Rear ride hight adjustment warning

Martin C · 08-05-2021, 12:40 PM

Whilst online researching ideas for making my ST4 easier to lift onto its centre-stand, I came across a lot of related information on Ducati.ms about rear ride height adjustment, which I will be trying out as a possible solution (if mine is in fact too low).

BUT, in my reading I learned that if a bike's rear ride height is raised above standard (by any method, not just hoop extenders), it also has the unwanted effect of reducing the drive-chain slack when ridden, even though it is "correctly" set when measured on the stand (it took me a while to understand the geometry of why this is so, but it is correct). I daresay many of you already knew this - but I certainly did not!

If the chain is too tight when riding this can of course increase chain & sprockets wear and risk overloading gearbox output shaft and/or rear wheel bearings. My 1995 M900 has for many years been fitted with rear-hoop extenders (hop-ups), and it has some history of such issues, so I rushed out to the shed to check.

Sure enough, although the chain slack is correctly tensioned when measured on the side-stand, with me sitting on the bike there is NO slack at all. This is the expected rear suspension position when a chain is at its tightest = with both the swinging arm pivot and rear axle in a straight line from the gearbox sprocket. When the rear suspension is either compressed or extended above/below this line, the chain inevitably has more slack., which is when it is normally measured and set as specified.

So, I'll be adjusting the Monster's chain to have more slack when measured on the side-stand so that it's not too tight when sat upon/ridden, and if I adjust the ST4's rear ride height I'll also increase the chain slack on that to compensate.

Mr Gazza · 08-05-2021, 01:07 PM

If you alter the ride height by whatever means, you also have to check and adjust the sag.
Like you with the chain slack thing, I had a 'mare working out why that was the case and even made a working model of the hoop rear suspension with all the relevant pivot points accurately plotted and ride height adjustment incorporated.
I never did really establish why the ride height also alters the sag, but from experience, in practise it does.

The suspension should have a working range, the chain slack should be correct if it is set at the full extension of that range, ie, on the stand.
If the sag is excessive it will settle to a position nearer the compressed end of the range and also limit the useful travel available.

That said the geometry of the raised ride height will eat into the chain slack when compressed, so more "static" slack would be needed, but I don't think that it should go taught with just your weight at standstill?

Incidentally the Monsters chain is checked with the bike on its wheels, which removes the static sag from the equation. (I do mine on a paddock stand, which is equivalent to on it's wheels.) . On your centre stand the suspension will be at full extension.

I might not be right but it's food for thought?

chris.p · 08-05-2021, 06:09 PM

I have always set my chain tension with the bike on a rear paddock stand as ^^^^, but I check the tension by laying across the seat so my full weight is over the bike and check the chain moves approx 25mm up or down, done this on all my road and race bikes. A slack chain is always better than a tight chain.

Dukedesmo · 09-05-2021, 09:25 AM

Quote:

Originally Posted by chris.p

A slack chain is always better than a tight chain.

Agreed, if the chain is good and not worn then no harm in having it on the loose side.

08-05-2021, 12:40 PM	#1
Martin C Bronze Member Join Date: Jun 2006 Location: Burton on Trent Bike: M900 Posts: 248	Rear ride hight adjustment warning Whilst online researching ideas for making my ST4 easier to lift onto its centre-stand, I came across a lot of related information on Ducati.ms about rear ride height adjustment, which I will be trying out as a possible solution (if mine is in fact too low). BUT, in my reading I learned that if a bike's rear ride height is raised above standard (by any method, not just hoop extenders), it also has the unwanted effect of reducing the drive-chain slack when ridden, even though it is "correctly" set when measured on the stand (it took me a while to understand the geometry of why this is so, but it is correct). I daresay many of you already knew this - but I certainly did not! If the chain is too tight when riding this can of course increase chain & sprockets wear and risk overloading gearbox output shaft and/or rear wheel bearings. My 1995 M900 has for many years been fitted with rear-hoop extenders (hop-ups), and it has some history of such issues, so I rushed out to the shed to check. Sure enough, although the chain slack is correctly tensioned when measured on the side-stand, with me sitting on the bike there is NO slack at all. This is the expected rear suspension position when a chain is at its tightest = with both the swinging arm pivot and rear axle in a straight line from the gearbox sprocket. When the rear suspension is either compressed or extended above/below this line, the chain inevitably has more slack., which is when it is normally measured and set as specified. So, I'll be adjusting the Monster's chain to have more slack when measured on the side-stand so that it's not too tight when sat upon/ridden, and if I adjust the ST4's rear ride height I'll also increase the chain slack on that to compensate. __________________ Owning a Monsta is not the beginning of the end, but it may well be the end of the beginning....

08-05-2021, 01:07 PM	#2
Mr Gazza Lord of the Rings Join Date: May 2013 Location: Norwich Bike: M900sie Posts: 5,848	If you alter the ride height by whatever means, you also have to check and adjust the sag. Like you with the chain slack thing, I had a 'mare working out why that was the case and even made a working model of the hoop rear suspension with all the relevant pivot points accurately plotted and ride height adjustment incorporated. I never did really establish why the ride height also alters the sag, but from experience, in practise it does. The suspension should have a working range, the chain slack should be correct if it is set at the full extension of that range, ie, on the stand. If the sag is excessive it will settle to a position nearer the compressed end of the range and also limit the useful travel available. That said the geometry of the raised ride height will eat into the chain slack when compressed, so more "static" slack would be needed, but I don't think that it should go taught with just your weight at standstill? Incidentally the Monsters chain is checked with the bike on its wheels, which removes the static sag from the equation. (I do mine on a paddock stand, which is equivalent to on it's wheels.) . On your centre stand the suspension will be at full extension. I might not be right but it's food for thought? __________________

08-05-2021, 06:09 PM	#3
chris.p Gold Member Join Date: Mar 2004 Location: lincoln Bike: No Bike Yet! Posts: 876	I have always set my chain tension with the bike on a rear paddock stand as ^^^^, but I check the tension by laying across the seat so my full weight is over the bike and check the chain moves approx 25mm up or down, done this on all my road and race bikes. A slack chain is always better than a tight chain.