# Tandem Cycling - Aerodynamics of Tandem vs. Single

Bikeforums.net is a forum about nothing but bikes. Our community can help you find information about hard-to-find and localized information like bicycle tours, specialties like where in your area to have your recumbent bike serviced, or what are the best bicycle tires and seats for the activities you use your bike for.

View Full Version : Aerodynamics of Tandem vs. Single

Smooooth
05-19-08, 06:31 PM
I think I read in one of Tandemgeeks posts that a tandems aerdynamics are not as good as you would think.

It seems to me that you are punching 1 long hole in the air friction category vs. 2 holes for 2 singles. You also have 2 tire frictions on tandem vs 4 on singles.

Can someone please chime in on this topic so that I can get some sleep tonight :D

TandemGeek
05-19-08, 06:47 PM
From the archives: http://www.bikeforums.net/editpost.php?do=editpost&p=731496

I thought tandems went faster than a non-tandem into a headwind because there are 2 people putting wattage vs 1 on a roadbike?

Yes and no. All things being equal except for the bike, (and despite myths to the contrary) a tandem road bike with two riders is between 30% to 50% less aerodynamic than a a solo rider on a road bike. Therefore, in order for a tandem to maintain a constant speed that is as fast (or faster) than a solo rider, the team must produce a net power output that is at least 30% to 50% greater than a solo rider.

Moreover, consider if you will that power production on a tandem is not necessarily linear since either one or both of the two riders on a given team may at any given time fall victim to poor form, individually or collectively. Put another way, even if you have two strong cyclists (1+1), they will lose some efficiency due to the nature of a tandem and, if they are not able to work well together, their efficiency will fall way off. So, in reality what you will often times find with a tandem is that you have two riders of different abilities who can produce variable levels of output which may or may not be as high as they are able to produce when riding alone. This is why some tandem teams who are weaker riders individually than a team comprised of two strong riders are often times as fast (or faster) than the stronger couple when both teams ride their tandems. The place where a team's combined efficiency level is most obvious is when they get to the hills where you have the same math problem with percent grade being substituted for headwind MPH and the added factor of team weight having more importance than aerodynamic drag. Less I digress...

So, to answer your question, if you have a team who can produce at least 30% to 50% more power as a team on a tandem they should be able to keep up with a solo rider. However, if a tandem team can produce a net power output above what is required to compensate for increased drag, the tandem pulls the train.

Hermes
05-19-08, 06:53 PM
I think I read in one of Tandemgeeks posts that a tandems aerdynamics are not as good as you would think.

It seems to me that you are punching 1 long hole in the air friction category vs. 2 holes for 2 singles. You also have 2 tire frictions on tandem vs 4 on singles.

Can someone please chime in on this topic so that I can get some sleep tonight :D

You need more time on the trainer.:D

I think the context of that post was the drag coefficient of a single bike versus a tandem. The tandem will have 30 to 40% larger drag coefficient than a single bike. The air hits the captain and some of it reforms and hits the stoker. The closer the stoker is to the captain the lower the drag.

Now two single bikes versus a tandem is another story. However, here are some numbers from the Berkeley TTT this year for two person teams versus tandems.

http://www.berkeleybike.org/events/bbcttt_2008-results.html

P/1/2 two man teams 39 to 42 minutes: Masters 90 + 40 to 63 minutes; Tandems 44 to 53 minutes.

merlinextraligh
05-19-08, 08:47 PM
so if you take 3 equally strong riders, and put 2 on a tandem, and the third on a single, the 2 on the tandem will smoke the single on the flats. ( in other word 30% more wind resistence, but close to 200% of the power is a good deal.)

On a sharp hill, they'll at best equal the single, and likely lag a little bit behind due to efficiency issues

uspspro
05-19-08, 09:17 PM
That's why I ride where there are no winds except 30+ tailwinds.... duh! ;)

Just like our rides that only have downhills...

Oh wait.... We are riding the tandem up Mt. Hamilton on Monday... DOH :D

cornucopia72
05-20-08, 10:24 AM
so if you take 3 equally strong riders, and put 2 on a tandem, and the third on a single, the 2 on the tandem will smoke the single on the flats. ( in other word 30% more wind resistence, but close to 200% of the power is a good deal.)

+1

Tandems pull the train in most circustances except uphill and with strong cross winds.

skibike951
05-20-08, 11:24 AM
+1 on the strong cross winds;

rode in them on Saturday with our normal group which is on singles and we were at a distinct disadvantage in the strong cross winds. normally we do pull the group but this was the exception since we had so much increased aero drag due to the cross wind and also not being able to get the stoker out of the wind and into the draft when we were in echelon drafting formation with the singles.

merlinextraligh
05-20-08, 12:27 PM
I can certainly see the physics of why a cross wind would have more relative effect on a tandem than a single.

However, if you start with the proposition that a tandem has 130% of the drag of a single bike, it would take a heck of a crosswind before the tandem had 200% of the drag.

Thus while the a crosswind erodes the tandem's areo advantage, I'm thinking the wind would have to really be blowing hard, before the tandem would lose all of its advantage.

Anybody with the math skills to model that?

TandemGeek
05-20-08, 01:15 PM

- 150% of the drag of a single bike, (I'm a realist, not an idealist or marketing guy).

I'm thinking the wind would have to really be blowing hard, before the tandem would lose all of its advantage.

Aero drag accounts for about 90% of the total drag on a bicycle and of that, a average size cyclist's total body frontal area drives about 75% - 80% of the aero drag. Therefore, once the benefit of the captain's draft is diminished by a cross wind condition, the total aero drag on a tandem begins to rise rather dramatically. The greater the wind velocity and angle of incidence, the greater the effect. Moreover, the larger the stoker the larger the impact of that loss of the draft effect.

Obviously, since the tandem only has two wheels vs. four on two single bikes, it will never lose all of it's aero advantage when compared to two single bikes. However, given my choice of dealing with a head wind or oblique cross winds on a tandem, I'll take the head wind.

brewer45
05-20-08, 01:39 PM
However, given my choice of dealing with a head wind or oblique cross winds on a tandem, I'll take the head wind.

I second that. A nasty oblique cross-wind had us calling for the SAG to haul us off our first organized ride. Made 30 of the 50 miles, but it was not fun.

Cheers!

merlinextraligh
05-20-08, 01:51 PM
- 150% of the drag of a single bike, (I'm a realist, not an idealist or marketing guy).

Aero drag accounts for about 90% of the total drag on a bicycle and of that, a average size cyclist's total body frontal area drives about 75% - 80% of the aero drag. Therefore, once the benefit of the captain's draft is diminished by a cross wind condition, the total aero drag on a tandem begins to rise rather dramatically. The greater the wind velocity and angle of incidence, the greater the effect. Moreover, the larger the stoker the larger the impact of that loss of the draft effect.

Obviously, since the tandem only has two wheels vs. four on two single bikes, it will never lose all of it's aero advantage when compared to two single bikes. However, given my choice of dealing with a head wind or oblique cross winds on a tandem, I'll take the head wind.

I get what you're saying, and clearly the tandem does better in a headwind as a relative matter to single bikes than it does in a cross wind.

I think the fact that a tandem has such an aero advantage to start with though may distort perception a bit.

Without the cross wind on the flat, a weaker tandem team (i.e. the average of the 2 riders power is lower) can keep up with a stronger (higher power) single rider. When a cross wind is added, it diminishes the aero advantage, and the weaker team falls behind because its lost some of the advantage that was helping them keep up with the stronger rider. But its not because the tandem is actually slower in the crosswind, its that its not as much faster as it is with no wind or a headwind.

Or taken another way, 3 riders same power, 2 on a tandem, and one on a single. I'm betting for most riding speeds, and most wind angles, its going to take one heck of a cross wind before the tandem team actually becomes slower.

cornucopia72
05-20-08, 02:27 PM
Or taken another way, 3 riders same power, 2 on a tandem, and one on a single. I'm betting for most riding speeds, and most wind angles, its going to take one heck of a cross wind before the tandem team actually becomes slower.

I would agree.... but, while on a pace line there is no place for the tandem stoker to hide...

merlinextraligh
05-20-08, 02:30 PM
^ If you had a true echelon going that would definitely hurt the tandem. But other than in a race, how often do you ride in an echelon?

TandemGeek
05-20-08, 02:47 PM
I think the fact that a tandem has such an aero advantage to start with though may distort perception a bit.

Aero + power on the flats is what distorts the perception; it's steep climbs that bring most tandem teams back to reality as there is no where to hide from gravity.

Hermes
05-20-08, 03:42 PM
In the results section from http://www.kreuzotter.de/english/espeed.htm#pv , he calculates an effective frontal area. For three same height (67.7 inches) riders two on a tandem and one on a single bike the effective area in the drops for each one is 4.28 and 3.26 for a tandem and single, respectively or a 31% increase in effective area for the tandem. YMMV

TandemGeek
05-20-08, 03:57 PM
In the results section from http://www.kreuzotter.de/english/espeed.htm#pv , he calculates an effective frontal area. For three same height (67.7 inches) riders two on a tandem and one on a single bike the effective area in the drops for each one is 4.28 and 3.26 for a tandem and single, respectively or a 31% increase in effective area for the tandem. YMMV

What happens when you punch in a 5'10" / 220lb captain with a 5'5" / 140lb stoker, neither of whom has ever had an inclination to touch their drops?

Hermes
05-20-08, 04:07 PM
What happens when you punch in a 5'10" / 220lb captain with a 5'5" / 140lb stoker, neither of whom has ever had an inclination to touch their drops?

I have no idea. I did not see a "no inclination to touch their drops selection". Did you?

TandemGeek
05-20-08, 04:48 PM
I have no idea. I did not see a "no inclination to touch their drops selection". Did you?

I was just being a smart a**. By the time you read the translated disclaimer that accompanies the tandem selection (Below in quotes) and adjust the various defaults (tandem with medium tires vs racing bike with narrow HP tires) you can find enough ways to jiggle the data to get any result you'd like.

However, if someone was so inclined they could extrapolate the difference by looking at the data for single bike rider on tops (4.8 frontal area) compared to drops (3.3 frontal area) after averaging the weights and heights of the notional couple.

The values of "Rider's Height", "Rider's Weight" and "Power" refer to just one of the tandem riders. If Captain and Stoker differ in any of these values, take the mean value of the couple. (The program will internally multiply the given values by 2 [1].)

Adequately, if calculating the power the program will deliver the mean value of the two persons, i. e. the half of the added total input power.

The type of tandem the calculation is based on:
Cannondale Aluminum frame. High performance equipment. 28 inch wheels. Equipped with lightweight mudguards and carrier, total weight is 39.2 lb (17.8 kg). Quite big timing rings (40 teeth) to reduce chain friction losses. Racing handle bars for Captain and Stoker. Riding position: hands on the drops (bottom section of the handle bars) front and rear, the upper part of the riders' bodies not being extremely bent, about 40 degrees to the ground.

[1] In reality, the total frontal area of a tandem riding team is not twice that of a single rider. The program's algorithm regards this fact.

uspspro
05-20-08, 04:58 PM
What happens when you punch in a 5'10" / 220lb captain with a 5'5" / 140lb stoker, neither of whom has ever had an inclination to touch their drops?

Probably safe to assume the same percentage increase applied to go from single rider in drops to single rider on the hoods can be multiplied with the results for the tandem category.

Question is: If you're one who is thinking about all this power/drag stuff, why wouldn't you be in the drops under windy and/or "wanting-to-hammer" conditions? :D

EDIT: I must type too slow....Sorry for the late reply

skibike951
05-21-08, 06:48 AM
^ If you had a true echelon going that would definitely hurt the tandem. But other than in a race, how often do you ride in an echelon?

We do this every Saturday morning when we ride with a group of singles. We ride in a flat area that has very strong cross winds this time of year. Last Saturday's winds were about 20 mph with stronger gusts and the angle was nearly 90 degrees. So it is a disadvantage, for sure, not being able to get the stoker out of the wind in an echelon pace line.

cornucopia72
05-21-08, 08:38 AM
^ If you had a true echelon going that would definitely hurt the tandem. But other than in a race, how often do you ride in an echelon?

Yesterday. My wife and I rode the triplet with an inexperienced mid stoker. It was very windy and the wind would change from front to tail and everything in between....It was a twisting route along the Kings river. On the way out, we basically were doing a five deep echelon on several sections. On the way back, the singles and a tandem loved when we pulled. With the triplet we provide such a nice windshield. When got to the only significant climb of the ride, Avocado Lake, a tandem team that could never climb with us on a calm day drafted all the way up with the captain just behind me and his stoker just between my two stokers. They stayed with us all the way up.

Four strong single ridders tried to do a "breakaway" using the climb as a slingshot. Breakaways such as this are usually brought back by the tandems in a few minutes. Yesterday it took us almost 30 minutes of very hard work to reel them in. I know that on a calm day we would have had those same four ridders hanging to our wheel as it their life depended on it!

merlinextraligh
05-21-08, 08:44 AM
We do this every Saturday morning when we ride with a group of singles. We ride in a flat area that has very strong cross winds this time of year. Last Saturday's winds were about 20 mph with stronger gusts and the angle was nearly 90 degrees. So it is a disadvantage, for sure, not being able to get the stoker out of the wind in an echelon pace line.

You must have a lot less crowded roads than we do if you get to ride an echelon on your saturday group rides.

Smooooth
05-22-08, 10:58 PM
So, in hill climbing, the biggest deficit for the tandem is that you are compromised on riding position? Let's assume 3 equal strength riders 2 on tandem. tandem is twice the weight of single.

The tandem wins up a hill?

TandemGeek
05-23-08, 05:48 AM
The tandem wins up a hill?

Only if the two riders on the tandem can climb as efficiently on the tandem as they would on single bikes, and even then it might only be a tie.

Remember, being successful on a tandem puts more emphasis on the ability to work as a team than individual performance. It's the sum of the tandem team's total power output as a team that must be considered and it's not necessarily 1 + 1 = 2. In many cases it more like 1 + 1 = 1.8 / 2 = 0.9 in hill climbing situations. I'm not sure if the 0.1 would be off-set by the decrease in rolling resistance because even though a tandem allows 2 riders to roll on two tires, the tires are still carrying the full burden of both rider's weight plus that of the tandem which is usually only about 1lb lighter per person compared to two single bikes of equivalent design / componentry.

Finally, stepping away from your notional models of having three riders of equal performance, in the real work most pairs who ride tandems usually have a fairly large disparity in on-bike performance. This disparity usually manifests itself on climbs moreso than in any other riding condition. Therefore, you're more likely to have a model that looks more like 1 + .7 = 1.5 / 2 = 0.75 in hill climbing situations.

jgg3
05-23-08, 12:34 PM
My wife and I can out-climb most of the riders on our regular riding group, but not the climbers (which would include me on a single). Our tandem is heavier than 2x my single, though about 2x our two former singles, both mid-80's full campy racing bikes. But there is no question that for a long climb, we are at a disadvantage just because of differences in output. When she decides she is "done", no amount of effort on my part can make up for it. To each rider, a tandem doesn't respond like a single bike, because the other rider is just that much out of synch. When we happen to hit it right in some situations, it feels great, but I think it will be years before we get that magic a lot of the time.

justcrankn
05-23-08, 08:37 PM
Can someone explain what it means to work efficiently on the tandem? If I put one ft-lb into the pedals and my stoker does the same then where does that two ft-lbs disappear to if we're not working efficiently?

merlinextraligh
05-23-08, 09:03 PM
When you're putting that ft -lb to the pedal, and your teamate is squirrely around on their saddle.

When you start to hammer,and their legs are locked in place,and part of your energy goes to unlocking their legs.

When you want to stand and they want to sit.

All these little incongruences eat energy, The more coordinated as a team , the less the loss, but there's alaways going to be something lost in the translation.

TandemGeek
05-24-08, 08:46 AM
Can someone explain what it means to work efficiently on the tandem? If I put one ft-lb into the pedals and my stoker does the same then where does that two ft-lbs disappear to if we're not working efficiently?

I'll let the physicists play with the theories here and attempt to address your question at the practical level.

In that vane, let's start with some overly simplified basics: Cycling efficiency is influenced by rider position, crank length, cadence, fitness, and technique. The ability to efficiently convert energy into work with the least amount of wasted energy requires cyclists to optimize those five elements. In other words, if any one of those elements is lacking or compromised, efficiency will also be compromised.

So, lets use the scenario suggested by the OP where we have two equally efficient cyclists with well-developed pedal stokes that were honed on single bikes and put them on a tandem and look at where the five elements are compromised:

1. Rider position: Few if any average-size stokers can replicate the riding position they use on their single bikes without planting their face in the back of the captain... and even then some riders may still not have enough room. For captains, most tandems are produced in three or four sizes instead of 1mm increments and use threadless stems. Therefore, fine-tuning the 'fit' usually requires access to a tandem that has not had the fork's steerer tube cut, a wide range of stems with different lengths and rise, and a handful of spacers. However, even then, adopting the same practiced riding position on a tandem as used on a single bike has been proven time and again to be something that must be modified to compensate for what I'll simply call "tandem riding dynamics", e.g., dealing with the added task load and work required to deal with the stoker's added weight and movement on a bike with a longer wheelbase.

2. Crank length: Almost all tandems come spec'd with 175/170 cranks. Dealers will often times suggest to captains who use shorter cranks on their single bikes that the increased leverage is favored by many captains: bovine scatology to be sure. Therefore, unless both cyclists happen to use these same crank lengths on their single bikes, this is another change that will alter their efficiency.

3. Cadence: There are spinners and there are mashers and within those groups are all different kinds of variations. Therefore, hoping on a tandem is somewhat analogous to a three-legged race where the ankles get tied together and must move in unison. And like a three-legged race, the variation in leg bone and foot lengths require compromise on both parties which also robs efficiency. In fact, I suspect this is probably the biggest hit to efficiency that very experienced single bikers may experience when they jump on a tandem or, conversely, when an experienced cyclist hops on with a newbie who's last experience with bicycles was slogging out 50-60 rpm on their 50lb balloon tire coaster brake bike.

4. Fitness: Different levels of fitness (4% lean not gaunt vs. 25% body fat) not to mention different types of fitness (weight lifter vs. gymnast) clearly factor into cycling efficiency, particularly with respect to acceleration, climbing and even aero drag.... however, of the three it's climbing where mass can be more of a hinderance than help.

5. Technique: This comes in a close second to cadence and really ties all of the other four elements together since this is ultimately what must be changed by both cyclists when riding a tandem. Changes in fit, crank length, cadence and compensating for different levels or types of fitness all manifest themselves in riding technique. Even the most skilled and clean-riding cyclist can find themselves bobbing and jumping around on the saddle when they captain a tandem. Cyclists turned stoker will usually struggle with the total loss of control and lack of forward vision which, when coupled with a completely unfamiliar riding position will have them fidgeting around, trying to look past the captain to see the road ahead, and struggling to remain focused / anticipating what they need to do to remain in sync. All of these things not only draw on the stoker's efficiency, they contribute to the added task load on the captain. Add to this more advanced techniques such as standing and pedaling -- something most cyclists do out of instinct -- sprinting, drinking, stretching (no hands can be a thrill even for the most seasoned tandem teams) and, well, you get the idea.

Bottom Line: If efficiency on a tandem was limited to simply converting energy into work by pushing on a pedal, 1+1=2. However, that's rarely the case and being very efficient on a tandem takes a lot of compromise and practice. Some pairs never get there.