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There are 2 iterations of this bike: what I call the IA single-digits and double-digits. These are the IA 1 thru 4 and the 10 thru 16. What is the difference? The single-digits is the superbike; double-digits is the mortal version. What is the aero difference? Little to none with a little work (like a TriRig front brake). But each bike has its appeal.
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As I travel around the country to Slowtwitch Road Shows the job I've given myself is not to be the emcee or the raffle winner announcer, but the bike fitter and prescriber. This has always been what people most want. It's what I teach in F.I.S.T. Workshops, and what I most hope happens at our Road Shows is that the fitters in these stores work with me and pick up whatever is helpful when I leave the store; and that brand reps who might not be entirely up-to-speed on the prescribing methodology add to their expertise.
I can pretty quickly prescribe most bikes for you based one or another among x/y measures. In the case of the IA (both IA styles) a guide for sizing is Felt's own Excel based calculator. There is always noise in these processes, but I think this calculator will get you close.
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You can download Felt's calculator
from its website. You type in pad y and x. You get results, and just as with most x/y calculators (Trek, Cervelo, QR, et al) you have a choice. Unless you know how the IA 1 thru 4 is made it will be hard to divine some of the calculator's prescriptions. The two images highest up are of the Felt IA 1 thru 4 pedestals below the aerobar. There is a 15mm and a 30mm pedestal. When you read “Stem: Integrated” on its Excel-based fit output that means it's the IA 1 thru 4 that's being prescribed. When you read “Stem: 31.8” that means it's the IA 10 thru 16 that's prescribed.
“Stem: Integrated w/15” means it's the IA superbike (IA 1 thru 4) with its integrated stem and the 15mm pedestal underneath shown above. The “w/30” is with the 30mm pedestal in the image above. But you still have more pedestal options, and these are pedestals under Felt's aerobar pads. If you see “+20+5” that means it's 25mm of pedestals and Felt is recommending you stack those two pedestals.
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Felt has a bridge, to keep things solid, and if the pedestal stack gets high it might prescribe the bridge that spans armrest to armrest. It might prescribe pedestals, the bridge, then more pedestals. That bridge just keeps things stable and strong, just like 3T's new bridge on its new Revo aerobar.
Felt has 1 and only 1 stem for its IA 10 and up, and it's about 100mm long. For all the IA bikes, super and mortal, you find your length through moving the pads fore and aft on the several sets of holes, and you move the extensions forward and rearward. There is a fore/aft dead spot I discovered over the weekend. You have 3 sets of holes in the armrest and you must use 2 of them, so, you have about 15mm of fore/aft adjustability by using the forward or rearward holes. When you need to go more forward with the pads then you need to put the pad clamp in front of the extension clamp. That's a 50mm jump, so, the delta between pad-forward on the clamp-rearward position and the pad-rearward on the clamp-forward position is 35mm. That's the dead spot.
There is a work-around, and that is to use a yet forward armrest cradle holeset than was originally contemplated, and a corresponding rearward holeset combo. It's in the image below.