Velomobile efficiency is one of the best reasons to own a velomobile. The subject is fairly complex and I will try to cover the subject in a few posts here. Today I will an overview of what characteristics of makes a velomobile efficient and address aerodynamics.
Out of the box, velomobiles are fairly efficient when you compare to an unfaired bike but there are still many things that will give you an even better performance including several options available from the manufacturers.
How efficient are production velomobiles? Again this depends on the velomobile you compare with but if you have a Milan SL or a DF, you have one of the most efficient velomobiles out of the box. As a rule of thumb I say that since 80% of the effort on a road bicycle is pushing the air, a velomobile should improve this by halving this effort. This gives a 40% advantage (50% X 80%) to the velomobile. As a result to maintain a certain speed a bike requires 300W while the velomobile requires 240W X50% +60W = 180W. I tried to see if my rule of thumb was close so I looked at segments that I rode and compared them to the next rider assuming he/she was riding a road bike. In one example a rider is pushing 255W for 12.5km on a somewhat flat segment, no wind and a speed of 39.5km. For the same segment, I rode the same segment with a 10km/h headwind. I needed 155W to travel at a speed of 47.5km/h. Leaving the speed difference and wind, my rule of thumb would give me an effort of 204W X 50% + 51W = 153W. While it appears to somewhat confirms the rule of thumb, the difference in speed and the extra work due to the wind has not been accounted for so the efficiency appears to be even greater on flat ground. Of course the difference on a hillier course would be somewhat different, as the power to climb the hills needs to be much greater for a velomobile.
I get it, many people are happy with the weather protection and carrying capacity of velomobiles. Others do not see a need for improvements or are unable to fully extract the performance of a stock velomobile or they opted for e-assist that gives them the boost they need. Still there is a large segment of riders who want to get as much speed as possible for the power they can generate. Getting the most performance out of your velomobile requires paying attention to the details.
There are four areas where improvements can normally be made: aerodynamics; rolling resistance; weight and mechanical efficiency.
There are many ways to improve the aerodynamic efficiency of a production velomobile but it depends on the velomobile you ride. It may be the low hanging fruit for improving velomobile efficiency. Every thing that sticks out like mirrors or holes in the shell like the hatch for the cockpit creates drag. I will provide some ideas on how drag can be reduced but these recommendations have to take into account your own velomobile and circumstances. Beyond these tips there could be many more improvements that may be possible.
I was reminded of this recently as I started my season riding without any significant modifications on my DF, it was essentially a stock DF. The only small improvement was a mini visor. I rode a few weeks until I decided to put on the hood. That single improvement provided me a 3 to 5km/h average speed improvement on the same courses; this is very significant in the order of 10 to 12% for the same power output. Going down hills where I had a maximum speed of 62 to 65km/h, I was now maxing out at 72 to 74km/h, a 10 to 14% increase.
Open hatch velomobile and the rider’s head/torso have a major impact on aerodynamic efficiency. Even a rider’s helmet increases the drag. First let’s look at the obvious. Many open hatch velomobile usually have a neoprene skirt that covers the hatch leaving only room for the head to stick out. This provides a significant improvement but it can be uncomfortable to ride with the skirt regularly. Another small improvement is the mini visor that is attached to the front of the hatch with a Velcro. The visor deflects some of the air around the rider. There is a small aerodynamic impact. A hood that covers the open hatch of the velomobile is probably the best way to minimize the drag of the velomobile. Some velomobiles like the Quest have several different hood designs and manufacturers to choose from. Each has its own advantage and disadvantage and have differing level of efficiency while others have a more restricted choice. The hood has the added advantage of weather protection in cold and wet conditions. On the other hand, visibility can be somewhat more restricted, it may be difficult if not impossible to wear a helmet, the cockpit can become too hot for some or it can fog-up. For racers, commercial hoods have also been modified to make them even more efficient for example by reducing the frontal area. While hoods are not created equal, as I mentioned above the gain from a hood can be significant, it is probably the single biggest aerodynamic improvement that can be made.
Open wheel velomobiles like the Strada, Mango, DF, Evo-K suffer to different degree from the turbulence created by the front wheels and wheel wells. There are several ways this can be addressed with different levels of performance improvements. First wheel cover help for open wheel velomobiles by covering the turbulence from the spokes. Some wheel covers are made of Lycra while others are made of fiberglass or carbon fiber discs glued to the rim.
The wheel wells are a source of air turbulence. Some velomobiles have tighter space between the wheel well and the tire to reduce this turbulence. Small plastic deflectors that essentially cover the gap in the wheel well between the shell and the wheel leaving just enough room for the tire to pass when turning can be added to minimize turbulence. Deflectors are normally taped to the shell just around the wheel wells. This provides a small but noticeable improvement in efficiency.
If you would like to further, increase efficiency wheel pants are the solution. Wheel pants essentially cover the whole wheel well making the airflow past the wheel well following the shape of the velomobile. This minimizes the turbulence significantly. Only the bottom of the wheel is visible. Unfortunately, there is a small price to pay because the wheel pants are restricting tire size and increasing turning radius. Some racing wheel pants or wheel pant extensions can be installed on racing velomobiles to further reduce drag as the wheel pant/extension are made to cover the whole wheel leaving less than a centimeter of air space between the ground. Even velomobiles where the wheels are covered like the Quest could see performance improvement with extensions of the wheel covers effectively hiding the bottom of the wheel and closing the bottom of the wheel well.
Velomobile with foot holes can see improvements by closing the foot holes with special covers that have bumps under the shell to give room for the feet to move freely on the pedals. While I don’t have a number to give, the improvement can be significant.
The perfect tail for a velomobile is shaped like a wedge. Unfortunately in order to provide a surface for rear lights and reflectors to increase visibility, most velomobile tails are somewhat truncated. Many riders have found a solution and added a tail extension transforming the blunt tail into a wedge using transparent plastics. The reflectors and lights are still visible but the airflow is better reducing turbulence. The improvement is small but noticeable for racers.
The nose is something that has attracted attention lately. The DF, for example is a velomobile that has an air intake at the front. While it is designed to minimize the aerodynamic drag but it still has an impact. Closing the hole will result in a warmer cockpit but that may not be an issue for a race or when the temperature is cold but may lead to an increase in fogging up inside the hood. To get the improvements, the cover has to follow the shape of the current nose and using clear plastics would not impede the headlights hidden inside. While this small modification provides improvements, you can go further.
Recently I was made aware of one rider who modified the nose of his Milan SL by extending it making it pointier and saw speed increase. This modification is not without drawbacks. The modification makes the velomobile more susceptible to side winds and this could make the velomobile difficult to control at high speed under windy conditions.
Beyond these modifications, those racing may also try to use tape to close seams and other holes and cracks in the shell. Anything protruding from the shell like lights, cameras and mirrors especially large ones are also creating drag. While I would not recommend removing any items used for safety, riders may choose to reduce their impact or removing them for special events like races.
Daniel Fenn is hard at work on the DF-4, the 4-wheel DF prototype. He recently went on a 170km ride with the prototype. Daniel even took his dog in the velomobile on this journey, the dog can fit in the luggage compartment just behind the seat is much larger than the DF. ICB has posted several pictures and video on their blog. The prototype will see more refinement before a decision is made to go to production.
There are a number of interesting innovations in the design. The pedals will drive the left wheel of the velomobile while an optional motor providing assistance will drive the right wheel. The DF-4 prototype has a mid-drive Rolhoff and a 10 speed cassette at the back. The wheel wells are larger than the DF/DF-XL and would enable the use of popular larger tires like the Shredda and the F-Lite.
Busy as usual, Daniel is also in the process of producing a new racing hood for the DF with no side windows. This is the type of hood that Milan riders have used in record attempts. The hood is apparently 33% lighter and hopefully more efficient than the original DF hood.