I could use the forum’s collective wisdom. I’m hoping this topic catches @Nate_Pearson’s eye, because I’ve heard he’s tall. It’s been mentioned before, right? 
I’m getting ready to drop some coin on a first TRI bike. I’m not as tall as Nate, but at 6ft 4in (193.04cm) tall, 215lbs/97kg weight, I’m looking for a TRI bike that offers me adjustability and compliance. I like a ride that feels strong and safe.
My eyes are turning to a QR PRSIX DISC with a 58.5cm frame. Good choice, or wave off?
Thanks for taking a moment to share your thoughts and opinions. 
Fit is important on a tri or TT bike. I’m 6’2" and used to be about 205 - down to 185 now (thanks TR). I had an old Cervello tri bike from 2006 that I got back into racing on last season. I got that bike in a 58 inch size around 2008(because hey I normally ride a 58) - thats what I had so it’s what I raced on last year. I was interested in a new bike and got a used frame that was a few years old I’m building up over the winter for next year.
Tri bike fitting is all based on stack and reach. Picture a graph with an x and y axis and the origin is your bottom bracket. reach is the x value and stack is the y value to reach a measurement on the frame that correlates to your positioning of your forearm pads or headset(different manufacturers I think can measure this a little different based on their proprietary definitions) but most often its to a point on the pads. The idea is once you know your stack and reach you can figure out what bikes and sizes work for you.
Before I got my new frame I got a bike fit on my old bike to figure out my previous stack and reach. Also had a “guru” bike fit where the fitter put me on a computer adjustable bike and he adjusted different positions for me and then they have a database of bikes and sizes they recommended for me. It was actually very enlightening as despite being tall and lanky with long arms I have a relatively short reach and fit better on smaller frames. I ended up on a medium 54" frame and normally ride a L/XL 58/59 inch frame on road and mtb.
I just finished up most of my build and got my new ride on the trainer and it’s spot on.
A couple hundred bucks on a bike fit to figure out your stack and reach is a good investment before dropping big coin on a new bike.
check out slowtwitch.com forums and you will go down a never ending rabbit hole on tri bike fit discussions
You just need to figure out your saddle height, stack and reach.
If your saddle height is really high, you’ll probably need to contact manufacturers individually and ask them if the saddle can go that high. If you don’t have a crazy inseam you’re probably OK.
If you know your stack and reach you can go over to slowtwitch and see what bikes would fit you.
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I tried to find the page(s) with the bike stack and reach info, but had no luck. I found general sizing comments, but no data for bikes with S-R or Pad X-Y.
Do you have a link?
Appreciated! Bookmarked for future reference. Thanks a bunch. 
From this fit report:
and looking at slowtwitch database it looks like I should use stack and reach in bottom left “Frame Stack 619mm” and “Frame Reach 380mm” is that correct?
That’s without a stem. So you have to then figure out what stem combo/handle bar combo you need.
There’s another tool on the site for that. You can say at this bike stack/reach, what would it be with a 100mm stem with a 7 degree rise.
Then boom, you can get everything super dialed.
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