Ducati Sporting Club UK
 
 
Thread Tools Search this Thread Display Modes
  #1  
Old 01-Sep-2003, 02:51
stephens stephens is offline
Registered Forum User
Montjuic
 
Posts: 77
Join Date: Jul 2003
9XX Spring Rate Help

I have swapped bikes recently and was running a 9kg spring in the rear. Although it handles well on the track, only 2/3 of the travel is being used and the bike is unridable on any sort of bumpy road. The bike sees about 90% track usage and I weigh 80kg (175lbs).
The rear shock is an Ohlins with hydraulic preload adjustment, so I would like to find a compromise spring rate that can be used on both road and track using preload to compensate, rather than having to swap springs all the time.
Quote+Reply
  #2  
Old 01-Sep-2003, 06:18
Shazaam!'s Avatar
DSC Member Shazaam! Shazaam! is offline
DSC Club Member
Big Twin
 
Posts: 1,167
Join Date: Nov 2001
WTF: You seem to have found the proper suspension settings to make the bike handle well at the track. Given that the bike spends 90% of its time there, any spring rate changes you make to help the bike handle better on the street will compromise its track performance. You can't set up your bike for the best track performance and still expect it to provide even a reasonable level of rider comfort for the street. A typical road surface is simply too rough for the stiff suspension settings that are warranted for the track.


Without knowing what specific behavior makes you consider the bike to be "unridable" on the street I doubt that it can be cured by a softer rear spring rate. Suspension travel is affected by damping as well as spring stiffness and preload. My suggestion would be to first record your track damper settings and then return these settings to the factory specs. Leave the other track geometry settings the same. Use the factory recommended tyre pressures. It's hard to solve suspension problems if you're running incorrect tyre pressures.

Any set of suspension settings is a compromise and the stock setup is no exception. The stock settings were developed by the manufacturer's test riders to give the best handling characteristics over the widest possible range of riding conditions. Consequently, they are not the best settings for a given road/track, or a personal riding technique. The stock settings will not provide the most comfortable ride nor will they result in the fastest track times. They will however, give you the best overall handling. Period.

So the factory recommended settings are simply the best starting point. From there you can use trial and error to fine-tune the damping to your favorite roads.

Leave the hydraulic preload setting alone initially. Ideally, you'll want to set the spring preload so that it doesn’t allow the suspension to bottom on the bump stops and it doesn’t fully extend (top out) either.

The steering head angle should only be altered after all the other suspension changes have been finalized. You need to first be comfortable with the overall suspension settings. If the bike displays any instability problems they need to be sorted out first, because the steeper steering head angle change will magnify these problems, and also because part of its effect mimics changing the rear ride height.

Experiment. One change at a time. Starting from the stock settings, try a different single setting until you develop a feel for the differences resulting from each change and the sensitivity of the bike to each click on the adjuster. Use the same road to evaluate your changes. This seat-of-the-pants testing is highly subjective so don't be surprised if your feelings about the changes are different the next time you ride. Even without any interim changes, there are days that you know that you're riding well and the suspension is just right, and days that you're not in the groove. Given that you're not an experienced test rider, you'll find that YOU are the biggest variable.

From a comfort point-of-view, the suspension is meant to absorb bumps. From a handling viewpoint it's meant to keep the tyres in contact with the road surface and maintain, as far as possible, the overall chassis geometry of the bike and performance of the tyres while it's doing those things.

Having the right suspension setup means the tyres will spend maximum time hooked up. The bike won't nose-down extravagantly under hard braking, tending to lift its rear wheel. It won't waste time wallowing and plunging as its rider tries to flick it over. Curing handling problems takes observation, some skills and a willingness to spend time testing and thinking what the symptoms mean.

Here's how Kevin Cameron puts it ...

"Motorcycle suspension spring rates are a compromise, soft enough to give useful bump isolation and good grip, but not so soft as to permit wild attitude changes that limit performance. Touring and sports-touring machines are not ridden to 100 per cent weight transfer (wheelies and stoppies) and can therefore be biased in the direction of comfort, with longer travel and softer springing and damping. In racing, this compromise is re-drawn at every circuit. If the circuit is smooth, less bump isolation is needed, and benefits can be had from a firmer setup, permitting less attitude change and faster response. If the circuit is bumpy, the suspension will have to be softened and bad side-effects of this accepted up to a point.

Remember that spring stiffness and damping force must exist in proportion. Too much damping overcomes the spring, slowing suspension response, making the ride too firm and reducing the wheels' bump-tracking ability and grip. Too little damping is overcome by the spring, and the vehicle oscillates after every disturbance. Damping has an undesirable side-effect now any bump gives the vehicle two upward shoves - one from the spring, the other from resistance in the damper. This extra push is unfortunate because it somewhat increases the bump's disturbance to the vehicle." (Ref. 1)

(1) Cameron, Kevin, Sportbike Performance Handbook 1998, Chapter 12,
Quote+Reply
  #3  
Old 01-Sep-2003, 07:12
stephens stephens is offline
Registered Forum User
Montjuic
 
Posts: 77
Join Date: Jul 2003
Thanks for the feedback. I have setup the previous bike after a fair bit of track time. After swapping bikes, I put in 1000 street km's with the setup from the old bike including the old race shock. This is too hard for street use because most racers tend to use 9-10kg rear springs to control squat. This is not condusive to any sort of suspension compliance on the street to the extent that the ride was not just uncomfortable to the point of having to ride the bike like a jockey over ant sort of bump and through turns with any bumps, the rear would skip and bounce. Now even though the bike is used mostly on the track, I like to be able to ride occaisionally on the street.
Changing fairings etc is annoying enough as it is (but much easier on the 999 than the GSXR!), without having to go through spring changes as well. The bottom line is I have a track setup that I am very comfortable with, but it is not useable on the street, so I want to find a compromise.
Quote+Reply
  
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes
Postbit Selector
Switch to Vertical postbit Use Vertical Postbit

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Recent Posts - Contact Us - DSC Home - Archive - Top
Powered by vBulletin 3.5.4 - Copyright © 2000 - 2024, Jelsoft Enterprises Ltd. - © Ducati Sporting Club UK - All times are GMT +1. The time now is 08:28.