OK, let's start over.

You're saying your *pad* stack is 537 and your pad reach is 416. That's low, and too low for a 51cm as well as too "short". So, you're a 48cm. You would need to be at a minimum stack of around 560mm and reach of around 435mm at that stack to make a 51cm work.

Eric

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Hi Eric - thanks again for your advice - going with XL was definitely the right decision. The position is great and I found I have plenty of options to adjust the position.

Unfortunately I couldn't race in Cozumel after the UK government made non-essential travel illegal in November. Ironman have been amazing and deferred my entry to 2021 without having any legal obligation to do so (the restrictions were announced post deferral deadline).

I have found a comfortable position close 15 degree tilt (slightly longer reach), but somehow I cannot get used to the short 165mm cranks ... my FTP seems about 10 watts lower with 165 cranks compared to 175 cranks (tried ramp test, 20 min FTP test and an hour in a Zwift race).. currently playing comparing the wattages of the crank pms and kickrs (one 2014 and a Kickr Core I got for free with the P3X from a Cervelo promotion) to see if it's the hardware. Might go back to 175mm but no rush at the moment.

One thing I noticed is that every roadie tries to drop you on the P3X, seems to stir up some bad blood in people :). I do most of the tough stuff on the Kickr, so not really too bothered (and at 51 I am no longer at my peak).

Overall an amazing bike - can't wait for the London perma rain to stop. Thanks again!

Have you tried 155mm cranks?

:-) ... no, I haven't. The 165s already feel strange, I'd imagine 155 would be like a kids' bike.

Maybe just a question of getting more mileage in.

Hi Eric. Just like Adub and Palmer45 above - keen to try TT/duathlon, good current P3 & P-Series deals, can't get a fit just now. 184cm, 88.5cm inseam, ride a lot but not super-flexible. Road bike saddle height 78.5cm, 7.5cm tip of saddle to BB (horizontal), 58cm tip of saddle to centre of bars/stem (sloping).

Realise it's a bad guess without proper fit but do you think a 56 could/should work, or more likely 58? (There are great deals on a 56...if it fits). Would a bar with lots of adjustment help? Thanks!

If you tweak your position a bit, you might be faster with 155mm cranks.

My younger son is 5' 11" and rides 155mm cranks.

Interesting - tweak in which direction? Seat up/down/forward/backward? I'm 6'3", so if I can get the 165s set-up well, I am not sure I would have the balls to try any shorter.

I would suggest considering what you gain for that 10w "loss" of FTP. There's three areas where you gain... time saved because of aerodynamics, time saved because of efficiency (comfort/power/aero longer), and time saved on the run.

Eric

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I think that at your height and given the adjustability options of the P-Series cockpit, not to mention the ability to replace the OEM stem with an aftermarket one, you could make a 56cm work, but it might be tight and I'd rather see you on a 58cm if I were to make a recommendation sight unseen. I'm guessing it won't be a problem for you until your TT position matures and evolves after a couple three seasons.

Eric

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With shorter cranks, the saddle goes up the same distance as the difference in crank lengths. You would also move the saddle forward a few millimeters (just check your saddle setback).

Wouldn't we move it back to maintain power stroke orientation?

E

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I was offering information on how to maintain the same saddle position.

So am I, indirectly.

E

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Thanks Eric. I’ll likely get the frameset to build up (most likely a 2018/19 P3), if that ends up a 56 would investing in something like a Vision TFA likely give plenty of scope to adjust position as I go, or would separate bars/stem and changing stem lengths if needed be more flexible?

Thanks again, especially as I know I’m not giving you enough to work properly with.

Vision TFA is a great system, as well as TriRig Alpha One. Both fast and both adjustable.

LMK if you need any fit help.

Eric

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Update on my side. Got the 58cm and very glad I did. Fantastic bike firstly.

Managed to get fit on it afterwards and got really nice and low, back angle of 18degrees but surprisingly comfortable! Option to get even longer for the shorter stuff as well so big thanks Eric! :)

Awesome, thanks. Great to hear.

Eric

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Hi guys, thanks for your input. I did what Jim suggested and put the saddle slightly up and forward. This feels better at sweet spot powers - I feel less strain on the calves. I will do a new FTP test after the next rest day and see. Will let you know what I find.

I am not convinced the 10 watts are real - could be a lot of things (4 different power sources which all have their own inaccuracies, maybe some unconscious bias against anything new/unusual, day to day fluctuations etc.). Also I have no clue what I am gaining from the new position in hopefully a better aero position. Would love to do a race, but seems somewhat off here still.

It's an interesting experiment - I didn't expect such fairly fundamental differences from small-ish adjustments.

Hi,
Arm Pad Stack: 618
Arm Pad Reach: 423
Arm Pad width: 221

Looking at P-Series model.

Thanks in advance.

Hi Eric,

I'm interested in upgrading to Cervelo P5X, P3X, PX or P5disc . As of now, i am riding 2016 BMC TM01 size Small with measurements below:

Pad X Reach (BB center - Saddle to back of pads): 425
Pad Y Stack: 605
ArmPad Z (CtC): 210

Saddle Height: 675 (with 170 crank)
Inseam: 79cm
Height: 169cm

Would appreciate your feedback for the best size for me on those respective bike models.

Thank you very much for all your help!

Hi, thanks for reaching out.

For a pad stack of 618mm and a pad reach of 423mm you are in the "upper left" quadrant... you'd fit on a 51cm P-Series bike, but if you moved just 15mm forward you'd be on the back end of a 54cm. If it were me, I'd explore with you your relation to your saddle as to how you sit on it and also your saddle in relation to your BB to see if you were "slack" and upright, in which case I'd probably push you toward a 54cm P-Series.

Make sense? A pic might help.

Eric

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Hi, thanks for reaching out.

With a pad stack of 605mm and a pad reach of 425mm you'd be in the upper-right quadrant of a 48cm P5d or the center-left quadrant of a 51cm P5d. Center-center for a size S P3x or lower-left for a size M.

For the P5d and P-Series I'd put you on a 51cm and for the P3x it's a toss-up between size S and size M, leaning toward size S, depending on how you think your fit would evolve in the future. Size S if you're pretty locked in and size M if you think you have room to grow so to speak. You'd have 35mm of reach to play with on the size S.

Eric

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Hi Eric,

Thanks for your reply. So if thats the case the safest bet for me would be size S for a P3x? What is the downsize if I pick the medium?

I think S is the safest and best bet, no downside to M other than weight.

E

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Thanks again Eric... S it is then... :)