suspension reviews

'hold that road'

author:  Rob Smith  AMT

	So far project Bonneville has gone remarkably smoothly. The engine is making more power and torque which, coupled with the slightly lower gearing, is giving a much crisper and snapper ride. The riding position is much less like a CB250 and although it's slightly sportier the open road comfort has improved and there's a useful increase in ground clearance.
	This month, having done the groundwork, we round out the package by taking a look at the suspension. In standard form the Bonneville has suspension aimed at being soft and pleasant for cruising between cafes and the occasional day trip. However, pillion passengers get a raw deal and when it comes to enjoying a day in the twisty stuff the rear suspension shows a little more than a tenuous relationship with the front.
	In order to get the best information we went to Pete "The Springs" Clements owner of ProMechA in Melbourne and a suspension guru of note. We told him what the Bonnie was doing.
	BUMPS A DAISY On bumpy, twisty roads the front rapidly runs out of initial travel and has virtually no compression or rebound damping. Apart from feeling overly soft, the springs themselves feel as if they're progressive, meaning that they are wound at different rates in order to offer a soft initial action before getting harder to compress as the spring compresses further.
	At the back, the same applies although the spring rate feels as it's at odds with the front and the two aren't complementing each other. In addition the shock looks and feels as if it's too short, meaning that the back of the bike sits very low, calling for a fair bit of effort at the handlebars. The Thruxton, which is fundamentally a sports Cafe Racer style Bonneville, has a smaller front 18 wheel compared to the 19 on the Bonneville and rear shocks that are 20mm longer. We figured we'd see what could be done with the standard shocks but at the same time got hold of a set of Ikon classic styled twin shocks with the required extra 20mm suited to a Thruxton.
	For starters Pete pulled the front forks apart, tossed the standard springs in the skip and set to work on the damping rods. The standard oil weight is a super-light 5W and the plan was to use 20W, so larger holes were drilled in the rods to facilitate flow that produced the required damping.
	Then a set of 0.85 weight linear-rate K-Tech springs was eased into the tubes, married to a set of (ProMechA) Provalves and the whole lot topped off with a pair of Thruxton preload adjusters. The result is that the forks now have the adjustability of a carriage-style fork. What does 0.85 mean? I didn't know either but apparently its the amount of weight in kilograms needed to compress the spring 1mm.
	As we said the rear shocks seemed to have fairly heavy springs but little in the way of compression and rebound damping. Sure enough the standard spring is rated at 150lb but, inside, the oil is just 5W meaning that on bumps, while the front was pitching and bouncing up and down the rear spring was constantly firing the back of the bike into the air. The answer was a ProMechA service. Pete keeps the details of the work to himself, not because it's terribly complex but because it's dangerous to start messing with a sealed shock. It's a job best left to the experts.
	CHANGE FOR THE GOOD Bloody brilliant! The first thing you notice is of course that the bike doesn't slump on its rear springs like a cringing dog the moment you get on board. The next is that pumping the forks feels much firmer and controlled. Down at the back the standard shocks, while looking and feeling low, feel as if there's far more control in their action.
	Out in the twisty stuff the Bonnie now feels taut and brimming with attitude. The front is maybe a little too slow to react for my taste so if it were mine I'd probably go for a lighter oil but overall the bike tracks with a ton of controlled feedback that just wasn't there before. Meanwhile, the rear's behaviour with the reworked rear shocks is a genuine surprise.
	Gone is the harsh bouncing replaced by a sense that the rear wheel is now in touch with the bitumen more of the time and the chassis is a far more stable platform. As a result, attacking bends is hugely confidence inspiring, allowing the rider to throw the Bonnie around and use the increased ground clearance afforded by the slightly higher footrest.
	If there's a downside, it's that the standard length rear shocks still contribute to slow steering and putting a limit to the ground clearance. Proof I suppose that you can't make a silk purse out of a sow's ear.
	Once we'd fitted the 20mm longer Thruxton spec Ikon shocks the Bonnie's transformation was complete. The bike now is a genuinely sporting piece of road equipment. The Ikons give a significantly better ride quality and are far and away more suitable for carrying a pillion. Without doubt, it's the extra 20mm that makes the most difference. Ground clearance is up significantly and the bike is far more willing to respond to the handlebar input needed to turn and drop into an apex. Once again I'd be looking at some fine tuning to get the thing turning even faster - but that's just me and it's possible that the extra 20mm is as much as can be used without sacrificing stability.
	My own preferences aside, the Bonnie's handling is improved by so much that the limiting factor to any bend is now the tyres. The standard ME33/MEZ2 Metzler combination are fine for general use but the MEZ2 in particular takes forever to warm up and the overall feeling is not one of confidence, especially in the wet. What the Bonnie needs now is some quality rubber like Avon Venoms or maybe even the Michelin Anakees I've been using on my BMW.
	SUMMING IT ALL UP Right from the start of project T120 Bonneville we knew that Triumph's torpid twin had potential and we had a very clear idea of the kinds of improvements needed. Rather than trying to make the Bonnie some sort of street racer and risk spoiling the overall usability, we wanted to do the sort of things an average Bonnie owner would actually want to do - subtle things that would make it more of a serious rider's machine using easy-to-access Triumph parts wherever possible. And being tight, we wanted to keep the costs low.
	Our mission has been a complete success. Power and torque are up. The riding position is slightly sportier but still all-day comfortable - and the safer handling will now let a good rider on a twisty road keep up with just about anything.
	AFTERTHOUGHTS It'd be great if Triumph made a serious road version of the Bonnie, they could call it the Speed Twin. Judging by the amount of people buying Bonnies and doing the things we've done, I'd say there'd be a market. Above and beyond our modifications and in keeping with the serious road focus, it would be nice to have a bigger tank range as the 16 litres in the admittedly attractive tank is good for about 200km. It'd be great to have 20 litres.
	The other thing I would like to see is a six-speed gearbox. Triumph made the same mistake with the first version of the T300 series Speed Triple, eventually reverting to a six-speed. Hanging onto the five-speed in the belief that, "that's all it needs" is a nonsense. This is the 21st century and people are used to a six-speed. No matter what gearing you run on the Bonnie, an overdrive top-gear would do no harm to the essential character of the bike and would improve the highway manners immeasurably.
	The MT Bonnie has reclaimed its rock and roll roots and personally I'm very pleased with the results. So much so that I'm hoping that the bank manager will let me buy the bike. Oh - does it do 120 miles per hour? Apparently it does! © 2005
	Reprint courtesy of Australian Motorcycle Trader Magazine  Issue 172 Sep/Oct 2005
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