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Switching to shorter cranks on the TT bike
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TOPIC: Switching to shorter cranks on the TT bike

Switching to shorter cranks on the TT bike 12 years, 9 months ago #575

  • rmur
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I'm hoping to get my TT position optimized (soon!) and looking at things I can do. I've got a decent set of aerobars mounted now with an adjustable stem - haven't had the opportunity to try them yet.

A couple of months ago I asked about the potential impact of going to shorter cranks on the TT bike. I'm 1.83m (6') with normal proportions but have always ridden 175mm cranks.

I'm seriously thinking of swapping out my 175 SRM cranks for 170's ....

The goal of course is to get my front-end lower on the bike. Currently I'm at 16cm saddle-pad drop (see positioning table) but feel I can get lower. I'm not near the horizontal torso ideal (or even 5deg for that matter) but suspect I will get thigh/torso clash soon after I start dropping the pads.

Any comments/experience? Good idea or waste of time?

Rick
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Cranks...saddle position 12 years, 9 months ago #576

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Hi Rick,

I always suggest that people try to keep their road bike and TT bike saddle positions and cranks identical unless they can verify an improvement in watts/CdA or if moving the saddle forward one cm optimizes reach. If w/CdA improvement is not verified, having a different saddle position or cranks on one's TT bike vs. one's normal bike is baseless.

Since most riders spend a massively disproportionate amount of time on their road bikes, it is best to minimize changes in the body-bike interaction.

As for changing to shorter cranks in general...there really isn't anything conclusive that I have seen which demonstrates a power/crank-length relationship as I'm sure you know! Like you said, shorter cranks may allow for more clearance between one's leg and torso at the top of the pedal stroke, but the saddle height must be raised to keep the same leg extension...so CdA improvements are not necessarily as beneficial as just dropping one’s pads an extra 5mm's.

Cheers, Kirk

Re: Switching to shorter cranks on the TT bike 12 years, 9 months ago #583

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rmur wrote:

Any comments/experience? Good idea or waste of time?


You know my opinion on the potential CdA benefits of shorter cranks.

I would do it a slightly different way than plunking down the $ for SRM cranks right away. I would get a cheap set of 170's (ebay, r.b.m., etc) take frontal pics from 5M away (identical setup except for cranks/seat height) with each set of cranks on, but raise the seat 3-5mm with the shorter cranks.

IOW, FA analysis of the best setup you can do with the 175's and with the 170's, and then see if it's worth making the switch.

JV

Re: Switching to shorter cranks on the TT bike 12 years, 9 months ago #584

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[quote]I always suggest that people try to keep their road bike and TT bike saddle positions and cranks identical unless they can verify an improvement in watts/CdA or if moving the saddle forward one cm optimizes reach. If w/CdA improvement is not verified, having a different saddle position or cranks on one's TT bike vs. one's normal bike is baseless.[quote]

Well I'm already resigned to a moderately different position on my TT bike. I need to get fwd to open up my hip angle enough to get modestly low. My TT bike saddle is 5cm (UCI) back of b-b. Plus that I sit ~8-10cm fwd on the saddle in the aero position. My effective position is well fwd of the b-b. Compare that to my road bike - setup traditionally with ~7cm setback and I don't ride on the tip! Taking that into account, 5mm on the cranks should be a breeze.

From what John mentioned - I could be able to raise my hips 5mm and drop my pads 5mm for a net reduction in 10mm torso elevation or about 1 degree reduction in torso angle. Seeing how I'm not near the ideal position now - that ought to help me.

I can't see any drawback whatsoever from a FA point of view. This should allow me to get 1cm lower than I otherwise would have. Raising the saddle don't have any negative FA effect, does it??

Kirk I should send you a link to my pics then you can see what I'm on about..

Rick
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Re: Cranks...saddle position 12 years, 9 months ago #585

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Kirk wrote:
Hi Rick,

I always suggest that people try to keep their road bike and TT bike saddle positions and cranks identical unless they can verify an improvement in watts/CdA or if moving the saddle forward one cm optimizes reach. If w/CdA improvement is not verified, having a different saddle position or cranks on one's TT bike vs. one's normal bike is baseless.

Since most riders spend a massively disproportionate amount of time on their road bikes, it is best to minimize changes in the body-bike interaction.

Cheers, Kirk


Easily solved - ride your TT bike a lot more. I do 60ish races per season inc 50s & 100s - that way you get very used to the position. My TT and road position are different 1.5cm behind on my TT bike and ~5cm behind on my road bike.

Bob
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Re: Cranks...saddle position 12 years, 9 months ago #586

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bob tobin wrote:
Kirk wrote:
Hi Rick,

I always suggest that people try to keep their road bike and TT bike saddle positions and cranks identical unless they can verify an improvement in watts/CdA or if moving the saddle forward one cm optimizes reach. If w/CdA improvement is not verified, having a different saddle position or cranks on one's TT bike vs. one's normal bike is baseless.

Since most riders spend a massively disproportionate amount of time on their road bikes, it is best to minimize changes in the body-bike interaction.

Cheers, Kirk


Easily solved - ride your TT bike a lot more. I do 60ish races per season inc 50s & 100s - that way you get very used to the position. My TT and road position are different 1.5cm behind on my TT bike and ~5cm behind on my road bike.

Bob


I agree with Bob here. I was from the "keep the seat in the same place" school for years, but as I've been able to build up and have a TT bike available to train on 1-2 times per week YEAR ROUND, I can easily stay adapted to that position (my 'cross bikes add a third position in the Fall).

Also, once you stick on aero bars and drop the front end, you've already changed stuff like thign/torso angle. If anything, moving the TT bike seat forward is keeping the body in a position closer to the one your relevant body parts are used to.

JV

Verification of w/CdA 12 years, 9 months ago #587

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Whatever allows one to maximize w/CdA is the best way to go!

Otherwise, moving saddles and such are baseless.

Most of the time, guys reach too far out instead of down which forces hip rotation and the impression that the saddle needs to move forward...and there is also a saddle height factor. It's more complicated and individual than just cranking it forward!

Cheers, Kirk

Re: Switching to shorter cranks on the TT bike 12 years, 9 months ago #588

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Thats the name of the game - but until you try different positions how can you know how much you will adapt and therefore what your power will be in a new position?

One of the things I have done in the last year or 2 is go longer, push the saddle forward and increase my hip rotation. The tri-bar pads have stayed where they were but my shoulders have moved forward and down and my elbows forward. This has had 1 major effect in that it flattened out a hump in my back. The greater hip rotation has reduced the difference in power between road and TT bike positions to almost nil. Getting rid of the hump has reduced my CdA I think, certainly my CdA seems to have gone down from my data but the cause is argueable. So I have done exactly the opposite of what you say and power ( absolute due to fitness and relative to a road bik position) has gone up, CdA down and speed gone up lots ) .

But i also tried moving the tribars up when i was sitting further back and CdA went up more than power so I ended up slower. I think you are right it is complicated and individual but without experimenting with different positions you dont know how they are going to affect your power production or your CdA ( most of us unlike you Willett brothers dont live in wind tunnels lol: ) . If I make suggestions to people it is always 'try this & that and see what happens' not do this and that and it will be better.

Bob
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Experience helps get the numbers 12 years, 9 months ago #589

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I think my point is being missed here. My point is that the wide-spread suggestion that "moving the saddle forward is better" is baseless if:

1) CdA improvement is not documented or not at least implied by a frontal area assessment.

2) Power is not documented to be equal between positions.

If one can document #1 and #2, then one has optimized position.

If you look at the "0.183" thread, moving my saddle forward will result in:

1) Increase the frontal exposure of my leg from hip to foot since that angle will move to 90 degrees.
2) Increase the amount my toes point towards the ground, increasing frontal exposure for the foot.
3) Raise the highest part of my back relative to the ground and the bottom of the crank circle...in accordance with the raising of the saddle as it is moved forward to preserve extension
4) Change the articulation of all biomechanical interactions with the bike relative to gravity.

So, a review of my photos implies that my frontal area will increase...(and I know I can produce the same power by matching my road saddle position)...so I gain nothing by moving forward. I have tried just about every position possible in my 18 years of dinking around with aero bars!

It is much more complicated and individual than just cranking it forward...but like I said...if w/CdA (or at least w/ frontal area) is maximized within the rules of competition...that is obviously going to be the best. Everyone is different though, so the art in the elicitation of performance is the use of experience to direct adjustments which can be then backed up by data...since not everyone improves from moving forward.

Cheers, Kirk

Re: Cranks...saddle position 12 years, 9 months ago #590

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Kirk wrote:
Since most riders spend a massively disproportionate amount of time on their road bikes, it is best to minimize changes in the body-bike interaction.


Oh, but you need to meet more tri-geeks. Like me!

I've had my road bike out three times in two years. Two of those times were when the tri bike was un-ridable for some reason.

I get on the road bike nowadays and feel awkward and weird. I feel like I'm falling backwards off the bike! This is not a good idea for actual bike racers, though -- only swimmers and runners.

As for Rick's concern about the thigh coming up to the torso (see Levi L. pics!), moving the saddle forward cures that in a heartbeat. You would also lower your saddle slightly. My torso is danged near completely horizontal, and the bottom of my kneecap does not come up past my elbow point -- ie, plenty of gap between legs and body.

I guess we all have to decide what our focus is. If it is pure TT (under UCI rules) that's one thing. Triathlon is another. Road and stage racing is another. I would hate to have to do all three.

w/Cda knows no boundaries! 12 years, 9 months ago #591

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Whether a multi-sport athlete or a one-trick spud...maximizing performance knows no borders!

Cheers, Kirk

Push it forward? 12 years, 9 months ago #593

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As for Rick's concern about the thigh coming up to the torso (see Levi L. pics!), moving the saddle forward cures that in a heartbeat. You would also lower your saddle slightly. My torso is danged near completely horizontal, and the bottom of my kneecap does not come up past my elbow point -- ie, plenty of gap between legs and body.


Have you determined the differences in frontal area between a "forward" position and a "back" position with the same leg extension?

I have watched in person a "celebrity" athlete/coach who has presented training "theory" at "conferences" suggest to athletes that they move their saddle forward to improve aerodynamics without any data to justify it.

All I could do was scratch my head and ponder how one can suggest such things without the data to support it. I then had to tell the riders afterward that they could very well be handicapping their performance by taking that suggestion at face value.

Check out these photos:

http://www.canadiancyclist.com/races04/ ... ameset.htm

Wohlberg complies with the UCI 5cm rule…which is a LONG ways BACK relative to body size…and very similar to myself, and we have no issues with hip-torso angles and the like. Imagine how far back this is relative to someone who is 6ft. tall! Maybe he trains to produce power with very tight angles and hip tilts by riding his TT bike often? Hmmmm. (if indeed he even loses any power…I have not identified any significant loss myself) Why would Wohlberg or myself want to move forward and INCREASE our frontal areas?

This Tuft guy has a lot of power, but he could do better in his positioning…even though over 7 cm behind the bb is FORWARD for him. He heard this “celebrityâ€

Re: Switching to shorter cranks on the TT bike 12 years, 9 months ago #594

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Simple question re Eric:
Why does he ride on his saddle tip while TT'ng? Doesn't that tell us where he'd LIKE the saddle to be positioned. Look Ekimov -one of the world's best TT'rs (when rested!) ... he's so far forward it hurts just to watch.

For me, moving forward:
1. Opened up the hip angle - biomechanically better IMHO for sustained power generation.
2. Allowed me drop my front-end relative to the road-bike - reduced frontal area and by inference CdA.
3. Did not cause any drop in TT versus road-bike power (once adapted)

If one's torso is particularly slender, I can see the benefits could be reduced but for most people fwd is the way to go.

Anyway, why DID the UCI limit the saddle tip relative to b-b if there was no performance advantage?

Rick
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DATA? 12 years, 9 months ago #595

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Simple question re Eric:
Why does he ride on his saddle tip while TT'ng? Doesn't that tell us where he'd LIKE the saddle to be positioned.


Sure. Heck, on my road bike when riding on the hoods or drops hard on the flats, I creep forward too when my saddle is too high or my reach is too long. From my experience, “tippin’â€

Data? Here's Heil's data... 12 years, 9 months ago #596

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-kraig

Re: Switching to shorter cranks on the TT bike 12 years, 9 months ago #597

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Kirk,

I wouldn't suggest that forward is more aero. I would suggest that lower shoulders are more aero (using the Pareto Principle here). If I start with the saddle 5 cm back, and get my shoulders as low as my biomechanics/comfort will allow (I have to ride 5.5+ hours like this in my event), my shoulders are "X" high off the ground, and my thigh-torso angle is "Y".

If I make a little Tinkertoy guy with that thigh-torso angle of "Y", I can rotate him forward -- and his shoulders get lower than "X". Along with that rotation, his butt moves forward (and up slightly). That is the basis for my suggestion that a non-UCI TT position might work better with the saddle up forward.

And, while I have not the data to support it, I think it goes without argument (?) that a more open thigh-torso angle is, all else equal, more powerful. Muscles contract most powerfully as they pass through their relaxed length. This is not an issue with the quads and gastroc/soleus muscles, but a more open angle benefits the hams and glutes significantly by reducing the length of the muscles during contraction back closer to their resting lengths.

Why else does almost everybody sit up when climbing hard? When the hammer goes down, folks like to shorten the length of the glutes by opening up the thigh-torso angle. I wouldn't take that observation lightly.

While the "reach" issue does cause riders to creep up on the nose (most seem to like their arms tucked up tight when hammering, contrary to the Superman position), we should not so quickly dismiss the possibility that they do this nose-creep in order to lengthen out the hams and glutes to spread the work around and add power. Perhaps Wohlberg creeps up to get more power out of his glutes.

It has been shown that the quads and gastroc/soleus muscles can transfer more force to the pedals than they directly produce (they must do so, actually), so extra glute power can add to total power.

Re: Switching to shorter cranks on the TT bike 12 years, 9 months ago #598

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Having said all that, Kraig's saddle is UCI-legal, and mine is 2cm in front of the bb. I can't get my bars any lower.

Today we went head-to-head in a 20k. His SRM read 240 watts; my PT read 230 watts. Given the usual adjustments, that made us nearly equal. He beat me by 1:22. Even adjusting for his disc (we both had aero front wheels), that is a big honking difference.

I've clearly got more work to do.

Re: Switching to shorter cranks on the TT bike 12 years, 9 months ago #599

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some good comments all 'round. I think Mr. Ashburn stated things very nicely for me I haven't got that way with words... but yes when I say fwd - I mean fwd insofar as that allows us 'typical builds' to lower our front end and thus decrease torso angle.

My effective STA is ~80 deg based on my saddle setback and where I ride ....

Let me see if I can free up my Yahoo position pics - they're nothing special but I'm trying to improve.

Rick
GIZMO marketing specialist

pics 12 years, 9 months ago #600

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http://pg.photos.yahoo.com/ph/rmur2004/my_photos

check the RM TT position folder ....

Rick
GIZMO marketing specialist

Assumptions 12 years, 9 months ago #601

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I think my point is being missed here. My point is that the wide-spread suggestion that "moving the saddle forward is better" is baseless if:

1) CdA improvement is not documented or not at least implied by a frontal area assessment.

2) Power is not documented to be equal between positions.

If one can document #1 and #2, then one has optimized position.


Add practice makes perfect too! Muscles adapt to the stresses imposed...including new hip angles just like any sport with a unique range of motion. So, if you haven't verified what #1 is and contrast it to adaptations to produce #2 (maybe adjusting to a tighter hip angle than you are used to takes a couple of weeks), then assumptions not based on an empirical assessment are being made and you have no idea if you have optimized your positon.

w/CdA is all that matters! Cheers, Kirk

rule of thumb... 12 years, 9 months ago #602

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I think the main contention here (from my perspective, anyway) is the broad brush recommendation of jacking the saddle forward to preserve hip angle. I dug up this abstract that I snagged off of lwwonline.com (I think that's it...) by klippel and heil - this is the study that inspired me to jack my saddle forward and try it out for 8 weeks or so last year...



The way I read this is that these guys suggest one should ride in the configuration you are adapted to (delta hip angle = 0- and also note that this adapted position _could_ be one that is an acute angle - for their analysis they chose to assume otherwise) to produce the most power.

For best aerodynamics (i.e, FA), STA should be LESS, or the saddle should be moved back). So, if you are adapted to a very tight/acute hip angle, moving your saddle forward is not the optimal solution according to klippel and heil. And, how does one know if they can ride an acute hip angle unless they try it and monitor power?

FWIW, my experiment last year with a moved forward saddle resulted in me moving it back since 1) I didn't gain any power, and 2) I actually documented a slightly higher FA with the seat in the forward position.

Also, as an aside, the klippel Heil abstract above is based on some debatable assumptions, IMHO. Their Cd equation is shaky, and their delta hip angle info has substantial scatter in the data -> which suggests individuality and hence, the difficulty (for me anyways) to blindly assume that moving the saddle forward is the right thing to do.

It's this thought process and this research/data that makes me flinch a bit when I hear the "steep" bike inventor say that the scientific literature has never produced data that questions the "forward is fastest" bike design concept.

My mantra is less complicated to understand:

"faster is faster".

Thank goodness there are tools available out there to let us find the way intelligently and efficiently.

And that's about all I have to offer on the subject - thanks for the discussion, fellas!
-kraig

Re: Cranks...saddle position 12 years, 9 months ago #603

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Despite my opinion that moving the saddle forward within reason works well for some, I have to say I also very much concur with both the spirit and the substance of this:

Kirk wrote:
If w/CdA improvement is not verified, having a different saddle position or cranks on one's TT bike vs. one's normal bike is baseless.


and this:

Kirk wrote:

"faster is faster".

Thank goodness there are tools available out there to let us find the way intelligently and efficiently.

They make it tough on us! 12 years, 9 months ago #605

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Another very important issue with the commercially available bike is their geometry. For example, almost everyone who buys the brand that Andy Coggan tested to be slightly faster ( http://www.biketechreview.com/aerodynam ... _frame.htm ) must go through all kinds of radical things to NOT have to jack their saddles forward…THOSE FRAMES HAVE MASSIVELY LONG TOP TUBES!

Look at Wohlberg in the photo link above…he basically has his stem at his head-tube…and if that is a not a custom bike, he has a 55 top tube with the 75 degree seat-angle needed to get 5cm behind the BB. No wonder he inches forward on that saddle…he can’t get his bars close enough if he tries to get low!

If you look at my position in the “0.183â€

Re: Switching to shorter cranks on the TT bike 12 years, 9 months ago #606

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I guess we all have our biases. Being of large frame and simply unable to ride at an acute hip angle - it was a quantum leap forward when I decided to try moving forward (in increments and especially when I got a decent TT bike). I believe I'm right in the Heil 'typical' road postion with 74deg STA and ~20deg TA.

I don't have readily accessible testing means so the only 'test' for me was to move forward, extend my aerobars to maintain the same upper arm angle, drop out a couple of spacers and see how my power was affected.

I don't have long stretches of flat road and very seldom have calm, light traffic conditions in which to field test. Structured CdA field testing isn't really practical here.

Since 2002 I've dropped my shoulders by 5-6cm, moved my TT position fwd by some 10cm and dropped my local TT time (21k) from 30:50 to 28:10. I figure my Watts/Cda ratio has improved 25-30% over that time
as my torso angle has decreased ~5 degrees.

Clearly I'm not going backwards ..... neither figuratively nor literally

One point that I haven't quite caught from you guys is: assuming one adjusts elbow-pad and extension position forward to match one's forward saddle shift - what is the mechanism for increased FA - and even assuming worst case that you don't drop your shoulders (which is the whole point of moving fwd)?

Are you picking up *significantly* increased FA just from a small increase in saddle height? While keeping your upper body angles the same? I must admit - I can't see that being significant - especially compared to the (for a lot of folks like me) significant lowering of the torso made possible by moving fwd.

In any case at ~80deg eff. STA and with UCI restrictions, I'm likely at the fwd limit now and can only hope to reduce my torso angle further via my shiny-new adjustable stem and A900's.

So back to my question, at a set STA of 80 deg, wanting to get lower but anticipating some significant thigh-torso interaction soon (as I drop) are shorter cranks a smart or dumb idea?

That was my original question. How did we get 20+ posts on here?

Rick
GIZMO marketing specialist

Re: Cranks...saddle position 12 years, 9 months ago #607

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JV wrote:
Despite my opinion that moving the saddle forward within reason works well for some, I have to say I also very much concur with both the spirit and the substance of this:

Kirk wrote:
If w/CdA improvement is not verified, having a different saddle position or cranks on one's TT bike vs. one's normal bike is baseless.


and this:

Kirk wrote:

"faster is faster".

Thank goodness there are tools available out there to let us find the way intelligently and efficiently.

But how can you verify the w/CdA improvement without trying the position? I think that you cannot give blanket guidelines to everyone of all bodyshapes about what arm angle, saddle position, whatever will be best for them. Everyone is different - the effect on their CdA and sustainable power from changes in position is not 100% predictable.
"try it and see if its faster" is my mantra

Re your 0.183 position Kirk - what I would try is dropping the saddle slowly and see if you can sustain your power - when you get to a point you cant then raise it slighly. Secondly try taking those fake breasts off...



I'll post a gif when i get home showing what moving my saddle forward did to my position and in particular my back.

Bob
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