I was doing a bit of stuff with me ST4s the other day and took off the vertical cylinder belt covers. I noticed that the belt seemed a fair bit looser than it should be. Belts were changed last year and are not due to be replaced till next year. Am I correct in saying that to tension an already installed belt, I don't need to line up markers etc as when replacing belts? The belts looked in perfect condition with no fraying or anything. A
The important thing to remember whenever tesioning belts, or checking them is that the relevant cylinder is at TDC and the valves closed. If not, the springs on the closing rocker are pulling on the valve train. That's why the inlet on the rear flicks round when you take the belts off and needs to be held in place when you refit them. Check horizontal belt with timing marks lined up, then turn the crank 3/4 turn anti clockwise to check the rear belt. If you fit new belts and set tension of both belts at timing marks, the rear belt will be too loose once the motor turns over.
Despite having owned my ST4s for over four years now, I have not had the bravery, time or motivation to change the belts myself. However, doing some recent maintenance I had a look at the belts, which were changed by a pro last year, and saw that they seemed to be a bit loose. This is not unusual as it has done about 8,000 miles since the change. Coupled with the fact that she had been just running a bit rough recently, I thought I’ll tackle this myself. The pro who did the belts last time said that it is good to check the tension on them at yearly intervals, especially as I do about 7,000+ miles a year on it. I found a good guide to belt changing and tensioning on the 4s here, which has good pics and detail. I did some further research and found that there has been a recent update from Ducati for all belt equipped bikes on how to correctly set the tension. More detail and certain methodologies here. The tension on all belts when fitted new, whether Desmodue (two valve) or Desmoquatro (four valve) should be that the belt vibrates at 110Hz. For used belts or re-tensioning, this should be 90-100Hz. For the ST4s that means that on the horizontal cylinder, the belt should vibrate at 110Hz (or 90-100Hz as appropriate) when plucked like a guitar string between the engine (drive) pulley and the tensioner pulley; and on the vertical cylinder when plucked between the engine pulley and the idler pulley. Now, various methods have emerged for how to measure this but I discovered that a free guitar tuning app for Android smart phones called gStrings, works a treat. In auto tune mode, a repeated plucking of the belt gives a very good average tension measure that is readable above any background noise and so works very nicely with good accuracy. Doing this, gave me the confidence to see how the timing marks work, establishing TDC for each cylinder and generally understanding the belts to take away the fear! I am now confident that I’ll be able to change the belts myself next year when the time comes and be confident that it is done right. I also cleaned and gapped my iridium plugs correctly and gave my K&N air filter a good clean and the overall effect is marked. She starts easily, settles to tick over quickly and just feels a lot smoother than it has done in recent weeks. All in all, I am very happy with the outcome. A
I found a slightly different tuning method having a guitar around... the A string is 110 Hz and that made it pretty easy to tune the belts. As you say it's pretty important to remember to turn the engine 270 degrees before tensioning the rear cylinder to remove the tension on the belt. br Jim
New belts are tensioned to 110 Hz. Old belts can be re tensioned but not to 110 Hz I cant quite remember the Hz of older belts but I think its 95 Hz. New belts wear off the sharp edges quickly and settle at about 90 to 95 Hz. Old belts dont have any sharp edges to wear off so will stay , near , 110 Hz . Too tight for long term. Not suggesting that anyone has re tensioned to 110 Hz just a reminder in case anyone thinks about re tensioning.
I did my 900 SS belts last year and checked them again a couple of months ago and they were slack like an old ruber band :-O Jim
I bought dayco belts of Nog last year, tensioned them using the 5mm / 6mm allen key method. Checked them last month and they were still spot on.
