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SPANNERMAN
- Australian Motorcycle Trader |
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| In issue #157
we gave our long-term Honda CB1300 some extra stomp
which highlighted the need for suspension tuning.
SPANNERMAN climbs into his suspenders to explain how
it all works... |
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If you compare the stock suspension on the CB1300 with that of its spiritual ancestor, the 1969 Honda CB750, it's not hard to be
impressed. Suspension engineers have climbed a long steep learning curve to try to keep pace with massive increases in horsepower
output and the desire to get it to the road efficiently. |
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The stock front suspension on the CB1300 allows the rider to adjust the spring pre-load and the rebound damping over a
wide range of settings. The twin rear shock units are also adjustable for spring preload and rebound damping. On the surface,
the range of adjustability appears to allow individual owners to get a setting just right for them so why did we consider additional tuning?
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COMPROMISEAnd here's the answer...Like all manufacturers, Honda thinks long and hard about the bikes it makes. It had a certain
market in mind with the CB1300. While suspension engineers know you can get better results with monoshock
rear suspension, the marketing brains-trust decided the target customer would prefer the familiarity of twin rear suspension units.
The potential owner would be knowledgeable but wouldn't want so much adjustability that he/she had to waste large
amounts of time thinking about it rather than just enjoying the ride. Comfort would be a high priority; otherwise the customer would
be looking at a Fireblade instead. He/she would probably be a social rider too, meaning pillion pillion comfort had to be taken into
consideration. These and other design criteria (including of course, price) lead to compromises. Rear springs which need to be firm
enough to handle a rider, a pillion and enough luggage for both may be too firm for a light solo rider. Similarly, front-suspension
springs which need to deliver a plush ride might be too soft for high-end sports riding.
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We decided our CB1300 needed firmer springs at the front, softer springs at the rear and more sophisticated control
of both compression and rebound damping. It's not much help to readers if we just throw money at the problem (for example,
replace the rear shocks with expensive aftermarket items) so we've decided to work with the existing suspension as far as possible.
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PROFESSIONAL HELPWhile it's possible to do some suspension tweaking in your own workshop, specialist help is often necessary. The owner's manual
for the CB1300, for example, has the following to say about the rear suspension units: "The rear shock
absorber assembly includes a damper unit that contains high pressure nitrogen gas. Do not attempt to
disassemble or service the damper: it cannot be rebuilt and must be replaced when worn out." This is necessary
advice in terms of public liability (and common sense) but just about all rear shocks are rebuildable which is
where specialists like ProMechA become useful. Peter Clements from ProMechA pulled the rear dampers apart, revalved
them with (ProMechA) valves and shim stacks and regassed them to better suit our needs. The standard valving
inside the rear dampers, which helps control compression (how the suspension unit regains its normal height), is
of good quality but of limited range in how it handles low, medium and high road speed demands.
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In simple terms, the new valving allows more efficient damping over the whole range of operating conditions. There's more control available.
The standard rebound damping adjuster still works as before but the changes you feel in performance are greater. Slightly softer springs
are the next step and this will allow us to accurately set the desired sag for the suspension and maximise the benefits of the revised valving.
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FRONTAL ATTACKPeter Clements is convinced that the springs are the key issue in effective suspension. The stock front springs
on the CB1300 are dual rate, meaning they're wound so that the initial response to bumps is soft but it gets harder as the
springs compress. While the theory makes sense, the reality on the CB was that we felt the initial response was too soft (lots of
dive under brakes and noticeable attitude change in and out of (corners) and the hard part of the equation was too hard. Changing
the springs for more suitable units (in this case K-Tech single-rate springs) is relatively inexpensive and has given
more predictable control to the front end performance.
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The front forks were also revalved using K-Tech hardware with the same results we expect to achieve at the rear
once we've replaced the springs: more subtlety and control over the over the entire range of operating conditions.
While both the front and rear suspension were apart, we changed the damping oil from what looks like a 2.5W product to Silkolene
5W synthetic oil which keeps its viscosity intact as it gets hotter, meaning the quality of your ride doesn't change
dramatically as the damping oil heats up.
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WHAT'S IT COST THENCost of springs varies with brand name but there's no evidence to suggest there's much difference in
performance. The single rate K-Tech springs we've used were $175 a pair. ProMechA will do a front end revalve for $285 (which includes labour)
but you can save around $40 if you bring your forks in without them being attached to the bike. To overhaul and revalve twin rear shocks will set you
back $240 a pair and new springs will add between $100 a pair for Ikons to $360 for Ohlins. A feature of ProMechA's service is that it does follow up
work with customers to get the best possible result from any changes free of charge.
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RESULTSSo, how much better is our CB1300 after all this? Well, the front forks dive less under heavy braking, meaning
the the attitude of the bike doesn't alter as much through corners in faster riding conditions. Small bumps in the road are handled
with the same kind of control as big bumps and although the front end feels slightly stiffer, there hasn't been any loss of front
suspension travel and compliance has been much improved.
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The new front fork springs have 10mm of preload already with the preload adjuster backed all the way off, allowing
an extra 15mm of preload if necessary. Compression damping isn't externally adjustable but the revalving combined with the new
springs has eliminated the sense of change from soft to hard we experienced as the original-equipment springs compressed. Without
having replaced the original rear springs, we're not in a position to accurately assess the effects of the revalving of the rear shocks
but watch this space for a comprehensive report as soon as we've finished.
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THANKS FELLAS
ProMechA for professional service and advice.
Honda Australia for its continuing patience while we
muck around with what is already a great bike straight off the showroom floor. © 2005
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Reprint courtesy of Australian Motorcycle Trader Magazine |
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