Asda claims its new bike is the country's cheapest. It may also be the nastiest and most poorly built

Earlier this month, Asda boasted that it was to start selling "the cheapest

bicycle in the UK". Having bought into the notion that to get a half decent new bike you had to pay at least £300 for it, I was intrigued to find out what you'd get for less than a quarter of that amount – Asda's adult machines cost just £70.

And so it was that I have spent the past month juddering around London on a 26in British Eagle 18-speed women's mountain bike in a patronising purple shade.

My first outing on the Purple Eagle ended on a sour note when the handlebars started turning in an entirely unhelpful and counter-intuitive way every time I rounded a corner. The headset was horribly loose, and I had no tools on me to fix it.

Herein lies the first problem with buying what bike snobs refer to as a BSO (bike shaped object): you have to build it yourself. The Eagle comes in bits, meaning you have to attach the pedals, front wheel, handlebars and saddle to the frame. Asda's PR folks made mine, but the lesson is the same. Are you sure you know how to put it together properly? If not, you can either take it to your local bike dealer and hope they won't laugh you out of the shop when you ask them to do it for you (and if they oblige, you'll pay at least £20 for it). Or you can risk getting it wrong. The best-case scenario is that, like me, you end up walking home. Let's not contemplate the worst case.

The second problem was the grip-shift gears, which are operated by twisting the end of the handlebars. Very quickly I wished that British Eagle had concentrated on getting three gears right rather than making 18 substandard ones. Every time I went over a speed bump I changed gear; even on the flat there was always an irritating clicking sound which spoiled every ride. On the scale of annoyance, it was rather like being at the cinema and having someone kick the back of your seat all the way though the film.

To test the bike properly I decided to take it on a grand tour of north London's Three Peaks: Crouch Hill, Highgate Hill and Muswell Hill.

Yorkshire folk will no doubt dismiss these bourgeois mounds as mere hillocks, but tackling them on my weighty (18kg), graceless machine felt like I was scaling Pen-y-Ghent on a pedal-powered tractor. The good thing about mountain bikes is that they have super-low gears, but as the Purple Eagle could never stay in any gear for long, I may as well have been on a single speed.

After 40 miles or so of gentle bimbling, I took the bike into my local bike shop, Two Wheels Good, and got the owner, Jonathan Boyce, to give it a once over. He groaned as I wheeled it in – "We see these a couple of times a week and so often the repairs cost more than the bike," he said, adding that he gave me "four to six weeks" before the bike was too jiggered to ride. Jonathan's advice for those on a budget is to scrape together £100 to buy a decent secondhand bike rather than waste money on the Purple Eagle or any of its relatives.

Here are some of the flaws Jonathan noticed:

1. The Purple Eagle is a ladies' bike. So why the men's saddle?

2. The components are rubbish and made out of the biking equivalent of a supermarket own-brand. The derailleur, gear shifts and more are made by a brand that sounds like Shimano but isn't. It's even written in the same font.

3. The brakes are made from plastic, rather than more expensive aluminium, and so will flex and bend, wasting energy.

4. The handlebar stem is the old "quill" style (instead of attaching to a steerer tube it fits directly into the headset and screws onto the forks), rather than an a-head stem.

5. The rear derailleur is hooked onto the axle, rather than bolted straight to the frame, making it almost impossible to adjust the gears properly.

6. The cheap plastic pedals will "simply fall apart before long".

7. The rear wheel was badly out of true.

8. The front wheel wasn't round, and was wobbling about the place as if the bearings have already gone. Apparently this shouldn't happen on a decent bike until you've done at least 1,000 miles.

But the biggest problem I had with the Lumbering Eagle was that it was horrible to ride. Every time I was due to set out on it, I cast a jealous glance at my lovely, nimble racer and prepared myself for the unpleasant ride ahead. This is the real downer with cheap bikes: they put you off cycling.