unravelling Gearing theory and formulae

[GilesM]

If you are seriously thinking of riding a Randonnee on a MTB with 2.4 inch wide tyres at 35 psi, prey the Lord above help you.

The rest of us will get out our Sports tourers with 700 x 23 110 psi jobbies.

As for geometry, are you planning to ride a Brevet on a Schwinn Stingray?

I don't think geometry comes into it. I have arranged gearing for a Raleigh 20" Shopper.

The mtb tyre comparison was obviously the extreme case as an example, however there will be a big difference across various road tyres, geometry is also important and I raised it as the actual weight of the bike seems to be the only thing you have considered, and any one who has a reasonable understanding of cycling will know that there are many more things to think about.

 

[jimboalee]

The mtb tyre comparison was obviously the extreme case as an example, however there will be a big difference across various road tyres, geometry is also important and I raised it as the actual weight of the bike seems to be the only thing you have considered, and any one who has a reasonable understanding of cycling will know that there are many more things to think about.

Yes, indeed you are correct.

Which is why the 'myth' is so simple.

If you include every variable you can think of, you will end up with a 12 mByte Excel workbook after eight years of studying.

I've trimmed the 'every day usable' one down to 387 kByte.

The 'myth', if you hadn't realised, is a 'quicky, rule of thumb' first guide to choosing a gearset. Not Exactly to the inch gospell.

As I have demonstrated, I perform the maths and put that gear 2 - 3 sprockets higher than absolute bottom.
This gives the bike capability to be ridden up 10% and a 12% sucessfully. 14% gets tough and a 20% is a heave.

If you regularly need to ride up 20% hills, do the maths and subtract another 15". 5 steps of 3".

A hefty 26.5 lb bike should have a 23" gear for a slow accent of said 20% hill.
A 28 ring to 32 sprocket will do.

Hey wait,,, That sounds like a Dawes Discovery 501.

Three cheers for the lads at Castle Bromwich.
I think they have a reasonable understanding of cycling, even if you imply I don't.

 

[peanut]

however there will be a big difference across various road tyres, geometry is also important and I raised it as the actual weight of the bike seems to be the only thing you have considered, and any one who has a reasonable understanding of cycling will know that there are many more things to think about.

you are quite right of course Giles I think for the purposes of applying any fomula to power to weight calculations the formula and the result needs to be an imperical rule of thumb and assume there are no losses due to

friction in the mechanism
Wind resistance
Drag ie road surface and tyres
weight of vehicle etc

just as when we measure power output in car engines we give a rating in BHP but at the flywheel, not the tyres where they contact the road because of all the variable losses between the flywheel and the road.

If we calculate the power output of a cyclist in terms of his physiology and fitness ie Watts we will arrive at a uniform result that can be applied to any cyclist riding any type of bike or recumbent etc.

We can then apply the various other factors like bike weight ,tyre drag ,wind resistence , incline % etc to arrive at an individuals gearing requirements.

I think Jimbo needs to extract the bike out of the equation because there are too many variables in the bike ie the method of applying the power

 

[jimboalee]

It really amazes me how Dawes Cycles still sell bikes.

They design a bike, weigh it ( they are one of the only manufacturers who quote the weight on there website, what are the others afraid of? ), put a gearset on it and launch it into the market.

The good people at Dawes always seem to get it right. They haven't got a wind tunnel, dynamometer cell or rolling road like a Motor car manufacturer.
What they do have are some clever guys who have been in the business for longer than I have who seem to know what gearing to use on the bikes.

For heaven's sake, these are bicycle gears were discussing here. If ever the government impose emissions legislation on cyclists, then maybe the subject can get serious.

What's the big hoo-har whether tyre pressure, frame geometry or wheel bearing friction should be involved?

What I've been trying to suggest for these last few months is :-

If a newbie hasn't got the equipment or education to do all the physics and build a spreadsheet, he can use a simple little equation, make some adjustment for his extra ( unwanted ) bodyweight, select a gear group that puts the '10%' gear as his No.3 or No.4 gear and GO!

It's as simple as that. No power testing. No dynamometer curves necessary.

 

[jimboalee]

you are quite right of course Giles I think for the purposes of applying any fomula to power to weight calculations the formula and the result needs to be an imperical rule of thumb and assume there are no losses due to

friction in the mechanism
Wind resistance
Drag ie road surface and tyres
weight of vehicle etc

just as when we measure power output in car engines we give a rating in BHP but at the flywheel, not the tyres where they contact the road because of all the variable losses between the flywheel and the road.

If we calculate the power output of a cyclist in terms of his physiology and fitness ie Watts we will arrive at a uniform result that can be applied to any cyclist riding any type of bike or recumbent etc.

We can then apply the various other factors like bike weight ,tyre drag ,wind resistence , incline % etc to arrive at an individuals gearing requirements.

I think Jimbo needs to extract the bike out of the equation because there are too many variables in the bike ie the method of applying the power

You are correct here. Brake Horse Power is on the dyno Brake.

Could you explain to me and for the interest of the readers, why the coastdown test to determine the coefficients for the chassis dynomometer roadload curve is performed on a test track with a whole rolling car?
Two, maybe three cars are tested in weather conditions that are not identical. If the timegate numbers are within 2%, the result stands.

The roadload curve they have just signed off is then used for legislative emissions testing for ECE and EPA regulations.

Now you are talking about every ( interested or curiuos ) cyclist putting themselves through a series of performance tests to pick his gearing.
The pros on the tour yes, I know its done, but Mr Randonneur???

 

[peanut]

sorry Jimbo you've lost me. What has auto testing got to do with human power output ? I only used that as an analogy to explain how we should remove the bikes ie variables from any human output calcs otherwise the calcs should be made using a standard set of bike parameters.

 

[GilesM]

It's as simple as that. No power testing. No dynamometer curves necessary.

I agree very simple, just remember, you're the one trying to make something very simple far too complex, I'm just pointing out that if you are considering tailoring the gearing so specifically that you consider the riders BMI, then surely considering the type of tyre and geometry of frame is sensible. When I stopped racing (many years ago) my road race bike had 42, 54 rings and a 13 to 21 block (7 speed then), my training bike which was only slightly heavier but with bigger winter tyres and mudgurad clearances had 42, 52 with a 14 to 26 block, climbing was always easier on the race bike.

 

[GilesM]

If we calculate the power output of a cyclist in terms of his physiology and fitness ie Watts we will arrive at a uniform result that can be applied to any cyclist riding any type of bike or recumbent etc.

We can then apply the various other factors like bike weight ,tyre drag ,wind resistence , incline % etc to arrive at an individuals gearing requirements.

I think Jimbo needs to extract the bike out of the equation because there are too many variables in the bike ie the method of applying the power

I agree, if you want to get sophisticated about gear selection then this would be the way to go, however you don't realy need to get sophisticated, as Jimboalee has already pointed out, of the shelf standard options give such a wide range of available gears that no calculations are really necessary, the only place gear selection is really critical and should be worked out scientifically is for track riders, obviously with only one choice of gear it has to be the right one.

 

[jimboalee]

I agree, if you want to get sophisticated about gear selection then this would be the way to go, however you don't realy need to get sophisticated, as Jimboalee has already pointed out, of the shelf standard options give such a wide range of available gears that no calculations are really necessary, the only place gear selection is really critical and should be worked out scientifically is for track riders, obviously with only one choice of gear it has to be the right one.

Yeh, lets go down this route. Establish how much power a rider can output first.

On another thread, I put forward the theory that when a cyclist is climbing a 10% gradient, the speed can drop to 30 – 35% of a ‘flat road cruise’ and his power output can double.

So let’s put a 90 kg geezer on a 11 kg beginner’s bike. He’s moderately fit and rides along merrily at 15 mph along a flat road. He’s doing well for a beginner. He’s riding on a 70” gear at 73 cadence.

He comes to a 10% hill. His speed drops to 5 mph. Instead of the 120 Watts he was putting out at 15 mph on the flat, he now has to find 240 Watts.

What gear might he be using up the 10% at 5 mph?

If he’s got a 52/42/30 chainset with a 13 – 25 cassette, he’ll have a 30 x 19 which is 42”, on which he’ll be slowly and deliberately pedalling 40 rpm.

Will he get up the hill?

 

[Sittingduck]

Don't keep us in suspense Jimbo... Did the geezer make it up or not?

 

[GilesM]

Will he get up the hill?

He should do, he may even change down, that's the advantage of such a wide range of different ratios, however, I expect it will depend how long the hill is, how knackered he is from his ride so far and how his head works, and to me, 73 rpm is a bit low, 85 to 90 is much better for flat riding on the road.

 

[jimboalee]

He should do, he may even change down, that's the advantage of such a wide range of different ratios, however, I expect it will depend how long the hill is, how knackered he is from his ride so far and how his head works, and to me, 73 rpm is a bit low, 85 to 90 is much better for flat riding on the road.

You've fallen into the 'spinning' web.

Someone has already criticised by mentioning 'muscle structure'.

If you watch the tour this afternoon, thare will be a wide range of pedalling rates across the members of the peleton.

Lance won the tour several times and his 'natural' pedalling rythym was and still is quite fast. This caused a 'spinning' craze, which has recently been conceded by the Cycling press 'spinning' DOES NOT suit everybody.

I have yet to encounter a 'mediocre' cyclist who can 'spin' efficiently.

 

[jimboalee]

He should do, he may even change down, that's the advantage of such a wide range of different ratios, however, I expect it will depend how long the hill is, how knackered he is from his ride so far and how his head works, and to me, 73 rpm is a bit low, 85 to 90 is much better for flat riding on the road.

The point is Giles, it is easy to simply SAY "wide range of different ratios", but to build a bike from bare frame, what ratios would YOU fit?

Most fortunately for you Giles, the work has already been done and all YOU have to do is copy what's being used by the major bike builders.

Maybe if I was to give you the task of spec'ing up a bike for a loaded tour through the Peak District, what gears would you retro-fit?

 

[GilesM]

Lance won the tour several times and his 'natural' pedalling rythym was and still is quite fast. This caused a 'spinning' craze, which has recently been conceded by the Cycling press 'spinning' DOES NOT suit everybody.

I'll just point out, most of my knowledge/experience of road or track racing comes from well before Lance's fame, and the only cycling press I have read much of in the last 15 years has been MBR, and 85 to 90 rpm is not high revs. So I can assure you that no "spinning craze" (whatever that is) is influencing my thoughts on this.

Just as a comment, is "spinning" a term used by people who also say "drafting" instead of following/holding a wheel.

 

[GilesM]

The point is Giles, it is easy to simply SAY "wide range of different ratios", but to build a bike from bare frame, what ratios would YOU fit?

Most fortunately for you Giles, the work has already been done and all YOU have to do is copy what's being used by the major bike builders.

Maybe if I was to give you the task of spec'ing up a bike for a loaded tour through the Peak District, what gears would you retro-fit?

Don't forget that you are the one who said you had a great formula for calculating the best gear ratios tailored for an individual and then came out with a super wide ratio spread that covered all possible options, and was definately lower than was likely to be needed for a road bike, and also much higher than would be required.
Today it is easy, the ratios available are so wide that nobody should ever have a problem, 30 years ago things were a tad different, the starter road bike for most was a five speed with a 46, or 48 chainring and a 14 to 24 block, this gave a bottom gear of 54" (for the 48 ring), with steel wheels and heavy 27 x 1 1/4 tyres, I remember climbing hills around about 1 in 7 was very tough.

I have no doubt that a 34 / 50 double ring and 12 to 26 cassette would be fine for me in the peak district on a loaded up touring bike, some may want a bit lower, so they could always go for a triple ring.
25 years ago I went cycling in the Austrian Alps for a week, we drove down there and stayed in one place, so the biles were not loaded, I rode with 26, 40 chainrings and a 13 to 17 straight through five speed block, not the lightest bike, but narrow (ish) light (ish) 700c tyres, never a problem on any of the climbs, and some were very long, obviously the top gear was a lttle low, but on the descents you go much faster by not braking than you ever do by trying to pedal.

All this is just a bit off topic, what I was still wondering is how you expect to specifically tailor the gears to an individual, yet you do not consider the type of tyre or geometry of the bike, this was my original point.