Do most of you use this formula that I found on the slowtwitch website:
C=.005D^2-.2D-1.5 +/-1.5
C=armrest drop
D= saddle height from BB
.
How about just removing all your spacers?
How about just removing all your spacers?
Not sure I can go that low at this moment. I would rather slowly get to that point. Is that bad?
I used it to check that my existing drop was accurate. I am right at the low (more drop) range of the formula if you ignore the first “minus 1.5”, because I have short legs and long arms, which necessitate more drop for a given hip angle. But, IIRC, most people would do well to fall within the range given by that formula, depending on the length of the race they’re doing.
that formula for me gives:
138mm
mine is currently:
187mm
so,
no.
g
I believe that you were “Fit” by a professional when you bought the bike… right ?
My advice for you would be to trust your fit. At least until you can get it out on the road for a couple months. If your bike feels like you could use an adjustment, take it in for a fix.
If your fit specialist knew what they were doing, you shouldn’t have to worry about the math. The drop is based on a lot of parameters that start at your toes and go all the way up to your fingers 
Just go with the 1 foot rule :-). Start at one foot and then post your picture on ST. Then lower your saddle by 2 cm. Then put on a LG Prologue Helmet and repost your picture. Make sure you shave your legs. Then remove all your spacers. Make sure when you post your picture your race wheels are on and that your ironman finisher picture is in the background and you have a stack of old VHS tapes starting from the 1989 Ironwar to the Luc Van Leirde vs Hellriegel 1996 classic when Hellriegel posts the fastest non winning Kona finish time…
Aside from that, just go out and train and keep tweaking…armrest drop is highly dependent on the length of your upper arm, flexibility, and saddle position.
I believe that you were “Fit” by a professional when you bought the bike… right ?
My advice for you would be to trust your fit. At least until you can get it out on the road for a couple months. If your bike feels like you could use an adjustment, take it in for a fix.
If your fit specialist knew what they were doing, you shouldn’t have to worry about the math. The drop is based on a lot of parameters that start at your toes and go all the way up to your fingers
Yeah I was fit for the bike, but it was only the initial fit. When I took the bike out yesterday and today, it felt really twitchy upfront even though my upper body was relaxed. I have never ridden a bike with this many spacers, so the handling is quite a bit different than what I am use to on my road bike. I am just trying to get a general idea of how much I might want to drop the front end when I go in the next time.
When people riding behind you can’t see the back of your head, it’s almost low enough. 
When people riding behind you can’t see the back of your head, it’s almost low enough.
Yeah that is one thing that I am questioning about my fit. He kept telling me how my position was really good since my back was nice and flat. However one thing that he forgot to realize is that I am going to be getting an aero helmet eventually. So shouldn’t I be able to drop my front end down even more and still remain aero with the use of the helmet helping to maintain that aero flow over my back.
Bike fit is tricky stuff. I have a strong opinion about my own setup, but have always hesitated about giving other people too much advice, even in person with them riding on a trainer in my garage.
I am a complete math moron.
Could someone tell me what he formula answer would be for a 69cm BB to seat measurement?
I am a complete math moron.
Could someone tell me what he formula answer would be for a 69cm BB to seat measurement?
8.5 cm (+/-1.5 cm)
.005 (69)^2 - .2(69) - 1.5 = 8.5 cm
I think…
I am a complete math moron.
Could someone tell me what he formula answer would be for a 69cm BB to seat measurement?
8.5 cm (+/-1.5 cm)
.005 (69)^2 - .2(69) - 1.5 = 8.5 cm
I think…
Thanks Dude. 8.5cm seems like a pretty small drop. I’m at 14cm right now and tryiong to get to 15+ cm
Shane D
two things:
-
you’re missing an important element to this, and that’s correcting for seat angle. this formula is set for 77° of seat angle. that (.2) changes based on seat angle variance. the change is .0075 per angle. if you were at 78° of seat angle, it would be .1925D. at 79° it would be .185D.
-
also remember that the -1.5 is sort of the age-group factor. if you’re a seasoned age-grouper, fully adapted to the aero position, fit and trim, and what’s keeping you from being a pro is the engine, not the chassis, then you’re probably best just disregarding that -1.5.
Don’t forget the anal lube on the table either.
According to the formula
C=.005D^2-(.2-Y*.0075)D
using D of 75, and Y of 3 (I’m at 80* STA), I get 14.8125cm drop. I’m currently at 17cm, comfy, and no apparent loss of power. So it’s a starting point, anyway.