During club rides I find myself getting dropped by riders with the same weight and power but the main difference is that they are riding road bikes and I am on a cyclo cross bike (with road tyres and wheels).
Is it my position on the cross bike or is it almost impossible to make a sensible decision with out knowing my co efficient of drag area?
The current bike I have is a merida cyclocross 3 with cantilever brakes (pictured).
On a cyclocross your CD (drag coefficient is not much different).
You are the same basic shape.
It is a little taller so you have a bit more more frontal area
Are you sure you are the same weight and power? Would any of them be willing to swap bikes and test.
I get dropped on rides on my street CX but it is because I am just not as strong.
Based on your comment I think you are confusing CD with area
Drag = CD x Area x Velocity x Velocity
If you are getting dropped while drafting as a team (they are not trying to drop you) you cannot blame that on aerodynamics. The guy(s) in the front have more more drag than you even if you are sitting a little taller. If you are getting dropped you are not helping the team. Take a shorter turn when you are in front. Try and line up behind the next tallest rider.
In most circumstances you should not be getting dropped by riders your weight and power. The advantage of drafting another cyclist is way beyond the differences in aerodynamics of your frame or brakes, even your position on the bike. If you have this problem in a sidewind it might be your position on the crossbike is too upright.
It would seem your problem is one of these:
A serious problem with your bike (brakes binding, bearing resistance or rusty/gritty chain)
Your position on the crossbike is too upright resulting in you getting dropped in strong crosswinds (but not other conditions).
The other riders are stronger than you, allowing them to drop you in a sidewind or up a hill (situations what minimize your drafting advantage)
A nocebo effect (the opposite of placebo) - you think you are at a disadvantage and give up earlier or are unconsciously unwilling to push yourself hard enough to hold their wheel.
I am in the same bandwagon, being dropped at high speed flat cruising, though I was looking for different suspects (pedals, clothes...). For aerodynamics I would expect having broad shoulders or narrow, for instance, to have more effect than what road - CX differences can imply. Even wider handlebars would have more significance than frame geometry differences (between road and CX that is)
Since you say that you have similar power output, it could be performance when approaching or being in a glycogen depletion state. I have marked this as the main reason of my dropping out the bunch, not sure if it can apply to you but think if the riders you go with are more used to long (60 miles and more) rides, and what is your average riding distance, and specially, can you follow them OK the first part of the ride or is it a no no from the beggining?
It may not have anything to do with aerodynamics, but your gear ratio.
The website for the Merida Cyclocross 3 does not list the specifications, but the Merida 4 does indicate that, like most cyclocross bikes, it comes with a compact crankset, with the largest chainring at 46 teeth.
You should be able to stick with the group, but you will need to be spinning faster than you might be comfortable with. The compact cranks make going up hills a bit easier, but at higher speeds you will spin a faster rpm than typical cranks for the same speed, meaning you'll be working harder then the group and may tire out sooner, especially on flat to rolling terrain.
For the higher speeds, make sure you have an 11 tooth cog in the rear cassette and that will help (again, specs for the Merida 4 indicate 12-28).
You can find some more in-depth info googling: "compact cranks vs standard" also: Why might a compact drivetrain be frowned upon?
