Engineering innovation that leaves modified cars for dead

Posted on February 24th, 2007 in Opinion by Julian Edgar

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I know that this is a car publication – and primarily a modified car publication – but forgive me if I digress.

As we’ve been covering in our series Building a Human Powered Vehicle, I have been making a recumbent trike. That is, a three wheeled, pedal-powered vehicle where the rider lies back at an angle and the pedals are relatively high up in front. My design uses full suspension (front double wishbones, front sway bar, rear trailing arm), is made from aluminium and uses a rear damper. It was inspired by the non-suspension Greenspeed series of trikes, of which I own one – the Greenspeed GTR.

Before building the trike, I knew very little about them – and very little about bicycle design as well. So, you might be thinking – what is there to know about bike design? Well, lots and lots. Things like gearing (crank length, front and rear cog sizes, gear ratios), steering (trail, castor), and of course frame design. With tadpole trikes, you can also add scrub radius, toe and Ackermann, and with a suspended trike, static and dynamic camber, anti-dive and so on.

Home builders of bikes and trikes have an extensive web presence, and while there are some simply horrible designs prominent, there are also some excellent engineering pages devoted to design. In fact, when I compare this scene with modified cars, in terms of uniqueness, engineering innovation and results, recumbent bikes and trikes show far more progressiveness than home builders and modifiers of cars. (The only home-built transport application which I think is even better can be found in experimental class aircraft.) Small, specialist engineering companies are also prominent in recumbent trike building.

Enough of the generalities – let’s look at a specific case.

I’m a member of a trikes mailing list – that is, a discussion group that works entirely by email. (There is a web compilation of these emails – see Over the last 18 months or so I’ve had plenty of arguments with members: those that have told me that clothes pegs clamping aluminium rods are all that I need as dampers, those that have said a lower roll centre will reduce body roll, those that have said that a suspension travel of 1 inch is all that’s needed. All completely wrong of course. (And those that have said that suspension will add weight and complexity to a pedal powered machine – completely right of course!) However, I keep reading the emails because you never know when something really good will crop up – and, to be fair to the members, that’s probably about once a fortnight.

So when one Sunday morning I saw an email from Trikes List member Ian Sims, I read it closely. Ian, the founder of Greenspeed, is seen as one of the patriarchal figures in recumbent trikes. His children now run Greenspeed, with Ian having retired to the Sunshine Coast. In the email, Ian said that after years of running oval-shaped front chain-rings (ie front chain cogs), he was now moving to smaller cogs and shorter cranks. (Using shorter cranks is one of the many big discussion topics about which I know nothing.)

But what was this about oval-shaped chain-rings? Ian listed as the manufacturer of the chain-rings and I had a look. Under there was a description of what these oval-shaped cogs do.

“As you turn your pedals, you encounter two dead-spots where you can achieve very little output. You will notice them when pedalling slowly up a hill – the cycle lurches forwards in spurts corresponding to each stab on the pedals, slowing down each time your feet pass the dead-spots.

“Ovals reduce this effect by providing a lower gear to speed you through the dead-spots. You spend longer in the power-stroke, where your legs can push the hardest, which means you can pedal more powerfully too.

“An Oval gives a smoother power transfer to your wheel, reducing the stop/start effect which causes wheelspin on slippery surfaces and the bobbing effect which occurs on some suspension bikes.”

That all seemed to make sense – in fact, I was quite blown away with the logic – and on a technical description also seemed right on the money.

Imagine: a gear ratio that was constantly and smoothly changing to match the torque output of your legs!

I re-read Ian’s email and noticed that the oval cogs he was replacing would exactly suit my suspension machine. They had the right crank length and they had the right tooth numbers. I emailed him, asking how much he wanted for the old set. Ian gave me a good price so I drove up to his house (400 kilometre round trip, but that why I drive a Honda Insight!) and collected them.

The system comprises 170mm cranks with a round 52 tooth large round ring, a 42 tooth middle ring with 10 per cent ovality, and an inner ring of 30 teeth with 20 per cent ovality. On my trike these work with a 9-speed rear cluster and three speed internal hub. I fitted the set (because Ian had ended up giving me the whole assembly, it was a simple 1 minute bolt-on) and immediately went for a ride. (Long day: 10 pm by this time!)

So to reiterate: the oval chain rings replaced round chain rings with the same tooth number. The crank length was also identical, and I swapped my old pedals over. The only change was the ovality of the smallest and middle front cogs.

And the results? Oh boy, are these oval rings good or what!

For the previous week I had been riding local roads which include very steep hills. (The gradients aren’t marked but the hills are clearly steeper than other local roads marked at 14 per cent. Walking up them is bloody hard work.) On one long hill, I had to stop for three rests. That is, I have been in lowest possible gear struggling up the hill and get so exhausted trying to turn the pedals that I stop three times – all within the last 200 metres or so of the hill. (It’s unfortunate that it’s the only way out of the suburb in which I live….)

But with the oval chain rings fitted, I did not stop on the hill at all!

That is quite incredible. I was damn’ tired at the last little bit, but only as tired as I previously was just before needing to rest three times. Another hill, where again I have always had to pause once, I went up without stopping.

No other change but the differently shaped chain rings.

Apart from the incredible gain in real world hill climbing performance, the other thing that surprised me is that the oval chain rings feel quite ‘natural’. In fact, if anything, they give the legs a more fluid reciprocating feel. Watching the chain wheel circumference going up and down is weird, but the leg movement feels fine.

The Highpath Engineering website makes the point that oval-shaped cogs are not new to cycling – but clearly 99.99 percent of pedal-powered machines don’t use them and very few people have ever heard of them. To me it represents – along with fully aerodynamically faired pedal machines normally called Velomobiles – real innovation in individually constructed vehicle design. Of the sort I simply don’t see with cars.

When did you last hear in automotive home-built or modification circles of something as incredible as a gear shaped to constantly change ratio?

3 Responses to 'Engineering innovation that leaves modified cars for dead'

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  1. Rob said,

    on September 13th, 2007 at 5:08 pm

    Shimano had these back in the early -mid 90’s … bthey called their system “Biopace” (this was obviously before StudlyCaps)

  2. Wave said,

    on December 11th, 2008 at 5:40 pm

    I have seen these oval chainrings sold for use on road bikes and even mountain bikes and was very interested by them. I must say though, to be fair to the modified car enthusiast, it is far more difficult and expensive to do any serious engineering changes to a legally road registered vehicle.

  3. Geoff said,

    on April 22nd, 2009 at 11:39 pm

    When I was a professional cyclist these were around, and I tried a set of the Biopace ones. Having already developed a very good circular pedal stroke on both fixed wheel track bikes and my primary road and time trial machines I found them uncomfortable and developed pain behind my knee-caps, which is a sign that the seat is too low. Further research after I had taken a half ownership in 3 bicycle stores found these were targeted only at the novice market, who actually did get benefit from the design.

    As an aside I retired from competitive cycling to study Engineering, only to suffer crippling nerve damage as my career was peaking so I no longer work or cycle, although I am still very interested in developing cycling technologies.