I made an interesting mistake the other day.

My Honda Insight has a ride quality and seats that are nothing fantastic. The Honda’s rear suspension is poor indeed and in general, the car rides like a badly developed, very lightweight car.

The seats are thin and not well shaped.

Over a long distance, the upshot is that the car is a bit uncomfortable.

So I thought I’d improve the ride and the seat comfort by installing new seats. But that’s easier said than done.

The main problem is that the car is unusual in that both lower anchor points of the seatbelt attach to the seat. That is, the seat belt loads are borne by the seat, and then by the seat attachments to the floor. Therefore, any replacement seats need to be of the same design – and this is very unusual.

Honda S2000 seats are apparently of much the same design – and some Insight owners have fitted these seats. But S200 seats are typically very expensive.

So one day I spent a full two hours browsing the local wrecking yard, looking at the seats in literally every one of perhaps 300 cars. And I found only two models that had similar seat and seatbelt designs – the Holden (Opel) Astra and Vectra.

But all the seats had pretty scummy trim, so I started heading off. However, in the entrance yard – an area that carries cars that have just arrived – I saw a Vectra hatch with seats in near-new black leather! I asked the price, was happy with the answer, so bought the seats. (I might add I’d brought with me full measurements of the required seats.)

Installing the seats required the fabrication of custom frames, frames that because of the seatbelt loads, had to be very strong. I was happy with the front frame but less happy about the back adaptor. In fact, I sent a pic of it to an engineer I know – and he wasn’t happy with it either.

So the rear seat frame needed to be upgraded – it was probably good for 500kg or so but not the absolute max required of a seatbelt mount. But anyway, I was happy enough to drive the car for short trips and revel in the new leather seats – the upgrade rear mounting frames I put on the list for later in the week.

In addition to the soft leather, the new seats featured adjustable lumbar support and a ratchet height adjust. I found that the seats needed to be set to their lowest level – and even then, they were a little higher – say 25mm – than the factory seats.

But what really concerned me was that whenever I drove the car, I felt sick.

I am susceptible to motion sickness, and the strong ‘new car plastic’ smell makes me feel ill, but I didn’t expect to be sick driving my own car. I thought at first it was perhaps the fact that I had inclined the base of the seat upwards at the front – perhaps it was squashing my (ever increasing) belly?

But then I flew interstate for a week and drove another car that had just the same squab inclination – and I felt fine.

Coming back after that trip, I decided to go for one more drive in the Honda. If after a week away, a short drive still made me feel sick, I’d need to examine this more closely.

And yes, driving the car still did make me feel ill.

So what was the problem? I drove around, feeling and thinking. The one thing I noticed more than other (and especially having been in other cars for the last week) was the level of high frequency vibration. In fact, if I held my teeth spaced just fractionally apart, the vibration made them chatter. I don’t know what the frequency was (5Hz? 10Hz?) but I figured it was making me feel ill.

But why the vibration with just these seats?  I think it’s their design.

When I had removed the Honda’s seats I’d been very surprised to find that they used metal springs. Metal springs – especially coil springs – were very widely used in seats in older cars. German cars kept using them right up to the mid Eighties. But pretty well all seats of the last 25 years use foam ‘rubber’ (not actually rubber but you get the idea). The Honda’s seats use a mixture of foam rubber and long metal springs made up of repeating S-shaped bends. (The S-shapes elongate when the spring is subjected to tension.)

The Vectra’s seats are fully foam rubber.

The harder the seat (defined as the less the seat drops in height with the weight of the person on it), the less effective it is at absorbing movement – including vibration. And the Vectra seat was harder – that explained why, despite my measurements, I was sitting about 25mm higher.

The original Honda seats used springs to provide the suspension and the foam rubber to provide the ‘body support’ function.

So I removed the new seats and went back to the old. (Actually, before re-installing the old seats, I changed the shape of the driver’s seat to provide better side and lumbar support – but I didn’t change the base springing.)

And, straightaway, no more sick driver!

I’ve changed the seats in two of my previous cars and never had this problem – it’s a really, really interesting one.

Still, I’ll now consider really carefully before changing any more seats….