Can someone explain geometry please.

dirkpitt74

Hi guys,
Time for another question.
Fork/steering and seat geometry obviously have a major part to play in handling etc.
Thing is I'm not quite sure what I'm looking at.
I think I understand the fork/steering - slacker angle (further away from 90deg) gives better stability and slower steering - presumably as the effective wheel base is longer. Tighter angle (closer to 90deg) gives better agility but a more fidgity.

Is the above right?
How does the seat angle effect handling?

As most of my riding will be over Cannock Chase - XC and FTD what should I be looking at?

Thanks
Chris

The Rookie

Seat tube angle moves your weight (seated) more laid back is better for descending and more upright better for climbing (broadly).

For Cannock you want an XC to trail bike, head angle around 69-70 and a similar seat tube angle, with 100-120mm forks, or at least that is what I would want!

benpinnick

Head angle (HA) primarily effects 'trail', the distance from the vertical line through the headset to the contact patch of the tyre. I say primarily as while it does effect wheelbase, wheelbase is subject to many other factors, whereas trail is only HA and fork offset. Most people ignore offset when buying a bike, but it can vary by enough from fork to fork to make a big difference. For example on our bikes we run Sweeps vs. Velvets as an AM option on some models. The offset of a sweep is sufficently different that the handling is changed more than the 0.5 degree HA change would lead you to believe.

Seat angle is more part of the overall package of a bike. Its a relative position really, and needs to be in context. SA has been getting steeper (more vertical) as bikes get longer, this keeps weight distribution more even and allows for more efficient pedalling.... However, and it is a big however, as you increase (steepen) seat angle, you do need to be more conscious of correct sizing for you. Running a long post on a slack seat will extend the cockpit and so give you more room. A steeper SA will not do this so much. It means that super long seat posts can lead to very cramped bikes if they dont size up properly. Heres a guide (and its only a guide) of to look out for:

HA/Trail - effect steering speed mainly, always work in tandem so dont rule out a bike for +/- 1 degree against another until you know the offset.

Top Tube length and reach - longer = more stable, more open so easier going, shorter = better for jumps/tricks. Expect
TT and Reach to go up a lot in the next 2 years as people figure out their bike was always too small.

Wheelbase and chainstay length - Long wheelbase is stable, but can be cumbersome. Generally though I would advise longer = better. Short chainstays = fun but can make a bike prone to wheelies (yes that can be an actual problem) meaning it can be light on the front end. Again though I say shorter = better, unless its a DH bike.

Seat Angle - mixed bag dependent mainly on the rest of the bike. Best not to judge a bike by its SA.

BB Height - lower is better (IMHO), but too low is a rock striking pedal-clipping nightmare.

dirkpitt74

Thanks for the reply. Most of what you say makes sense.

I have no idea how to work out what sort of geometry will work for me.
Looking for a trail friendly set up that's not gonna tip me over the back up hill or throw me over the handlebars downhill.
Have narrowed it down to rockrider 8.1 or Calibre .50.

Head angle is slacker on the Calibre but chainstay and tt are within a few mm.
Thanks

The Rookie

The RR is more XC biased, the calibre more trail (centre) biased...have a go on both and decide.

paul.skibum

benpinnick wrote:

Lots of info - all cut just for the sake of length of post!

Interesting stuff Ben but I would question some of it - chainstay length in particular is causing me some issues.

When I stand my Bronson and my old Cove Handjob side by side and measure some distances I find it interesting that with the front axles side by side, the overall cockpit - is not that different on two very different bikes - the bars fall at the same point, the seat is in the same place two - in fact despite some fairly different angles and wheel sizes, the axles to axle distance is the most marked difference. With the rear wheel ending up being a little over 25mm further back behind the saddle.

The net result for me is that as a general rule whilst riding the Bronson, I have to make a more concerted effort to get my weight back on steeps and for any manual/wheelie activities which for me is making technical terrain a little harder (although some of that is also fork collapse related).

I don't have an issue with a wandering front end on climbs on my hardtail despite its shorter wheelbase so I am not convinced that a longer wheelbase especially resulting from a longer chainstay is better. Bloody 650B - should have got a 26er Nomad.....

benpinnick

Paul, you're right on the effects of your chainstays, although the wheel up issues relating to your bronson are as much down to the way a VPP shock behaves as much as the CS length. As I said in my post though, as a rule, shorter = better when considering chainstays (perhaps you missed that )

paul.skibum

benpinnick wrote:

Paul, you're right on the effects of your chainstays, although the wheel up issues relating to your bronson are as much down to the way a VPP shock behaves as much as the CS length. As I said in my post though, as a rule, shorter = better when considering chainstays (perhaps you missed that )

Fair enough - I did misread that line a little.