Aero versus lightweight?

Hi all,

I am planning a new bike project and am on the search for suitable wheels...I do not race but ride regularly, including some long distance events (100 - 200Km) as well as plenty of miles in the Adelaide hills.

I am basically at the point of trying to decie between the aero advantage of a deep dish wheel versus the less aero but weight advantage of a lower profile wheel e.g. Dura Ace C35 (35mm carbon/alloy) at approx 1600 grams versus Dura Ace C24 (24mm carbon alloy) at approx 1300 grams.

Taking price out of the equation, I am interested what riders out there think as the relative merits of going with a heavier, more aero wheel versus a lighter, less aero wheel, keeping in mind that this will be my only wheelset and needs to work in the hills as well as fast riding on the flats as well as long distance (100km +) endurance rides - a lot to ask i know!

I look forward to anyones thoughts on the subject.

Cheers,

Lachlan

Tags: Wheels, aero, deep, dish

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Permalink Reply by Tim B on April 5, 2012 at 16:20

I went through a similar question at the beginning of the year. I had the Ultegra version of the C24's (RS80's) and liked them but started looking around for something more aero, and just because I wanted to buy something new and shiny :-).

I was looking at the C35's and the C50's and ended up going with the C50's. My thoughts were that the C35 ended up being a bit of a nothing wheel stuck between the other two. Not exactly aero, but not exactly light either. If you want light, get the 24's, if you want aero get the 50's. Personally I love the 50's, took me a while to get use to some of the cross winds in the hills earlier this year. They are possibly a bit heavier in the hills but I'm slow anyway and don't honestly notice any difference, but once you get them wound up over 25kph or so, they are very fast. (I'm told most of their extra weight is at the hubs any way and not noticeable) I've been cranking out PB after PB in the hills of late, so they cant be THAT slow. I think in winter I'll stick the RS80's back on the bike to slosh around in the hills and with some wind around etc

If you are looking at the custom route, you could try this guy http://www.twebikewheels.com.au/

uses Reynolds or Enve rims with DT spokes and high end Japanese hubs

Permalink Reply by Pete Roberts on April 5, 2012 at 18:15

I went down this same path last year. My riding is very similar to what you do, (hills / long distance) & I ended up buying Zipp 303's.  These are a deep dish, but not that deep, so they do not suffer so much from cross winds, which can be a real problem with the deep profile wheel.

They are still quite light weight, and pretty much an "All Rounder" as far as I am concerned. 

For me, I think I made the right choice.

Permalink Reply by john mills on April 5, 2012 at 23:55

I reckon a simple 'wind tunnel' experiment could be used to find out the advantage of the aero wheels compared to the lighter wheels. But you would need to be able to 'borrow' a set of each to compare them.

Before I go any further I will say that I am not an aeronautical engineer, or car designer. Just a physics teacher. So any feed back on the suggested experiment and theory below would be appreciated.

Here is the procedure for my experiment;

Find a gentle slope where you can adopt a certain position on the bike that you can repeat for both sets of wheels. Maintain this position and let the bike roll down the slope until the speedo reaches a steady terminal velocity. No pedalling! Wait until you reach a terminal speed and note what it is. Change wheels, making sure you use the same tyres and pressures for each set of wheels. Repeat the roll - same hill, same starting point. Hill needs to be constant slope for whole run. Could be the tricky part - finding a long constant slope.

Record the maximum speed ( terminal velocity ) for each trial. 

You would need to make sure you have the same weight on the bike for both trials. So weigh yourself before using the aero tyres and record your weight. Then weigh yourself before using the lighter tyres. Drink enough water to bring your weight up to the same weight used for the aeros. About 600ml according to your info. 

In a simple analysis you can just look at the extra speed the aero wheels provided down this gentle slope.

But you can also calculate the ratio of the two drag co-efficients for the two wheels. And then this ratio will give you a rough estimation of the relative forces ( aero wheels Vs lighter wheels ) that you need to put on the pedals to 'fight' air resistance for any given speed.

To calculate the ratio of drag coefficients of the two wheels use this formula;

          DA / DL  =  vL squared / vA squared

   (  where   DL = drag coefficient of light wheels    DA = drag coefficient of aero wheels

        vL = terminal velocity achieved by light wheels   and    vA = terminal velocity of aero wheels )

 

 Here is how you could apply this information to decide whether the aero wheels are worth using;

Pretend you are busting your gut pushing along at say 35km/hr on a flat road 'wearing' the light wheels. The forces you are exerting on the pedals are balancing ( equal to) the mechanical friction in the gears and chain PLUS the friciton between tyres and road PLUS the air resistance.

You would need to take a bit of a guess as to what fraction of this total force could be attributed to air resistance. Maybe a bit of Googling could find the answer. But let's say air resistance accounted for 70% of the total push/pull on the pedals. Call this force the Air Force!

Then you could say that if you swapped to the aero wheels you would only need to use DA / DL of the Air Force needed for the light wheels to ride at this speed of 35 km/hr.

Or, if you want to keep the same push as before, and just go faster, the speed you could achieve with the aero wheels is;

                             35  /  square root of  ( DA/DL )

Sorry, nothing on the TV, and I was bored. But if you don't mind a bit of maths, and you could be bothered to change wheels and tyres, it would be an interesting science experiment. If you are into science experiments LOL. 

Permalink Reply by Sam Young on April 6, 2012 at 1:20

Looks good, and sorry to be a trasher of such a long analysis in a short space, but you've missed out on something really important. 

When going up hill, air resistance is less of an issue - say you're pushing up Norton Summit at 20 k's an hour, instead of going along military road at 40.  What does become an issue is weight, because you need to lift every gram all the way up the hill. Increasing your weight by half a kilo and dropping your wind resistance by (say) 10% may be a pretty good tradeoff on the flat, but it's not so peachy going up a hill. 

If I'm lifting 75kg instead of 75.5kg, I'll go up Norton Summit 20 odd seconds faster. Less power is needed to lift less weight, in the same time, or the same power can lift the same weight in less time.  

I think this is the crux of the question - and something the proposed experiment doesn't account for.

Of course, the other option is to go the aero wheels and lose weight. Until you're an elite athlete, The changes you can make to your body far outweigh the changes you can make to the bike.

So buy the wheels that look prettier or last longer.  

Permalink Reply by john mills on April 6, 2012 at 19:09

Well I was only considering what I thought was the trickier, and IMO more interesting, half of the aero Vs light quandry.

But the weight issue is not as simple as it seems. For a start, if you are interested in the extra force needed on the pedals to push that extra 600g up a hill, its not just 600g. You would need to do a bit of simple trigonometry and calculate the component of gravity that is parallel to the road surface. For small angles, since tan theta is almost equivalent to sine theta, you could approximate that the force required is just the percentage gradient of the hill multipled by the 600gm. So for a 10% hill the extra force required on the pedals to push the 600g extra mass is approximately 10% of 600g = 60g.

The second aspect that is interesting, IMO, about the 600g is that if you consider the relative force on the pedals required to lift an average cycler, plus his/her average bike, say 75kg plus 12kg = 87kg, then this 60g of extra push is relatively small. i.e. for a 10% hill, the force needed for the mass of the biker and bike = 10% of 87kg = 8.7kg = 8700g. Compared to 60g for the extra weight of the heavier wheels.

( I have not factored in the gearing system of the bike, which would increase these values. But the relative difference would remain the same. And of course I am using grams as a unit of force as a simplification that  most serious physics people would frown upon! )

So depending on just how good these aero wheels are at reducing drag, and considering that drag forces increase in proportion to the square of the increasing velocity, I am assuming the relative difference between the forces required to counter air resistance, between aero wheels and normal wheels, would by much greater. Especially at higher speeds, where the difference will be even greater.

The third intersting factor is the fact that when you lift extra mass up a hill on a bicycle you are storing up gravitational potential energy. So that extra effort is not in vain. You get your reward coming down the other side of the hill. This is not the case with the extra effort required to push through the air with less efficient wheels. All that energy just wasted as it disappears as hot air!

But as far as racing results are concerned, this energy storage effect is is not necessarily a non issue. I think the maths would show that the extra time needed to climb slowly up a hill is not paid back with an equal number of seconds on the fast downhill run afterwards. Energy is given back equally as far as gravitational energy is concened, but the faster speed down the hill is negated in a significant way due to the higher drag forces that are in proportion to the square of the higher velocity. And I suspect there is some time = distance / vlecoity calculations that would show that even without this drag force issue, the 'payback' time is less than the 'pay' time.

I have noticed, over my short time in group biking, that although I enjoy 'catching' my lighter fellow riders down hills, since I weigh in at over 100kg and have another 20kg for my bike, I don't think I really make up for the gap created climbing the hill.

So I hope I have now covered both parts of the problem. If you try to say that hill riding would suggest the weight factor is more important, you still need to remember that the higher speeds involved in the down hill run. If the hill is not so steep that brakes are needed, then the aero advantage will still be a significant factor.

Permalink Reply by Sam Young on April 6, 2012 at 22:34

Thanks for that. I reckon that post just taught me a whole lot. I must admit I'd not looked into the effective power difference needed, and thought it was a more direct relationship. 

Permalink Reply by Frank on April 6, 2012 at 7:44
I have done an experiment similar to this but to see the effect if I use very very good lubricant in my hubs as opposed to having the shops grease them!

I used the road connecting top of unley to goodwood rd! Sheppards hill I think!

Anyhow, I compared different wheels, different riders, mostly heavier but similar technique!
My conclusion is that a really well lubricated hub given the bearings are perfect and you can access them and lightly lube with say slick honey lubricant that they will roll faster than stock or shop lubricated in 95% of the time.

One set that rolled passes me was a guy on a pasoni with lightweights but he was also 15kg heavier!

I concluded that the hub makes a big difference, the total weight, the tyres seem to have had little affect although we mostly ran schwalbe R1 and being tucked in as low as pos and knees squeezing top tube.

I used campag neutron wheels that are 4 years old but with serviceable hubs and they spin forever!
Reason I did this is cause a bike shop serviced them by filling the grease point at midway of hub and just filled it up, it was like riding with handbrake on!

In group rides I find when everybody rolls on, I need to constantly apply some handbrake, a tad annoying but good to see that even relatively cheap($700) wheels can go very well!
To add to that I must say that deep dish is really designed to speeds over 35km/h and more and given enough space etc and time perhaps they would have come into their own!

A bloke on a Willier with shimano wheels was not far behind though, he is also only a tad heavier!

Perhaps more time etc and it could be more scientific but if you can service your existing hubs, do it and be light on the grease, use super high quality stuff and you may be surprised!
A few tools needed like a chain whip, socket to suit shitmano or campy nut, grease like slick honey and laptop with video on how to service hubs!

Trust me, I am a builder!!!!
Permalink Reply by Tim B on April 6, 2012 at 8:41
Sounds a good experiment for your physics class!

One other thing, which way is the wind blowing and was it constant for both runs?
Permalink Reply by Don (Who's lost?) Nairn on April 6, 2012 at 2:46

I would think as an all round wheel don't use aero.

Cross winds can be unpleasant so I have been told.

Also are they too stiff to cope well with rough roads?

 

 

Permalink Reply by RD6 on April 6, 2012 at 8:25

I'd go for C24's = good long mileage and climbing wheels, I am 85kgs

Permalink Reply by Craig T on April 6, 2012 at 9:41

I've got RS 80 which I really enjoy and ride mainly in the hills, the problem is that the wear and tear on the breaking surface will soon take its toll. I would consider having a cheap set of wheels for those shorter 'training' rides and keep your upgrade wheels for the longer rides and special events.

Permalink Reply by easytiger on April 6, 2012 at 12:14

35mm I would not consider deep dish, 50mm and up is deep dish. 35mm is considered a good compromise between aero and light. Heavier wheels are slower to get up to speed, but maintain their speed with less effort, ie. more rotational mass.

For rolling hills, aero is better. For long climbs, low profile (ie. lower weight) is better. On the flat, aero is better.

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