ThermionicScott 

This seems like the best forum to ask...

Why is it that frames with slacker angles are considered to be more comfortable? Is it because the fork and seat tube will flex rather than transmit the bumps directly? Or because the longer chainstays will flex and absorb the shocks?

Or is it none of the above?

Thanks!

veryredbike

With seat tube angle, it has a lot to do with your position relative to the cranks. As you come forward you put more weight on your hands.

With head tube angle I'm a bit out of my depth, but I think it has more to do with the style of riding. If you're making a comfortable bike, you don't make the steering aggressive and twitchy, you leave that for "more pain equals more gain" crowd ;-) It'll affect the relative handlebar position... but that's something correctable with the stem length and top tube length... so yeah, I think it has to do more with what you're aiming for steering wise.

Road Fan

laid back seat and head angles usually imply more front end flex to cushion bumps, and a longer wheel base that (all other things equal) leads to a cushier ride.

Andrew R Stewart 

It is common thought that the rider's stance should be the same regardless to the frame's geometry. Or, another wards, the frame's angles shouldn't effect how the rider is positioned (seat post being set back or straight to get seat set back over the shell the same for differing seat tube angles). But as the frame tubes become more horizontal (blades included) the vertical forces are more in bending then compression. Also in the past slack angles usually were combined with longer wheelbases. And the longer the wheelbase is the further out the lever the bump starts. This is one reason why tandem stokers have seat issues,. In the attempt to keep the tandem handling quick designers try to keep the wheelbase as short as possible. And since few tandem designers ride stoker (yes, I know about Mr Santana) they don't consider the stoker with as much concern as the y do the captain and his steering the bike. Andy.

fietsbob

being a triangle the rear end does not flex much, .. unless like Richey or Hetchens,
you make the tubes not straight, so there is some bending,
rather than straight down the tube compression.

hueyhoolihan

i find it hard to believe that a more vertical seatpost tends to put more weight on the hands. if so, a unicyclist should have the most weight on their hands, yet they have none!

c_booth

The comparison doesn't stand up. Unicycles (by their very nature) have no handlebars, so you have to sit bolt upright to keep your centre of gravity over the cranks in order to ride the thing. Extend that 90deg geometry to a bicycle and it would shift your centre of gravity so far forward of the cranks you'd be placing a massive amount of upper body weight on your arms. Even the most extreme TT frames don't go that far.

c_booth

This is OTM. And just to follow on from my last post, the steeper the headtube angle, the more upper body weight is thrown forward onto the handlebars, which itself creates a twitchier ride. It also means less fork rake is required to hit the trail sweetspot (which again will vary depending on the type of ride you're aiming for) which in turn pulls the front axle back towards the cranks - this of course shortens the wheelbase, resulting in a more responsive ride (a good thing on a track bike), but it also places your hands/weight further over the front axle, which also effects handling (to what degree I'm not so sure), and the only way to shift that weight back would be to shorten the stem, which would undoubtedly effect a further increase on the twitchiness of the ride.

So designing a frame/fork set i always think is kinda like filling in a jigsaw puzzle - sometimes the bits don't quite fit perfectly, in which case it's about compromising in a way that will least jeopardise the type of ride you're aiming to achieve.

fietsbob

If you are racing then there is arm and upper body effort applied to the bars,
as well as the strong trunk muscles from weight training to stay on the game.

slacker seat tube part typically of a bike made to sit upright,
perhaps part of looking good, on the streets of CPH ..

race bikes have custom parameters if you need them ..
seat angle related to thigh/femur length, in relation to the crank.

Headtube /fork rake is determining the handling .
Town block circling Criteriums, or stage racing 5 + hours, between cities ,
has different needs.

hueyhoolihan

my post referred to the seatpost. as most of the thread was a discussion of the seatpost angle.

most of us can ride a bike without the use of handlebars (like a unicycle). so i'm asking what does the angle of the seatpost have to do with comfort unless the discussion is equally focused on where one grips the handlebars. which in turn is determined by the toptube length, stem length, shape of the handlebars, whim of the rider at the moment, and where the saddle is clamped on the post?

veryredbike

The difference is that, unless you are riding no hands ALL the time, some of your weight is forward of the cranks, which means that the amount of weight behind them determines where the point of balance over them is (while pedaling with any real force). Try this: stand straight up, lean forward at the waist, see how far you can lean forward. Now stick your butt out and try the same thing.

I've helped hundreds of people with riding pain while working at a shop, and this is a pretty typical issue. People move their seat forward to reduce the reach to the handlebars, and make things worse by shifting their balance over their hands more. You CAN compensate by moving the seat around on the post, but only to a point. The angle of the seat tube determines the range you can work with.

c_booth

Edit - that should read steeper the seat tube angle throws weight forward.

c_booth

The angle of the seatpost is determined by the angle of the seat tube though, which in turn effects where your centre of gravity lies relative to the cranks which, assuming the top tube etc is the correct length, has the greatest determining effect on the comfort/feel of the ride, because it determines how much weight you're arms have to bear, has a knock on effect on trail/fork rake and chain stay length which all in turn have an effect on the way the bike handles.

The unicycle comparison is a red herring, since you're never required to lean forward onto a set of handlebars as on a bicycle. Notice when riding no hands that you still have a fair range of forward/back upper body movement, due to the cranks being set forward of the saddle - ride no hands on a bike with a 90deg seat tube angle (same as a unicycle) and you'll barely be able to move beyond the vertical without falling forward onto the handlebars.

hueyhoolihan

i think i understand and agree with this.

if so, to remain balanced it would seem that the smaller the angle of the seat tube the longer the reach, and everything that entails, must be. thanks.

hueyhoolihan

this was my point. a steeper seattube angle eventually results in no hand pressure at all.

c_booth

I think we must talking at cross purposes here. I'm pretty much in agreement with veryredbike. By steeper seat tube angle, do you mean a seat/top tube angle approaching 90deg (eg as on a unicycle)? In which case no, on a bicycle it would result in far greater hand pressure.

veryredbike

C_booth: exactly. As you move the seat closer to being over the bb, you have less to counterbalance your weight. Your reach to the bars decreases, but the weight on your hands increases.

The no hands thing isn't relevant to normal riding, because riding with no hands requires you to bring your weight back above the BB, or at least much closer to it. Steeper angle would only lead to no pressure at all if you didn't have your hands on the bars or if the bars were to rise and come backwards as the seat tube tilted towards the front of the bike. If you just rotate the tube until it's going straight up from the bb, you can't lean forwards without requiring hand strength to balance you.

Road Fan

Actually it should read "inadequate saddle setback throws weight forward." Seat tube angle, seat tube setback, and saddle rail adjustment are all tools for getting the saddle setback the rider wants.

Andrew R Stewart 

I don't completely agree here. But you point about no compression in a straight tube VS a curved tube is mostly right. But some of my comments had to do with the length of the rear triangle and where the load is WRT your butt. The further rearward of your butt the load enters the rear triangle the less displacement you feel.

On another fitting aspect the amount of seat set back will effect the weight you feel on your hands. (Somewhat independent of your reach length). When you butt (I love that word) is well positioned above (and behind by the "right" amount) your feet you can support your upper body with your core and not only your arms/upper body. The way my boss shows this is to ask riders to go from the hoods to letting go and sitting upright. With poor core usage one tends to push back with their hands as they sit up. With good positioning that employs your core the sitting up motion is fluid and needs no help from your arms. Additionally when dropping back down to the bars the motion is not one of "falling" and "catching. It took some wrapping of my mind around this idea but I've seen it during fitting sessions many times. Andy.