory.
http://www.pinkbike.com/news/liteville-601-mk-2-review-2015.html