Current Projects chat and miscellany

if the clearance is tight enough to the point that a slight flex on the fork cause the tyre to touch the downtube and end up flipping the cyclist over the handlebar, then it's perfect.

2nd attempt
http://www.lfgss.com/picture.php?albumid=1243&pictureid=7577

Lovely Condor!

thanks very much. 42t chainring is about where I want to be.
Not sure about the bars yet. Put the risers on as I already had them and wanted to take it for a ride.
First time in a coaster hub for ages and makes a nice change from riding fixed.

i thought 42t was where i wanted to be too until i went 52/19, now i wouldn't go smaller than 50t.

It reminds me of my first fixed, except mine had silly white tires.

i thought 42t was where i wanted to be too until i went 52/19, now i wouldn't go smaller than 50t.

It reminds me of my first fixed, except mine had silly white tires.

i have never gone past a 48t and that seemed bit much.

i was hoping the 700x28 tyres would fill the gaps a bit more really. not that bothered as it is a nice ride.

I think the point was about smooth drivetrains rather than bigger gears. 53/19 and 42/15 are the same gear (within 0.2%), but the big ring version is smoother and lasts longer.

Cheers for the tip, i have already had a puncture so lesson learned there. They are Veloflex Pave

...which have a kevlar belt under the tread for puncture protection. Just keep them pumped up, and keep those lovely soft rubbery sides away from kerbs, holes and buses.

i love how this forum is obsessed with clearances and gaps. it's really neat. like it's some mystery that can only be figured out by putting the bike together.

It's a well proven fact that every 1.76mm of tyre to fork gap reduces your speed by exactly 2mph for any given power applied. Tyre to downtube is 1.5mph per 2.3mm. So you can see just how important this is to anyone wanting to keep their calorie burn down on their ride home. Saves £££ in pasta over the years.

It's a well proven fact that every 1.76mm of tyre to fork gap reduces your speed by exactly 2mph for any given power applied. Tyre to downtube is 1.5mph per 2.3mm. So you can see just how important this is to anyone wanting to keep their calorie burn down on their ride home. Saves £££ in pasta over the years.

have you any proper data for this? not trying to disprove you but i want something to show people when i say i can ride quicker on my fixed gear than on my roadie

have you any proper data for this? not trying to disprove you but i want something to show people when i say i can ride quicker on my fixed gear than on my roadie

Ah. You have me there, as I made this up. I wouldn't be at all supprised if a larger gap actually had better aerodynamics due to causing less turbulance. Damned tricky stuff, air.

but surely having less space for the air to get caught up in would decrease drag (thinking of the back wheel here) wouldn't that be why modern TT frames have horizontal dropouts to allow the wheel to be run as close to the seattube as possible? if thats a load of bullshit i don't care, tiny clearance on frame and tyres are cool

but surely having less space for the air to get caught up in would decrease drag (thinking of the back wheel here) wouldn't that be why modern TT frames have horizontal dropouts to allow the wheel to be run as close to the seattube as possible?

Odd things happen in that gap, which is why Look designed the new 596 Triathlon bike to have a gap of about 12mm, a counter-intuitive decision in the light of a decade of ultra close wheel cutouts. The reason for the frame to follow the line of the tyre as closely as possible is to make the seat tube act as the leading edge of an aerodynamically integrated body including the rear disc wheel - this works really well as long as the wheel isn't turning, which is why the likes of Cervelo etc. provide horizontal dropouts and adjusters to allow you to optimise the gap. Even with the disc spinning, the bulk flow over the whole system stays nice and smooth, and you get low drag, as long as you only measure the drag force pushing backwards on the bike. Traditional wind tunnel tests don't measure how much power is used to actually keep the wheel spinning, and this is where it gets interesting, and where Look made a discovery which has influenced some track teams to start running bigger gaps between the rear wheel and the curved seat tube.

Essentially, what happens is that the rear tyre boundary layer gets pulled into the tight gap and is then acting as a thin shear layer; the bit by the tyre wants to keep up with the tyre, the bit by the tube wants to stay still, and as any fule kno, the friction in a layer a fluid being sheared between two surfaces goes up as the thickness decreases.

As a result, increasing the gap a little bit makes the drag pushing on the whole system a tiny bit bigger, but the relatively large increase in the thickness of the shear layer (about a 4 or 5 fold increase in thickness compared with typical "tight" clearance) makes the power required to spin the wheel a tiny bit smaller. One of those tiny bits turns out to be a little bit bigger than the other if you judge the increased gap just right, and the design becomes an act of balancing one set of drag against another. The same effect also explains why some of the latest time trial frames have more clearance around the tyre as it go through the fork crown than was fashionable as recently as a few years ago.

^ v interesting, cheers.

I was going to add that you should look to mdcc_tester for the informed science. Thanks for that.

I was thinking more about the air going between the front tyre and fork crown, I reckon a larger gap with the right shape would have a lower drag coefficient than a 1 to 4 mm gap with a blunt profile, as air doesn't like being forced into little gaps and tends to bounce about.

As a slightly serious question, although these factors are obviously interesting to an engineer, and to a bike manufacturer wanting to demonstrate having the best possible science, how much difference does it really make to the cyclist? I know aerodynamics is a major factor in cycling, but how much of what we're talking about is perceivable or measureable outside of the lab?

As a slightly serious question, although these factors are obviously interesting to an engineer, and to a bike manufacturer wanting to demonstrate having the best possible science, how much difference does it really make to the cyclist? I know aerodynamics is a major factor in cycling, but how much of what we're talking about is perceivable or measureable outside of the lab?

i'm not certain in terms of frame aerodynamics but i can definitely feel the difference in speed when using a deep section wheels on my race bike rather than shallow rims, other than that i haven't felt too much of a change without the use of forward facing rest bars

i'm so happy my sarcasm went somewhere serious.

I was going to add that you should look to mdcc_tester for the informed science. Thanks for that.

I was thinking more about the air going between the front tyre and fork crown, I reckon a larger gap with the right shape would have a lower drag coefficient than a 1 to 4 mm gap with a blunt profile, as air doesn't like being forced into little gaps and tends to bounce about.

so did LOOK

http://cdn.mos.bikeradar.com/images/news/2008/08/01/Look_2009_596_crown_clearance-850-65.jpg

Perceptible to the rider? Not at all - the small refinements are at a scale well below the sensitivity of a seat-of-the-pants measurement. It's actually hard to measure outside a lab too, when you're talking about changes of less than about 5W at 50km/h, because repeatability on the road to that level is hard to achieve even if your Science grade SRM has the accuracy to measure it.

At 50km/h though, 1W saved is worth about 0.025km/h or about 2s in a 50km time trial. 6W of power saving through aerodynamic tweaks would have accounted for the whole of the difference between GB and Australia in the final of the team pursuit last week. There is that much difference between two aero helmets, the lesser of which is still regarded as pretty good.

Other things worth about 5W at 50km/h: Switching from Mavic Aksiums to the equally cheap Shimano R560, switching from Conti GP4000 to Vittoria Open Corsa CX, switching from butyl tubes to latex and, if you're running tubulars on a wooden track, increasing pressure from 9bar to 14bar.

In short, there are a lot of small gains available, none of which you can feel when riding, but which are nonetheless of vital importance to the competitive cyclist as they can be the difference between victory and defeat.

But as far as not having those ultra-tight clearances on my commuter go, it isn't a major issue.

MDCC, you're (slowly but surely) derailing the "current projects" thread to "bicycle science". Good stuff (<-- not a joke).

In a few weeks time I should be done writing my thesis and will have time to return to several bike projects. Hurrah.

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mdcc is bringin it right now.

mdcc is bringin it right now.

Bringing it like Euclid.

My not so current project, but very recent.
To save some space.

lovely 3 wood you got there, i bet that makes a lovely sound when u hit the ball right.