Geometry - Stack and BB Drop what does it mean!?

carms28

Hi,

I need educating in geometry...in particular between the Meta Am 29 vs Instinct BC. On paper the geometry is very similar except for 2 factors (3 including seat post angle but i assume that doesn't really make a difference when you're pointing it downhill!) Stack and BB Drop! Can some one explain these measurements? I don't fully understand the measurements and how they play apart on the bikes overall feel.

The Rocky has a 12.5mm lower stack than the meta but has +19mm bb drop, the meta on the other hand has a -25mm BB drop. The fact one is negative and the other positive makes me think these 2 bikes would handle very differently and be perhaps aimed at riders looking for different things in a bike.

Thanks for any explanation in advance guys!

Rich

JBA

Stack is the vertical measurement between the centre of the bb and the top centre of the head tube. It is effectively a measure of how 'tall' the frame is. However, on a MTB reach is more important as riding stood up makes the stack height pretty much irrelevant.

BB Drop is the vertical distance between the wheel axles and the centre of the bb. A larger drop lowers the centre of gravity resulting in a more stable ride. However, it also means the bb is closer to the ground so the chances of pedal strike increases. This can be compensated for to a degree by fitting shorter cranks.

steve_sordy

To add to what JBA says (and I don't disagree with him).

Riders got used to the height above ground of the 26ers. When the 29ers came out the axle height was 1-1/2" higher. So if they kept the BB (bottom bracket) in the same location, then everybody would be higher up ie the centre of gravity of bike and rider would be higher up. So they sensibly dropped the BB to compensate. If you keep dropping the BB height because it improves the CofG, then eventually you will be having pedal strikes. The BB drop is also influenced by the amount of travel on the suspension. The more the bike compresses, the nearer your pedals come towards the ground. This shouldn't be a problem if your cranks are flat, but you can't guarantee that you won't be pedalling over bumpy ground. So the BB drop tends to be lower on bigger travel bikes. Shorter cranks arrived to counter this. With the advent of electric bikes, where too many riders stay seated for longer, pedal strikes became a big problem for the less skilled rider and supply met demand and really short cranks (135mm anyone?) came onto the market.

As Stack is the vertical distance from the BB to the centre of the head tube, then Reach is the horizontal distance. Reach is more important to me than Stack because I have a long body for my height, so I need more room. Also look at seat post angle because this moves the saddle forward with increasing angle, reducing the ar$e to bars distance.

The thing with geometry is to compare what you are contemplating with what you know works for you. The problem with that approach is that you may fail to take advantage of something better. The answer of course is to test ride the bike wherever possible.

steve_sordy

This will be useful.

Bike Geometry, what do the terms mean?

Chainstay length
Chainstay length is the horizontal distance between the centre of the bottom bracket and the centre of the rear axle. On most bikes, the chainstay length changes as the bike moves through its suspension travel. However this change is never communicated in geometry tables.

Head angle
The head angle is the angle between the ground and the steering axis of the head tube. Slacker angles generally mean that the steering is more stable, whereas steeper angles give a more active steering feel.

Reach
Reach is the horizontal distance from the centre of the bottom bracket to the centre of the top of the head tube. Reach tells you how long the bike will feel when you are standing, but due to different seat tube angles is not a good indication of how long a bike will feel while seated.

Stack
Stack is the vertical distance measured from the centre bottom of the bracket to the top of the head tube (where the fork steerer exits the frame). Stack gives a good idea of how tall the front of the bike will feel on the trail.

Reach / stack ratio
The reach/stack ratio of a bike gives a good indication of how your position will feel while riding. Bikes with a lower ratio (under around 0.75) will generally feel taller with a more upright riding position, whereas bikes with a higher ratio (above 0.75) will feel lower and more stretched.

Seat tube angle
This is the angle measured between the ground and the line drawn from the centre of the bottom bracket to the centre of the seat rails at a given seat height. Many brands measure this differently. Due to kinked seat tubes, this angle can vary wildly at different seat heights so it is not easy to compare one eMTB to the next.

Seat tube length
The seat tube length is the distance measured from the centre of the bottom bracket to the top of the seat tube. This gives us an indication of what length dropper post can be inserted into the frame. However on many frames, pivot locations and kinks in the tube reduce the maximum insertion depth.

Wheelbase
The wheelbase is the horizontal distance measured between the points where the front and rear wheel contact the ground.

Effective top tube length
The effective top tube length is the horizontal distance from the top tube/head tube junction to the seat tube. While this measurement does not take into account the amount of seat tube extension it is the best indication of how long a bike will feel when seated.

whyamihere

To finish off what has been said above, the reason one is listed with a negative BB drop and the other is listed with a positive BB drop is because the manufacturers choose to represent the same number in a different way. The BB is below the rear axle on both, the actual difference is 6mm.

carms28

Thanks all, this is great information, interesting stuff! Seems odd why companies reference BB drop in different ways, that -/+ threw me. So based on that the RM is a slightly taller bike and because the Meta has more travel and lower BB it would have a better centre of gravity? Like you said though steve_sordy best test ride them both if possible,.

The Rookie

steve_sordy said:

When the 29ers came out the axle height was 1-1/2" higher.

Ahem, 1 1/4" higher (a 29er being 28.5" if measured the same way a 26" measures 26"!)

steve_sordy

I am not disagreeing with you, I'm only surprised that the 26ers actually measure 26"! But that 1/4" difference will also be influenced by the size and make of the tyres and the rim width.

My mate and I both had 26ers with Continental Trail King 2.2" tyres. He bought a 27.5er. He said it didn't feel any different, so we measured the wheel diameters.

My 26er measured 26-3/4. His 27.5er measured 27-1/4" His tyres were Schwalbe Rocket Rons (may have been Racing Ralphs I can't be sure now).

The Conti TKs are big tyres, the RRs were not and his rims were 25mm, mine were 19mm which helped to push the tyre outwards. He very soon ditched the RRs and bought TKs, not to get the extra diameter, but for the extra grip; the diameter gain was a bonus.

The Rookie

Well yes, the nominal size is carcass not tread, so a more knobbly tyre has a larger rolling diameter than slicker one of the same nominal size and then tyres of different models from different manufacturers will measure differently as well, plus rim width affects how 'tall' a tyre inflates. The 26" originated from the rolling diameter using a 2" tyre.

Not disagreeing when different tyres come into it, but using the same width and model of tyre the 650B will be 1" bigger rolling diameter than a 26".