Looking for new smaller frame, how to transfer set up from current bike
 

Hi All, I'm looking to pick up new road frame & fork. Right now I'm riding a 58cm frame that is a little too big. The top tube is 575mm horizontal with a 70mm stem (-7 deg rise) on the current set up. Current bike is set up pretty well for me (just did 75 miles with no upper body pain). Top of bars are level with saddle.

Question: is transferring dimensions simply horizontal top tube length + stem length? So if I found a new frame with a 565mm top tube length would I most likely need an 80mm stem? 560mm TT + 85mm stem, etc.

Is it really that simple?

If the current bike is set up pretty well, & you ride it with no pain, then why are you looking for a smaller frame?

Trying to understand what you're trying to solve with a differently sized frame.

Must have found a snazzy frame that he wants to transfer his current set up onto.

If the new frame is the same geometry, then yes. Your method should put you pretty close. However, if you're switching to a frame with a sloping top tube or one to a flat, your fit is going to be thrown off a bit. I'd still recommend being refit to the new frame.

if it's a little too big for you than you probably want that total number to be a little bit less right? get a frame with that 1 cm shorter top tube and keep your existing stem, otherwise your going to pretty much be back at the same point.

The current frame is actually a hybrid frame set up as a road bike and meant to be temporary. Canti brakes and a fair amount of frame flex. Want a proper road bike frame. I'll take the current (big) frame and build it up as a commuter. I'll transfer as much as I can from the current frame to the new one.

The current fit on the big frame is good (hence the short stem). Current frame is a 58. I'll be looking to buy a 56 or 55.

I'll likely be buying the new frame used on eBay/online so getting it fit isn't an option.

You can do that if you like, but if you are going to keep your currect bike, be aware that components are cheaper when you buy them with the frame, i.e. the entire bike. The bike companies, by buying in bulk, get a better deal.

I would recommend you look for a road bike with the components you want on it. Your LBS can set a bike up very close to your current configuration.

As others have mentioned the only sure fire way to exactly transfer your fit is with a professional bike fit but you can get close on your own.
I just matched a fit from one bike to another bike that I just built and the measurements I took to get my position close to the original were:
-Seat height from Bottom Bracket
-Drop a Plumb line from seat nose, measure distance to center of bottom bracket.
-Measure how far from level your seat currently sits. i.e I run mine slightly nose down to where the level bubble doesn't sit in the center of the two lines but barely touches the inner line on the one side.
-measure reach from seatnose to center of bars
-run a level from seat to bars and measure drop from bottom of level to top of bars. If the seats not level it's ok, as long as you are using the same seat with the same angle then the drop to the bars will be relative.

It's important to note that i used a seat on the new bike that very similar in size as the one on the first bike. I also used one of the specialized adjustable stems which uses different shims to get different angles. I bought a 120mm and 110 mm and the shop let me return the one I did not need. This allowed me to get my fit close without guessing or buying and trying multiple stems. The shims I had allowed for +/- 4 +/-8, +/-10, +/- 12 degree angle changes. Because the bikes had different top tubes, headtubes and angles I went from a 110 -8 stem, to a 110 +12

Only thing I would "alter" in the above is how you measure your seat height. I measure from the center of the Bottom Bracket but follow the Seat Tube up through your "Seat Post" to the center and top of your saddle.

Of course we are assuming that your crank arms will be the same length, if not, add or subtract the difference to equal out.