Riding pillion on the Mother Road, Sarah Barrell heads for Milwaukee, home of the world's most famous motorcycle, to join the centenary celebrations

Sitting in a quiet roadside café on Water Street it's hard to imagine Milwaukee as the host of the "world's largest rolling birthday celebration". On a muggy evening in mid-August dark dock houses loom over the recently regenerated canal-side area of this modest Mid-West city, the new waterside walkways as deserted as the surrounding concrete shopping precincts. With several hundred miles of highway still ringing in our ears, the silence is oddly deafening.

Sitting in a quiet roadside café on Water Street it's hard to imagine Milwaukee as the host of the "world's largest rolling birthday celebration". On a muggy evening in mid-August dark dock houses loom over the recently regenerated canal-side area of this modest Mid-West city, the new waterside walkways as deserted as the surrounding concrete shopping precincts. With several hundred miles of highway still ringing in our ears, the silence is oddly deafening.

This weekend Harley-Davidson turns 100. To celebrate the centenary, 250,000 bikers from as far afield as Barcelona, Tokyo and Sydney will arrive in Milwaukee, the final stop on their pilgrimage to the birthplace of the bike. This epic two-wheel tour, dubbed "The Ride Home", is being billed as the biggest American road trip ever planned.

Earlier we had picked up a rental bike several hundred miles south of Milwaukee, at the Surdyke dealership near St Louis, Missouri, to make our own ride home. The plan: to travel the eastern end of Route 66, into the heartland of the Harley-Davidson Owners Group, or Hog.

As a consummate pillion passenger, I couldn't care less about gas tanks and the complexities of Harley's heel-toe manipulated gear shifter. I fidget with endless racks of Hog merchandise - biker boots, biker jewellery, biker underwear - and wonder on what occasion a pet dog would require a miniature leather biker's cap embossed with the company logo. Out on the forecourt I'm greeted with a sharp reminder that despite its merchandising muscle Harley-Davidson is still as much about the bike as it is about the brand. My companion is test-driving our nominated rental, the Electra Glide Ultra Classic, at 850lb the biggest motorcycle that Harley-Davidson has ever made and, for the uninitiated, no easy rider.

Monster trucks scream past us on the nearby highway and my designated driver begins to look more Woody Allen than Dennis Hopper. Surrounded by the kind of men whose bulk is complemented by nearly half a ton of bike it seems rather emasculating to suggest my companion might be more comfortable astride something a little smaller. We eventually manage to make a diplomatic shift to the 250lb lighter Heritage Softail, which unlike the Ultra Classic (complete with intercom system, stereo and more padded leather seating than a three-piece suite) is a bike that according to manager Matt Surdyke "you can really feel the road on". As my rear end would latter attest he was not lying. But for the moment, as we finally get our motor running and head out on the highway, the only thing I am thinking about is how to contain an enormous yelp of idiot glee. High-fives and whoops of "Yee-haw" may be uniquely American, but girly squeals, I suspect, aren't that credible in die-hard biker country.

The most distinctive thing about a Harley is its noise. Like cheerleader chants and slide guitars it is pure acoustic Americana. Sit on the back of one, however, and the characteristic combination of rumbling vibrations means that for the first half-hour you're constantly looking behind you for a chapter of Hell's Angels. Harley-Davidson is the motoring equivalent of a Tuvan throat singer; you can't believe all that noise is coming from one set of tubes. Burning out of St Louis it would be nice to say that we confirmed the tourist-board tag that there's "more to St Louis than the arch" but once on the highway with the exhaust pipe rumbling contentedly there was no stopping us. Gateway Arch is St Louis' monument to the American West, where wagon-train pioneers paved the way for Route 66 with a dusty frontier road stretching from Missouri to California. Centuries later but before the advent of the multi-lane American highway, Route 66 replaced the dirt tracks, a 2,381-mile road connecting Chicago to Santa Monica.

Since its inception in 1926, the adventures of the open road and American popular culture have been nowhere better embodied than Route 66 - "the mother road". We scoot over the Mississippi river casting a glance at the old Chain Rocks Bridge, Route 66's original river crossing. Though decommissioned in the 1970s you can still follow the brown and white Route 66 Heritage Highway plaques that indicate where to jump off the main Interstate (highway 55) on to what remains of the old Mother Road and its roadside attractions. But with vast green-tipped cornfields and distant, towering silos flanking the open road ahead we are reluctant to stop.

After a few hours on the Softail, however, my own tail is anything but soft. Road-worn, windswept but feeling undeniably cool we stop at the Ariston café, Litchfield. I'm walking with the kind of wide-legged swagger not usually associated with nice English girls. Perhaps it's this or our windblown grins that prompts instant questions from the waitress as we eat a slice of phenomenal cheesecake. Like travelling with a precocious child or pedigree puppy, travel with a Harley and bystanders can't help but come over for a stroke and a coo. Most are well into their 50s, a testament to Harley's grown-up baby-boomer fan base. We find ourselves in the parking lot talking to a friendly septuagenarian about a legendary spill he took on his brother's bike back in the Sixties, but escape just in time to ride into the sunset, a sudden cloudburst drawing a curtain of pink rain across the cornfields.

Route 66 seems built for bikers. Though well in sight of the main highway, this two-way road is almost deserted and as long as you watch out for the odd farm vehicle clattering toward you, scythes poking out of the trailer at rather inconvenient head height, you are King of the Road. We roll into our next stop, Springfield, limbs intact, feeling like royalty. Much lampooned in The Simpsons, every American state has a town called Springfield. At the Illinois branch we find a seedy Route 66 bar called the Curve Inn where people eat a lot of late-night travel food and drink a lot of take-home pay. Jackrabbits high-tail it through grass verges and the sound of countless Harley engines compete with the squeezebox tones of the late-night freight train. An evening scripted by Steinbeck couldn't have been more perfect.

Further along the road in the town of Odell we stop at the best preserved of the Mother Road's old gas stations, complete with 1930s pump and clapboard canopy roof. We park and watch a Harley with father at the helm and young son as pillion do a slow rumbling drive-by, acknowledging us with a friendly salute. So much for the mad, bad posters boys of Route 66: Nicholson, Hopper and Hunter S Thomson. Today's Harley riders, flying stars and stripes, are for the most part as far from the Gonzo lifestyle as their lawyer/doctor/accountant occupations would suggest. Hog members pride themselves on being keen community workers, volunteering their riding time to good causes.

It's a shame, then, that these community-spirited souls aren't around on the final morning of our ride home. Entering the motel car park at dawn we find an empty spot where the bike had been. The bewildered conversation of the recently robbed and patently stranded in the middle of bloody nowhere ensues. This culminates in my companion crumpling in a heap on the curb tearfully proclaiming that he had christened the beloved bike Bob, after his brother, and hotel staff telling us not to worry because they're going to "call the cops". Three minutes later the Illinois State Police arrive. Very nice they are too (not that they find our bike) but the police officer-cum-town fire chief is very sympathetic and calls me ma'am a lot. He even drops us at the local Budget rental car office from where, for want of a better option, we continue our journey in a pickup truck.

The last leg is maddening. Harleys rocket past us, and from the truck the Mother Road seems dominated by bland motels, franchise fast-food joints and bleak billboards championing gun ownership. We roll into Milwaukee in sulky silence, the redundant crash helmets taunting us from the back seat.

On a replacement bike we thankfully manage to find the time for a few victory laps of Milwaukee, stopping off at the Juneau Street Harley-Davidson HQ where the first bike was built at the turn of the last century. The original workshop, a tiny wooden shed, will be bought out of archive wraps for centenary celebrations. But for now we make do with the modern factory tour, happy to be back in the saddle and inspecting the Harley-Davidson production line. Sportster and Buell engines gleam on the conveyor belts while outside in the car park, men with beards as long as the handlebars on their well-worn Low Rider bikes gather to pay homage to the spot where it all began. We follow them out in formation, and even manage a smile as one of them, on hearing our story hollers, "Got yours nicked, on Route 66, eh?"

The Facts

Getting there

Sarah Barrell travelled as a guest of American Independence, Harley Davidson and Cellet Travel Services.

American Independence (0870-241 4217) organises tailor-made tours throughout the US. A fly-drive package to the eastern end of Route 66 costs £798 per person. This includes seven nights hotel/motel accommodation en route (two nights in St Louis, one night in Springfield, Illinois, two nights in Milwaukee and two nights in Chicago) and flights departing from London via Newark or Chicago, to St Louis, returning from Chicago. The hire of a Harley-Davidson for the week costs an additional £485 plus a $200 (£140) fee for one-way bike rental dropping off in Chicago. Bike insurance starts at $14 per day. Harley-Davidson has 220 authorised rental locations worldwide, including 30 in Europe. Bike rental in the US starts at around $80 a day. For more information about organised tours and rental pricescontact Harley-Davidson Rentals at www.hdrentals.com or contact the Harley-Davidson Motor Company (0870-904 1450).

Further information

The Visit USA Association (09069 101020, www.visitusa.org.uk). For detailed tourist information about the Illinois section of Route 66 and Milwaukee, contact Cellet Travel Services (01564 794999, www.cellet.co.uk) or see www.milwaukee.org. For St Louis, Missouri, contact the Missouri brochure request line (0870-9000 996) or see w ww.VisitMO.co.uk and www.explorestlouis.com.