Comfort Comparison - Geometry Input

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GideonReade
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Comfort Comparison - Geometry Input

Post by GideonReade »

Hi Folks, this is one for the experts, I guess.

I'm considering the two bikes I use for "road" riding (as against just riding on the King's Highway). Today's exercise is looking at the frame geometry, because I want to make the slower bike a bit faster, while not losing its greater comfort. So, I've been out there with a tape.

Q1. Here's a seat tube, and here's a saddle, with some rails, and a curved shape of compressible material. Is there a standard process or method for considering where on the saddle I take measurements from? I've tried to wing it, using a point where the width is about halfway between nose width and rear width, but I'm not sure that's good.

Q2. Bike 1 is a 1986 LBS hand built, 531ST trad British tourer, 23". Bike 2 is approx 2002 Pinnarello Angliru (ally frame), 53cm I think. Anyone care to guess at their seat tube angles?

Those above are to help me compare the setup. I'm clear that the Pinnarello position is longer & lower, so more aero. But I'm not sure by how much, and the above will help me compare.

Q3. Bike 1 was built crooked, but around 2002 a respectable framebuilder rebrazed the back end for me, 135OLN, standard dish. So I think it's very standard & correct for use with that era MTB gears. The current 3x8 Shimano gears operate very well. But, I have no trust that the existing BB is optimum in terms of chain line, since I did all the purchase and assembly myself, without the aid of t'internet. If I want to rebuild it with a modern Campag BB and 2x11 or 2x12 gear set, how might I determine which length BB to buy? I mean in the sense of getting the chainline right.

Fingers crossed!

Thanks in advance.
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Paulatic
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Re: Comfort Comparison - Geometry Input

Post by Paulatic »

Q1 what are you wanting to measure Saddle height or setback?
For height I measure, if different saddles, to the dimple area made by sit bones. Same saddles I hook my tape measure on the rail next to clamp and pull down to pedal or BB whichever you work on.
Setback a pendulum through the BB and measure to front of saddle.
When comparing a set up on different bikes I put the back wheel against a straight wall. Measure BB to wall and back of saddle to wall.
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GideonReade
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Re: Comfort Comparison - Geometry Input

Post by GideonReade »

Hi Paulatic,

I want to measure the height of my bum in relation to the top of the hoods, and the BB centre, and the reach from my perch to where my hands almost always are. Then I have a comparison between my two bikes and can fiddle with stems and posts and rails to, well, optimise both, really.

These measures may or may not have common correct names, or there may be common correct measurements I can use just as well to the same effect.

I'm not trying to determine the difference in frame geometry, as the stems, posts and saddle rail settings are all in play too. But if I can get a reasonable seat tube angle to go with a saddle-BB length, I can get Excel to draw me a comparative layout with the BB correctly positioned. Else I need a helper and a long spirit level to measure the X axis BB-saddle. And it's dark and cold outside :).
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531colin
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Re: Comfort Comparison - Geometry Input

Post by 531colin »

Do you know if the one bike is really faster, or does it just feel faster because its less comfortable/ more bumpy ??
Are the cranks the same length? Is the saddle to bar drop similar?

Unless you have identical saddles, its pointless measuring to the front or the back of the saddle; the only thing that makes sense is measuring from the "bum bone dents".
these are the measurements I would take, with the bike level....
1.. horizontal distance from bum bone dent to bottom bracket axle (drop a plumb line from the bum bone dent)
2.. direct distance from BB axle to bum bone dent, at an angle behind the seat tube
3.. direct distance from bum bone dent to hoods (or wherever you normally hold)

If you measure those 3, I think you will start to understand the differences in set-up between the bikes.

BB axle length is determined by the chainset. Modern external bearing cranksets are outside my experience, but isn't the axle attached to the right crank/chainwheel assembly?
GideonReade
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Re: Comfort Comparison - Geometry Input

Post by GideonReade »

Hi Colin,

1. Slower? Well, I don't do Strava or suchlike. But I got dropped on the tourer by some folks I can usually stay with on the Pinnarelo. Generally, I keep up with my friends much better on the Pinnarelo. And, the tourer ought to be substantially slower as presently configured. And true, it's a gentler ride. But it doesn't need to be for its new mission.

The reason for now looking at the geometry is to see if it's worth upgrading all the other slow bits, repurposing it from tourer to fastish day ride. Or if not, it may guide purchase of something newer.

2. You're right, but I need to recheck if there are any bum dents on the two synthetic saddles. Don't think so. They're very different saddles, too. Not today, it's raining.

2a. But even with rough measurements, it's clear the tourers hood position - my normal home - cf the saddle, is about 40mm shorter and 25mm higher.

Pinnarelo has 172.5mm cranks, Overburys 170mm. I think Pinnarelo's perch-BB is 10mm more, but error, in the engineering sense, in that measure is excessive. Ditto that it appears 10mm further forward perch-BB, which one might expect.

I need to do some testing, too. There's maybe signs that I get less backache climbing hills on the tourer. Could be geometry, could be the difference in bottom gear, 35" versus 45".

3. I think you're right about Campag's modern BB's, but does that mean there's no scope for variation? Will have to find out.

Thanks so far.
iandusud
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Re: Comfort Comparison - Geometry Input

Post by iandusud »

GideonReade wrote: 30 Sep 2022, 5:38pm
1. Slower? Well, I don't do Strava or suchlike. But I got dropped on the tourer by some folks I can usually stay with on the Pinnarelo.
How do the tyres on the two bikes compare. On long steady rides I don't really think that geometry or weight differences (within reason) make a lot of difference in overall average speed. What can make a big difference is tyres, and "touring bikes" generally have tyres that are biased towards comfort and puncture resistance, whereas "racing bikes" have tyres tyres biased towards low rolling resistance. You might actually find the touring bike faster with the same tyres if you are more comfortable on it, particularly over longer distances.
GideonReade
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Re: Comfort Comparison - Geometry Input

Post by GideonReade »

No, no, stop getting ahead!

There are plenty of components on the tourer that will, inevitably, make it the slower. But the sum total is too complex for one thread. I intended this thread to just discuss the (overall, changeable) geometry.

a) I think if I sit signicantly higher on the tourer, that's more drag. And it's on fast flat, or fast gentle inclines, not hills, that I struggle.

b) I may be able to set them geometrically the same. Hopefully for not much spend, although with a quill stem to "lengthen", quite a lot of hassle!

Then I can get onto the leaden tyres, load hauling wheels, huge gear jumps, slowish gearshifts...

Cheers
Last edited by GideonReade on 30 Sep 2022, 9:49pm, edited 1 time in total.
peetee
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Re: Comfort Comparison - Geometry Input

Post by peetee »

To talk about comfort, specifically.
To my mind I struggle to see the direct correlation between seat tube angle and comfort.
I’m not sure where the notion that seat angle affects comfort originated. Perhaps it is to do with the relationship to intended use; in the 70’s and 80’s it was common to spec racing frames with 74 degree seat tube and tourers with 72 degree. Tourers have greater fork rake creating comfort and longer stays that introduce a degree of vertical flex by virtue of their length and the fact that the seat stays are inclined more towards horizontal so the action of vertical movement at the rear wheel translates to bending rather more than it would if it were vertical. Conversely, for any given wheelbase, a more laidback seat angle gives a more upright seat stay angle and less give in the tubes.
There are other components in the mix, however. A seat post and saddle with greater layback will give more when rider weight resists vertical wheel movement. Longer overall wheelbase will reduce pitching and impart a more stable feel to the bike that may be perceived as greater comfort. Whatever the differences that conventional geometry choice can impart to a frame with any given material (and I concede that a frame is a dynamic structure where all the angles and elements interact and affect each other) it is unlikely to exceed the comfort achievable by an educated choice of tyres, saddle and handlebar covering.
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Jamesh
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Re: Comfort Comparison - Geometry Input

Post by Jamesh »

Tourer 72°
Race 73°
TT 74°

That's the old guide!
GideonReade
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Re: Comfort Comparison - Geometry Input

Post by GideonReade »

Cheers!

I can't imagine my measuring will get to 1° accuracy, but it's good to know!
PH
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Re: Comfort Comparison - Geometry Input

Post by PH »

Q2) If you have a smartphone, there's an app for that. I use the aptly named Angle Finder. There's probably a choice, this one was top of the list when I searched and is free. Couldn't be simpler, calibrate it with a spirit level, put a straight edge of the phone against whatever you're measuring, that's it. Checking against a protractor, it hasn't been more than 0.2 degree out and that's probably due to the bike not being perfectly upright, that's close enough for any use I have. Prior to that, I've done it with a sheet of A4 paper, a spirit level and protractor, several ways to do that, either off the floor or a horizontal top tube (real or imaginary).
However you measure, you can use good old Pythagoras to confirm the result. If your maths isn't as rusty as mine, you could probably use that in the first place!

Rather than ST angle I use a set point, I don't know what you'd call it, midpoint? - A right angle from a real or imaginary horizontal TT that dissects the BB axle and the top of the head tube, measure everything from that point.
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531colin
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Re: Comfort Comparison - Geometry Input

Post by 531colin »

Set the saddle forward, it will throw your weight onto your hands, but it will make it easier to get a low aero position; and pedalling will load your quads more than hamstrings. If you pedal hard enough, the "equal and opposite reaction" of Newtonian physics will help to support your torso and relieve your hands of some weight.
Set the saddle back and the weight on your hands is reduced, but its harder for somebody of average flexibility to get into a low aero position.; pedalling will load your hamstrings and glutes.
(cleat position also influences the way the load is shared between the different muscle groups)

Saddle height (or more accurately saddle to pedal distance, because the setback makes a difference) is for me the most critical dimension. My winter boots have thicker soles than the summer shoes, and I'm driven to alter the saddle height by less than 5mm.
Saddle setback I keep the same within about plus or minus 5mm, but this is with identical saddles on all the bikes.
Reach is much less critical for me; stems usually come in 10mm increments, and thats accurate enough for me.
Handlebar height creeps up as the years roll by, but again I don't notice small differences; I'm happy to set the bar height by putting 2 bikes next to each other.

A bike with a long wheelbase takes a bump "one wheel at a time" and the motion is rather similar to a rocking horse. A short wheelbase bike bumps a bit more.
I'm pretty sceptical about some of the other claims for bike comfort. Long before disc brakes were common, Chris Juden reckoned that lightweight handlebars would make as big (or as small) a contribution to comfort as flexible forks. Inch and eighth steerers rather than inch, ahead stems rather than quill, oversize handlebars....all these increase the stiffness, and increase the shock through the handlebars; although the removable faceplate is a boon for handlebar or stem swaps.
A lot of exposed seatpost will flex a bit, but much less than the saddle rails and saddle top. (Ride behind somebody and watch!)
I don't think you will get much vertical compliance from the rear triangle, simply because its a triangle. The seat stays run (usually) directly from the dropout to the seat cluster. So unless the seatstays compress along their length the dropouts aren't moving relative to the seat cluster.
GideonReade
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Re: Comfort Comparison - Geometry Input

Post by GideonReade »

Thanks Colin, that all has the ring of sense and actual engineering know how.

Today, I thought I'd try and drop the tourer's stem 20mm to be more similar to the road bike. No joy, won't go down (it's not stuck, down is the only way it won't go). Don't know why, cables prevent me pulling it out. Tomorrow I might put the saddle up 5mm.

But interestingly, did a hardish ride last weekend on the tourer, hmm:

On both bikes, I get a minor amount of ache in neck/shoulders, after a half day. On my heavy tourer, over a long trip, I crept the stem up daily until neck ache stopped. Comfy, but way too high for fast. So for these two bikes, I guess I just endure some ache.

On the road bike, I get some ache in the muscles either side, at the back, in the lumbar area. On the tourer I don't. Why? Has to be from:
  • Reach longer, bars lower
  • Roughly 10mm longer bum-pedal
  • 172.5 versus 170 cranks
  • 45" versus 35" effective bottom gear
I wonder which?
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531colin
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Re: Comfort Comparison - Geometry Input

Post by 531colin »

Do you mean on the road bike the saddle is effectively 10mm higher than the tourer? (ie 10mm further from the pedal at bottom to the bum bone dent?)
If the saddle is 10mm higher, I think you should check that you are not rocking your hips to reach the pedal at the bottom.
see Steve Hogg here.. https://www.stevehoggbikefitting.com/bi ... n-it-be-2/

I don't and wouldn't put up with neck ache. (in my mid-seventies, my rides aren't very fast or far now.... but I don't have neck ache)
Do you have a bike which doesn't give you neck ache?
Instead of having your hands "nearly always" on the hoods, you could set up the bikes so the hoods are a relaxed position but you use the drops when you are working at it. I know thats more 1960's than current fashion, but it would give you some variation.
GideonReade
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Re: Comfort Comparison - Geometry Input

Post by GideonReade »

Thanks Colin, Well, I think the saddle is 10mm higher, but it's very hard to tell as neither saddle shows any sign of a bum print, despite quite a few miles. And they're wildly different saddles.

(The roadie's was made I believe by shaping a house brick and painting it white. Reasonably comfy though. The tourer's I bought 20 years ago mid tour in frustration with some dreadful but expensive Ti railed Specialized thing, it says Selle Royal, but I think it came from a sofa factory, it's super squidgy and very forgiving, I'm not sure how deep I sink in)

So the 10mm is a very unconfident measure. But, yes, I wondered if hip rocking on the roadie is my problem. Perhaps I should drop the saddle slightly tomorrow or Thursday.

Neckache, yes, "On my heavy tourer, over a long trip, I crept the stem up daily until neck ache stopped. Comfy, but way too high for fast."

I was out with a nice paced group this morning, on the tourer, everyone else on a guardless road bike, probably all carbon. Quite noticeably, I struggled to tail chase the fastest rider (hello Roger) in headwinds, but on middling hills I was fastest by some margin, eschewing the granny ring, nothing below 42-24. It really does feel like something's draggy at pace. No backache though, didn't notice my neck either...

I'll get there.
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