“ Are you referring to a situation where you are riding out of the saddle?”
Absolutely, I don’t ride anything downhill without the saddle dropped (usually by the max 185mm that my dropper allows). Likewise any flatter stuff with turns. Or technical uphills. To me, and many riders nowadays, the saddle is pretty much for pedalling in straight lines only.
There is absolutely no way that I could ride the trails I do at the speed I do with the skills I have whilst staying in the saddle.
“ It would be interesting to find some video of bicycle slalom’s to see what techniques the rider’s use.”
Dual slalom bikes tended to be full-sus 4X bikes, with very low BB heights.
Well here's another theory...
Doing really quick turns in the woods (slower and tighter than on a slalom track) the bike and rider don't behave like one thing. Initially the rider rotates the bike (about a longitudinal axis) and moves it under them, staying relatively still themself until the bike is set up for the corner and starts to bite. This is achieved by a combination of counter-steering, unweighting, jumping if you are that way inclined. Possibly preceeded by a Scandinavian flick. So approaching a right hand corner, the objective is to get the tyre contact patch off to the left of the line of travel (and the rider's cg) so that a turn can happen.
Let us assume that even the rider's feet don't move much in this process (probably they do move a bit more than the rider's bum does but let's just assume). The longitudinal axis of rotation (of the bike, not rider) I just mentioned will go through the bb (where the feet are). The more bike there is below the bb, the greater lateral displacement of the tyre contact patch for a given angle of rotation of the bike. This will make a higher bb bike feel easier to flick through this type of corner. Or to think of it another way, trailside trees permitting the rider's (higher) body can straighten out the curves more than with a lower bb bike.
You can see the technique that chiefgrooveguru is talking about between nine and fifteen seconds into this video. How to corner fast video
I tried my own method of leaning the bike into a turn on a Geoff Apps' bike with a fifteen and three quarter inch high bottom bracket. However, I was put off by the nose of the saddle hitting the back of my leg every time I lent the bike. I need to put on a short nosed saddle and try again.
With the dropper post down it was still possible to lean the bike a fair angle whilst remaining in the saddle. This cured the problem of the saddle hitting my legs and might be a lazy man's way of cornering faster.
^That demonstrates the thing I was trying to describe - the bike moves a lot, flicking from one lean angle to the opposite, the rider much less. Complete with little jumps.
I also agree with the @chief's point about controlling drift. Once relatively settled in a turn, altering the lean angle of the bike relative to the body can move the tyre contact patch more or less underneath the rider's cg, and bb height will affect how that works.
Slap in a normal headset and that Pole is ripe for a mullet.
Well here’s another theory…
Exactly, and what I was getting at with my poorly worded comment here earlier on,
Yet having seen how fast some riders get through closely linked corners ..
With the BB higher, for how much the tyres move more side to side underneath you, your body weight has to move less. From the BB to the contact patch has increased radius. Gets complex with grip and balance as the speed goes up. Seems you could get weight inside in a corner a bit more easily at a lower lean angle – how/if a skilled rider uses that for speed, grip or agility I’m not so sure. There’s a limit on all these things but in slalom-like turns or similar rapid edge to edge riding I can see a theoretical advantage to a higher BB bike.
But a lower does BB seem to help catching slides more quickly, both the MTB and gravel bike I have with low BBs are good in that situation.
Sorry I managed to miss your comment jameso. Exactly. I guess there are a number of things going on, some favouring higher some lower bbs.
I asked Geoff Apps about the cornering technique shown about nine seconds into this video.
Faster cornering video
He said that this was why from 1977 onward all his bike designs had had sloping top tubes. He said that it was essential that the rider can easily and quickly move their body-weight around. In comparison the early US mountain bikes had relatively high top tubes that were parallel to the ground.
He then went on to talk about the wide variety of cornering techniques that motocross, speedway and motorbike-trials riders use.
Geoff's designs are based on decades of experimentation with different geometries and what feels best. Give him any style of bicycle and he will take it off road to find out how it handles. By doing this for the best part of six decades, he came to the opinion that large wheels, high bottom bracket, steep-steering-angled and short wheelbase bikes work best for his purposes. He believes that shorter bikes are work best because it takes less time for the rider to move his body's centre-of gravity relative to the tyre contact, patches than with longer wheelbase designs. So his design's principly focuses on it being as easy and quick as possible for the rider to move his weight around in all directions. As I have said before, Apps bikes are fundamentally big wheeled BMX bikes. For him, speed is far less important than technical capability and machine reliability.
