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Re: Descending, bike fit, weight distribution

Posted: Thu Jul 25, 2019 9:00 am
by Multebear
Mr.Gib wrote:
Thu Jul 25, 2019 4:10 am
Multebear wrote:
Wed Jul 24, 2019 11:52 pm
I'd definitely choose a stem as long as possible. You´ll put more weight on the front wheel which stabilizes steering and overall bikecontrol.
Is this what you want when you have the bike leaned over at the limit? Just about anything works going in a straight line, but the ability to corner at speed without feeling like you are going to slide out is what makes the difference. How does loading up the front end help in this situation?
Yes, it is. If you´re holding the perfect line for your speed while cornering, you can go faster when leaning forward.

Re: Descending, bike fit, weight distribution

Posted: Thu Jul 25, 2019 10:06 am
by stevod
In my (limited!) experience stem length hasn't made much difference (all else being equal), but reducing the stack height (again all else being equal) made an extraordinary and unexpected improvement when descending. Reduced after a bike fit. Unexpected as that wasn't the aim of the fit or something we'd discussed.

Total 30mm in the fit, reduced on the road by 10mm per month over three months. Even the first 10mm was amazing, then it just got better and better. Much more stable and confidence-inspiring.

YMMV of course, and likely depends where you're starting from too.

Re: Descending, bike fit, weight distribution

Posted: Thu Jul 25, 2019 10:06 am
by Weenie

Re: Descending, bike fit, weight distribution

Posted: Thu Jul 25, 2019 6:47 pm
by GrumpyOldPizza
Mr.Gib wrote:
Wed Jul 24, 2019 10:32 pm
I started reading the replies and became infuriated so I quit after about 4 or 5. Apologies then if I repeat what subsequent replies already covered.

To the OP - the engineer in you should have known not to ask about what saddle position is best for descending. You have to ride your bike and as such there is (with very minor latitude for personal preference) a correct postition for your saddle based on the proportions of your body. Move it 10mm in any direction and it's wrong. You won't pedal as effeciently, you will be less comfortable, and you may injure legs, knees, hips, etc.
Ok, perhaps I am saying now a lot of stupid things, but here it goes. I own 2 bikes a CAAD10 and a SuperSix. Both 54cm, compareable geometry. On the CAAD10 I had a Retul fit, along with a separate session by one of the local gurus. On the SuperSix I had a GURU fit. Retul moved me quite aways to the front. The local guru mostly said "zero-offset". And the GURU fit ended up with a 40/60 (at the time). Upper body wise I never felt comfortable with the Retul fit. However Retul and GURU roughly ended up with the same distance between center of BB and where the femur joins the pelvis. I would assume (with my physics background) that what really matters for the saddle fit is this distance. If you move the saddle back, you need to lower it, if you move it more to the front you need to raise it. Just to keep this very distance the same. Simple geometry. If you move your saddle back (and rotate this axis), then the stem needs to be shorter and higher up, while if you move your saddle to the front your stem needs to be longer and lower. Simple geometry dictates that. What changes though is your upper body angle relative to the horizontal plane, which affects your spine (my personal problem here), which in turn affects the load on your arms, wrists and fingers. So IMHO after you found the correct distance between center of BB and your femur/pelvis, you still have a whole lot of room to play with your saddle position.

So that brings me back again to the tradeoff in weight distribution vs. steering (of course in addition to comfort and aerodynamics).

The riding part is kind of the tricky thing. Right now we have way more thunderstorms than I'd like, hence not enough opportunity to simply ride a test series. Also before doing so, I'd love to understand what I'd be looking for. Where would what change make a difference. I can feel stiffness of a wheel, because I just can off a set of wheels that was not stiff enough. So there is a point of reference ...

Perhaps including the term "bike fit" was the wrong choice, because it's really a matter of physics ...

Re: Descending, bike fit, weight distribution

Posted: Thu Jul 25, 2019 6:59 pm
by zefs
Well you need to do bigger changes to be able to feel the weight distribution difference, but you don't have to since you can move further back when descending. One thing that will happen if you do move further back but keep the same knee angle is the glutes will work more and your cadence will be slower, too like with the older pros' fit. It might feel like the bike is larger and it will give you more stability on the descends.

Re: Descending, bike fit, weight distribution

Posted: Thu Jul 25, 2019 8:16 pm
by Hexsense
Many mention about wheel base, but skip on more important parameter: trail value. a few mm different in trail value make more different in corner than a couple cm different in wheel base length.

Trail value is the result of (mostly) Head tube angle and fork offset.
So There can be one bike with slack head tube angle and normal short fork offset, which result in high trail and handle slowly.
Another bike can be the same slack head tube angle but pair with high fork offset, resulting in low to normal trail number and it feel fast and twitchy. Therefore looking at headtube angle isn't representative of how the bike will handle, without knowing fork offset (and wheel, tire size) to calculate trail value.
=======

About OP's question, i also play around positions. I know we all can get the same hip angle on various pelvic rotation which resulting in various seat position set-up. Therefore one perfect position doesn't actually exist. Get one of correct position from a fitter, Look from the side, rotate everything around crank center as pivot point for a few degree, that's also another perfect position for the leg to push the pedal. It's more of hand pressure and upper body comfort vs aero trade off.

There is a point where my handling get worse in hard crit corner when my seat is too far forward. There, i back off the seat and, lower saddle and raise the bar to keep the same leg and hip angle. I'd change stem length too, but it's impossible to shorten stem by a few mm instead of the full cm.
You don't need to test it on actual descend. Try to do sharp corner on flat would already tell if something feel off.

Re: Descending, bike fit, weight distribution

Posted: Sat Jul 27, 2019 11:28 pm
by Mr.Gib
GrumpyOldPizza wrote:
Thu Jul 25, 2019 6:47 pm
However Retul and GURU roughly ended up with the same distance between center of BB and where the femur joins the pelvis. I would assume (with my physics background) that what really matters for the saddle fit is this distance. If you move the saddle back, you need to lower it, if you move it more to the front you need to raise it. Just to keep this very distance the same.
yes this is the basic idea.
GrumpyOldPizza wrote:
Thu Jul 25, 2019 6:47 pm
Simple geometry. If you move your saddle back (and rotate this axis), then the stem needs to be shorter and higher up, while if you move your saddle to the front your stem needs to be longer and lower. Simple geometry dictates that.
NOT necessarily. There is a very good argument that if you move the saddle back you should move the handlebar further forward! (And vice versa.) This increase in reach is necessary to maintain good fore and aft balance. Steve Hogg wrote an good article about this idea of fore and aft balance on a bicycle. This is a sport. We are athletes. Our body positions should be balanced, poised, and athletic to allow us to perform various movements as efficiently as possible. How do you stand when you are preparing to receive a serve in tennis? Same idea.

This bring me back to the point that there is more or less a correct position for your saddle on a given frame. Any other position is wrong. Find the correct position for the saddle and then make everything else compliment that position.

And when I wrote that "you have to ride your bike" I didn't mean to test the position, I meant instead that a bike should be set up so you can pedal it most efficiently. In other words, set it up for pedalling, not descending. When you descend you will put your body in the position that works best at that moment, a position that will change as you enter and exit corners, straight run, etc. Because of this, and the fact that you don't have to pedal while descending, location and relationship between contact points are somewhat irrelevant to the task.

Re: Descending, bike fit, weight distribution

Posted: Sun Jul 28, 2019 12:16 am
by gurk700
Stueys wrote:
Wed Jul 24, 2019 11:01 pm
I’ve an r5, wouldn’t say it’s a rock solid bike at speed but I wouldn’t describe it as twitchy.
Personal preference and what you're accustomed to. My point wasn't to pick on R5. That's just how I felt about the bike. My "twitchy" standard might not match yours. :beerchug:

Re: Descending, bike fit, weight distribution

Posted: Sun Jul 28, 2019 3:19 am
by Mr.Gib
Multebear wrote:
Thu Jul 25, 2019 9:00 am
Mr.Gib wrote:
Thu Jul 25, 2019 4:10 am
Multebear wrote:
Wed Jul 24, 2019 11:52 pm
I'd definitely choose a stem as long as possible. You´ll put more weight on the front wheel which stabilizes steering and overall bikecontrol.
Is this what you want when you have the bike leaned over at the limit? Just about anything works going in a straight line, but the ability to corner at speed without feeling like you are going to slide out is what makes the difference. How does loading up the front end help in this situation?
Yes, it is. If you´re holding the perfect line for your speed while cornering, you can go faster when leaning forward.
You haven't answered the question - why does having more weight on the front wheel improve traction? (we're talking about faster cornering right?). Wouldn't the best traction come from a more even distribution between the tires?
Also makes me wonder why the best descenders shift their weight back a bit when cornering on a descent?