A 5 second turbo bike might be further away than you think.

Five years ago, in 2015, Cycledrag.com posted this article about the possibility of a 5 second turbo bike. Some commentators are now suggesting that this achievement is imminent, and could even happen “this year”. Here are 7 reasons why I personally think it will take longer. These are not intended to belittle anyone’s efforts in this arena, my aim is merely to identify the challenges involved and to stimulate discussion.

1. The timeline. Gary Clarke ran 6.45 in 2000, a record which stood for nearly 10 years. Rikard Gustaffson ran 6.36 in 2013, followed by others into the low 6.3s in the last few years. Now we are at 6.20 but by this measure alone, two more tenths to make a five second pass is some years away, not months. When Top Fuel bikes made this same journey, from 6.20 to 5.99, how long did it take, 10 years?

2. Technical advances. The 6.3Xs funnybikes of a few years ago used standard blocks, mechanical injection and simple boost controllers. Cutting edge ECUs, sophisticated boost controllers and billet engine components have taken, at best, 1.5 tenths off these times. What kind of technical advance will it take to run another two full tenths quicker?

3. The eighth mile time fallacy. An argument has been put forward that bikes running 3.9 second eighth miles just need to continue the full distance to run a 5. I believe this argument originates in the fact that a 5 second quarter mile can indeed have a 3.9 second eighth, just as Larry McBride did when he ran a 5.99 quarter and 3.94 second eighth on the first 5 second pass in history. However, this argument doesn’t work in both directions. Let’s look at why.

Bikes geared for the eighth mile have more rear wheel thrust due to mechanical advantage of the low gearing. When you gear them up for the quarter, that advantage is lost and the 3.9 second bike becomes a 4.0 eighth mile bike again – not fast enough for a 5. 

To convert a 3.9 second eighth into a 5.9 second quarter requires a 2.0 second back half. Simple maths tells us that a 2.0 second back half requires an AVERAGE speed over the second eighth of 225mph – faster than any funnybike has ever gone at the finish, let alone as an average over 220 yards.

4. Power deficiency. This brings us to an obvious truth. The lower the entry speed into the second eighth, the more speed you have to make up in the second half. On McBride’s 5.99, he was at 179mph at the eighth – relatively slow for a fuel bike, and maybe a dubious figure given that the bike usually ran in the 190s, but an achievable speed for a turbo bike. However, he was then able to gain 64mph by using 1207hp (his calculation) to cross the line at 243mph for an average speed of 219.5mph in the second half. The first eighth was 3.94s, second half 2.05s. Turbo bikes do not currently have the luxury of that much power. As drag increases with the cube of the speed it takes 30% more power to go from 220mph to 240mph.

5. Weight. “But turbo bikes can be lighter!”, comes the cry. Well, that’s true, but is it an advantage? Power to weight ratios are chased almost religiously throughout motorsport, but in drag racing weight=traction. In the absence of any meaningful efforts to create downforce aerodynamically, the heaviest bikes are also the fastest bikes because they can transmit more power through the tyre. Building a turbo bike light may not be an advantage at all in terms of traction available.
Also, the faster the speed the less light weight is an advantage. Why? Because the weight of the bike/rider remains constant during the run (apart from using fuel), but aerodynamic drag rises exponentially. At the start, 100% of the energy used is in moving the weight of the bike. This favours a light bike. At the finish, the majority of the energy is being used to overcome drag. You might not need 1200hp like Larry, but a funnybike is not far off in terms of drag. Power is much more important than weight in the second half.

6. Engine strength. It seems then, that any proposed 5 second turbo bike will need to make at least 1000hp, and probably more like 1200hp, to reach the speeds necessary. A 1.5 litre turbo engine can certainly make that much, we saw it in F1 in the eighties, but with most of the efforts revolving around unit construction engines with streetbike origins there are numerous questions around the strength of the driveline at these power levels. The way forward is far from clear.

7. Finally, if you ever need a sobering reminder of how hard it will be to make a turbo bike run a 5, look around the fuel bike pits at all the bikes that have never made that number. Those guys running 6.1s and 6.2s with sometimes decades of development and experience, and often way more power than any turbo bike too, yet still the 5 eludes them. Do you still think the turbo 5 is imminent?

Lorcan.