New Math (Frame Geometry)
 

I have been an avid road cyclist for more than 25 years.

I would like to think that I have a good basic understanding of frame geometry, but with the movement towards compact frames I feel like my knowledge is moldy. I still think in terms of Top Tube, Seat Tube, Head Tube lengths...and HT angle/ST angle.

However I have been hearing more lately that since manufacturers are lying about their geometries it makes more sense to look at "stack" and "reach."

I want to learn a bit more about this but I suspect it will require a change in my thinking.

Anyone know a good website to help me convert my old math (geometry) thinking into the 21st century??

Thanks for the link.

Looks like you still need to know your frame angles, though.

But it's a start.

With compact frames, the only thing I pay attention to is the "effective top tube length" measurement. If you can get that within the ballpark, everything else works out (usually).
If you are flexibility challenged (like me), you can also pay attention to the head tube length.

Good Luck

You can use "effective top tube" instead of "top tube."

Otherwise, most geometry characteristics haven't changed much. E.g. trail, wheelbase length, seat tube angles are basically the same.

The main differences won't be reflected in the numbers though. A frame with smaller triangles will be stiffer; frames and bikes in general are lighter than they were ~25 years ago; compacts might have 1 or even 2 fewer sizes than traditional frames. You can use geometry as a basic guide, and then do some test rides to see how it goes.

In my short experience though, I have found few manufacturer's that list effective top tube. :(

I've almost always found this in geometry charts.

OP...if you want a meaningful response, you will have to be more specific. There is no difference between todays's frames and yesterday's frames in terms of geometry other than sloping top tubes no longer makes head tube length proportional to seat tube. A level top tube allowed frames to be sized off of the seat tube because head tube always correlated in length. This is no longer the case...especially with comfort or endurance frame geo. Most compact bikes are sized 'effectively'...but the number many times relates more to top tube than effective or virtual seat tube. There is no substitute for evaluating the entire frame geometry. I will say that most on the 41...or many that got a C in geometry in school will never understand frame geometry...they don't have it in them. They may be strong riders however. The refrain mostly heard from C students on 41 is 'size by top tube' which is basically BS.

I call BS on this.

In the context of road bikes, if you get the ETT and HT right, then you've gotten a bike that can be fitted to you, regardless of its STA or HTA.

EG: for me, as long as a bike as a 565-570mm ETT and a 175-185mm HT, I can get it to work regardless of STA and HTA.

I can "try" to be more specific. I have a body type that is off the bell curve and ride custom geometry bikes.

However I am always on the lookout for the rare stock frame that might fit me. I already found an aluminum one (Nashbar) that nearly duplicates my custom geometry.

I am interested in (some day) trying a CF bike...but custom geometry in CF is way too costly. So I want to have some "grounding" in current monocoque frame geometry language so I can look at a bike frame and determine if it's close to what I have...or not.

Generally the answer has been "or not" so far...

p.s. here is the geometry for my current Ti road frame...

http://bikeforums.net/attachment.php...hmentid=258756

p.s. wow that's a tiny image...click to enlarge

and a pic of the bike as I have it set up:

http://i1228.photobucket.com/albums/...s2/bike002.jpg

And this is the geometry of the Nashbar Aluminum frame. If you compare the 60cm, it's VERY close to the specs on my custom Ti frame.


http://media.nashbar.com/images/nash...BAF-SIZING.gif

Of course you call BS on that. I didn't mean to insult your poor geometry comprehension. Half that enroll in engineering school flunk out and they were good math students in high school. Sta?...hey who needs that? Must be like choosing a color for a guy like you..lol.

You may not like my comment Datlas about your bike. But there is no way you can be close to a classic or average road bike position on that bike. To me this is classic custom bike sizing gone bad. You have to ride it bolt upright with that short a stem and top tube with your leg length. Don't care how short your torso is unless you don't have any arms. ;)
Bet your back angle is 20 degs if that on the hoods. Can't put any power down and catch too much wind in that position.
Btw, I ride more upright than many good cyclists on the 41.
If you can do it, post a picture of you on the bike with your hands on the hoods to prove my point.
If you move toward a more traditional fit on a roadbike, the world of frame availability will open up for you.

So since you are a math genius, quick - what is the fore-aft difference between a 72.5 degree STA and a 74 degree STA on a 82cm seatpost (BB to saddle top), which is longer than what most people will ride?

And what is the length of a typical saddle rail?

You do know that you can move the saddle back and forth to get the right pedaling position, right? And stems do come in more than one size, right? So given that the vast majority of people do not ride with their saddles slammed all the way on one end of the rails and with a 140mm stem on their bikes, what does that tell you?

But hey, stick to splitting theoretical hairs on the interwebz with your superior geometry skills.

I will have to look for a good photo of me on the bike but I guarantee you this fits me properly.

p.s. When someone as knowledgeable as Harry Havnoonian agrees with the design of my frame, I think that speaks volumes that the geometry is correct for me.

Pfffft. Who cares what some HavNOOBian thinks. The internet is where truth is, man. If someone tells you on the internet that your bike fit is wrong, then it's wrong.





figured I should put this: :D

:lol:

I agree with you, GZ. Head and seat tube angles, and chainstay length affect handling but NOT fit. Fit is all about stack and reach. Effective top tube length is hugely important in determining reach.

Yeah, what was I thinking?!? Thanks for the reality check.

I know better than to argue with you Mr. below average. :)
You do what you like. Be sure to pick red for your favorite color of sta...red is faster.

HTA as you know affects handling more than fit...although a slighty more laid back HTA on our Roubaixs does bring the bars back a bit more fractionally resulting in shorter reach when increasing spacer stack height under the stem... but...lets say that HTA is pretty insignificant relative to fit.

STA however is a pretty big deal. The reason is...indeed reach has to be satisfied as you believe...and stack...beauty of why we like our Roubaixs....but STA is very important for fore/aft weight distribution which is really the starting place or cornerstone of fit. STA in turn affects net reach...especially if deviating from a stock seatpost which some need, including me for optimal balance on the bike. For the Roubaix...Spesh sells two varieties of seat post...a single bolt with 21mm of setback and a 2 bolt post which is 25mm. So depending on which post your bike came with...Sworks has been sold with either, on clamp center, your net reach will be affected by 3mm. Neither are enough for me to balance my weight however. Many amateur rides including pros like Hincappie, Schleck, Boonen etc prefer in excess of 100mm of setback...saddle tip to BB center. I am long legged and ride with my Speedplay cleats a bit rearward therefore I ride with a slightly lower saddle height for my leg length. This reduces setback even fruther with the stock post that came on my bike of 21mm. My 58 Roubaix has a sta of 73 deg...and I believe Colin your 61 has a sta of 72.5 deg. Ideally I have the leg length which works better with a 72.5 deg STA like you have....but a 61 Roubaix is a hair big for my overall size...you are a bit taller. So...I run a 32mm setback post...see pic below. I ride it just forward...about 3-4mm of center for a net setback of 28-29mm or so...or about 5-8mm over stock. This dramatically changes the weight distribution on the bike and for the better. But by having a stock 73 deg STA and increasing setback, this effectively elongates my effective top tube length. This btw is OK in my case as I ride with a 120mm stem...I have long arms to match my long legs and choose to ride with a higher handlebar which effectively shortens reach. Reach should always be served independent of what bar height you ride. Most importantly over and above reach...is fore/aft weight distribution must be served for any chance of a good fit. This is a function of STA and saddle setback as it relates to net reach and why so vitally important to answer your question.
The thing about fit is...it is much more complex than just 'top tube' or head tube height. STA really affects overall reach but most importantly, it affects fore/aft position on the bike AND effective reach to the handlebars. So STA is significant.
Hope that makes sense.

Try "virtual top tube" or "horizontal top tube," then. These are still more widely used than stack and reach.

Actual top tube and actual seat tube are virtually useless unless you like to ride bikes that are too large for you.

C4L: Guadzilla and I are clearly talking about effective top tube length. Which in fact accounts for the seat tube angle, although they make an assumption about seatpost type and extension.

Weight distribution is true, but it doesn't vary much and isn't that significant on a road bike. (Unless a MTB especially when climbing.) Weight distribution is heavily driven by body posture and chainstay length. Same person should be fit the same way on different road bikes, minimizing these contributions.

TL;DR
I still disagree. Reach and stack are not easy to use, but they're what matters for it. Handling is huge, no doubt, but it is something different than fit.

Thought I maybe wasting my time with you Colin. You are right in there with the rest of the guys.
Have fun.

Given the situation, I don't think anything really has changed. Again, you just have to look for "effective top tube" with the compacts.


It is, but my recollection is that road bikes have largely settled on the 72º-74º range for seat tube angles. (E.g. the seat tube angle on the Roubaix and Tarmac are nearly identical.) And a 0.5º difference over 580mm only results in a 5mm difference.

I think you'll see a lot more variation with head tube height, trail, wheelbase and the like than seat tube angle, and you'd only see something like a 77º ST on a dedicated TT bike.

I'd think that if he needs a radically non-standard seat tube angle, that's going to alter his weight distribution on the bike, and you're almost certainly back in Custom Geometry Land to really get everything right.

What this tells me is that bikes have a lot of adjustability. The puzzling thing is why you think ETT is important.

If you want to get the "right" frame size... which means you have standard parts in "aesthetic" adjustment ranges... the most important aspect to look at is head tube length. This analogous to the old frame sizing method of looking at seat tube length when top tubes were horizontal.