Ducati Sporting Club UK > DesmoDue Race Series > DesmoDue - General Questions and Chat > FAO Tonio600, Fork spacer

[skidlids]

And for anybody else wanting to do something about the springs in their Class B bikes front forks

Sping is a Ohlins single rate spring as supplied for a Honda VTR1000 Firestorm

Spacer is 18mm long with a OD of 35mm and a ID of 27mm (4mm wall thickness)

[Tonio600]

Thanks Kev, that's brilliant

[Tonio600]

Kev, did you get rid of the black plastic spacer? Shouldn't it at the top of the springs?

[skidlids]

Yep did away with the black plastic spacer, as the Ohlins springs are longer and therefore a 18mm spacer as per the pic adds the right amount of preload

[Tonio600]

Thanks Kev.

Now I don't understand what's the importance of the length of the spring... What does a longer/shorter spring means?

On one hand there is Kev who has found a solution using (much) longer springs and much shorter spacers than standard, and on the other hand there are those Ohlins springs sold by a mate which almost match the size of the standard springs but they are 20mm shorter. So I will need to add more spacer (about 20mm I'd say....).

What's the difference between those 2 solutions? Which one will be easier to ride for me? It's either more spring less spacer or more spacer less spring...

Thanks guys.

Tonio.

[Paul James]

Springs can be pretty complex things Tonio, there are quite a few aspects to consider. You have spring rate, number of coils per unit of linear measurement, preload, whether the spring is a constant or variable rate etc. The wire diameter will have an influence on all of these things of course. The spacers will give some effect on the preload. When you consider how little actual preload travel you have on adjustable forks relative to the free length of the spring you can see how critical it can be.

I'm not a suspension expert, in fact I'm looking to take my 996R to someone who is very soon as the front end is just not right as it is and I'm not too sure how to set up the combination of spring setting and damping. That said I have done a lot of engineering design work on springs in the past.

Where the spring coil pitch is constant you have constant rate; so if your spring has a rate of lets say 5N per cm you will compress the spring 1 cm for every 5N of force applied, if variable rate the "softer" part of the spring will come into play first then the harder section giving you a variable suspension rate hardening up during compression. If you had a spring of this rate and by your selection of preload spacer had say 20 cm of preload you would in effect have 100N "pushing" up the forks, to then compress the forks by 20cm you would have to apply another 100N of force. (Just simple figures to illustrate the theory, not likely to be relative to your fork springs)

If you want to really stiffen up the front forks get a higher rated spring not just a longer one, it might actually be shorter and require a longer spacer. Theoretically you could end up limiting the fork travel if you used a spring of small pitch if you put on too much preload. You can simply work out the available spring travel by multiplying the spring wire diameter by the number of coils, take the product of that calculation from the free length and you have the maximum travel available, take off any fixed preload length you apply to give you the actual travel available before the spring becomes coil bound.

If you know the spring rate and the length of fork travel you can calculate the force required to compress the spring to any given length. If you know the data for a range of springs you can calculate the effect each would have.

If you want to find out the rate for your spring get a known weight, measure the free length, apply the weight, measure the compressed length, take the compressed length from the free length and relate it to the weight you have applied and you have it !!! This is usually expressed in Newton/Metres.

You could do that with the bike all assembled I guess. Put it on the rear paddock stand, stick a set of bathroom scales under the front wheel and note the weight displayed. Apply a weight to the top of the forks, doesn't matter what you use as the scales will tell you the difference. Measure the fork deflection from unloaded to loaded to give the travel. Divide the weight by 2 (cos both fork springs are having equal effect here) and you can work out the spring rate of the ones you've got in there now without even taking the bike to bits. I guess this is slightly flawed as to be precise you would need to raise the back of the bike to overcome the rake angle of the steering head and ensure that the forks were acting in a truly vertical plane. A certain a,mount of riction would come into play as well but oif you "bounced" the front slightly once compressed it should overcome that.

Then you come to the mysteries of damping which is influenced by oil viscosity, oil volume and the rate at which it can pass through the damping valves.

Well that's the theory, how the hell you apply it all in practice depends largely on how you like the bike set up I guess.

Probably best to go back to the basics on the spring bit, decide what you are actually trying to achieve then see what you have now and in which direction you want to take it to make the desired changes.