Calling all suspension experts
 

After two years of trying to get the rear suspension set up on my 996sps I am about to give up. Is there anyone who knows exactly what they are talking about when it comes to suspension who can help me?

My problem is this:

I weigh about 8 1/2 stone or 62kg approx. I have already changed the spring rate to one level below standard to 15/62. This on the ohlins chart would relate to a rider weight of 65-70 kgs, my presumed weight with gear. However the rear ride is still hard with the dreaded 'kidney shakes'. The ohlins unit on the bike isn't the original with a code of DU8440 which doesn't match anything I've read so far. I am considering going down a further spring rate because with the statis sag set up correctly I get hardly any change when i get on or off the bike ( free sag = 90% rider sag). However I have been told before that dukes need a strong spring to resist the torque action from the motor. I use the bike for road and track use.

One feeling I get while riding is that the rear end often feels like it is twisting, could this mean more or less rebound?

Do i have the correct shock with correct damping rates? and do I have the correct spring for my weight?

thanks
Bryan

Hi Bryan,

I dont much about ur supension problems but 62kg is about is 9.7 stone nearly 10 stone actually.

Just so who ever answers knows ur weight as theres about 1.5 stone difference there:o:o

[Edited on 4-6-2005 by Lee1980]

Phone the experts
http://www.k-tech.uk.com/contact.htm

Really helpful guys/girls there ,sorted me out :D

Yeh, dont spend anymore money on it if you dont know what your doing. Get K-tech or Maxton to set it up for you.

Agree with the above comments. Call K-Tech or Kais Ohlins.
One thing with changing the spring in isolation is that the shock body is still valved for the heavier spring. You're probaby suffering over damping now on both compression and rebound.

Where abouts are you, I can recomend Steve Jordans in Bookam + he is an Ohlins dealer :D

thanks for all your comments, nelly I agree with what you say about the shock being valved for the heavier spring, I am currently running with no compression damping and after a quick ride yeaterday, very little rebound also.

I think i will give steve jordan a visit as he is quite local, if you recommend him gaz.


thanks

Bryan,

Steve at stevejordanmotorcycles.co.uk is excellent, he rebuilt and resprung my Ohlins rear, also a lot of the BEMSEE paddock usehim for suspension set up.

Another option if you are a little further east is Perry at www.hmracing.co.uk, also very very good.

Hope that helps.

I don’t know the length and stiffness of a DU8440 but your front and rear springs are likely still too stiff for your bodyweight. Also, you shouldn’t use increased damping to compensate for an incorrect spring rate. If your damper has incorrect damping rates for your bike it will add to the handling problem and ride harshness. You first need to set sag correctly, front and rear.

Ducati’s superbikes are designed to use a 160mm long rear spring that will allow 75 to 85mm of deflection. If you’re too heavy for the spring, the spring will sag too much under your weight and use up the suspension travel needed to absorb bumps. For severe bumps the suspension will bottom-out (hit the bump stops) if the spring stiffness isn’t in the proper range for your weight.

Conversely, if the spring is too stiff for your weight, the spring won’t be compressed enough and the suspension will (too often) fully extend and top-out.

The rule-of-thumb is that you want the spring to be about 25% compressed when you are just sitting on the bike at a standstill. Ducati recommends 40mm of rear sag for street riding, 30mm for the track.

In the table below, the sag under just the bike weight without rider is called static sag, with rider it’s called rider sag.



I’m assuming here that when you use the term “free” sag you mean static sag. Using your rule of thumb (free sag = 90% rider sag) is incorrect. For use on the street, the free/static sag should be (30/50 to 30/45) 60% to 66% in the front, and (10/40) 25% in the rear.

Using a percentage is incorrect, you need to achieve BOTH static and rider sag measurements. If you can’t, you need to change your spring.

Here’s an example:

The factory OEM rear spring on a 996 has a spring rate of 64N/mm (365 lbs/in, Ohlins’ stiffness code -16) which is adjustable for rider-plus-gear weighing between 155 and 165 pounds.

So, the first thing you need to do is to adjust your spring preload to achieve the proper rider sag.

If you fall within the 155 to 165 pound weight range then when you adjust your preload to get the correct rider sag, when you check your static sag it’ll also be correct.

However, if your weight is more than 165 pounds, you’ll need to increase the rear spring preload to get the correct rider sag, so when you check static sag you won’t have enough because you have preloaded the spring too tight. So you need to replace the stock unit with a stiffer spring.

Conversely, if your weight is less than 155 pounds, you’ll need to decrease the rear spring preload to get the correct rider sag, so when you check static sag you’ll have too much (i.e. more than 10mm) because you have removed too much preload from the spring. So you need to replace the stock unit with a less-stiff spring

The same logic applies to the front springs. Front springs just have more travel to absorb bumps better.



Ohlins Spring Code

Example: Ohlin's spring marked 1091-29/90 090

1091-29 is the Ohlins code for a 160mm (6.3 in) long 57mm I.D. spring having a spring rate of 90N/mm (514 lbs/in)

/90 = 90N/mm stiffness
090 = production batch number

This would be the correct spring for a Ducati pre-999 superbike monoposto rider weighing between 245 and 265 pounds (including gear), or a 155 to 165 pound biposto rider plus passenger (with the spring preload increased to obtain 30mm sag)

The first number refers to the length:

1093 - 150mm
1091 - 160mm
1092 - 170mm
1095 - 180mm
1096 - 190mm

The next number is a code for the spring rate, and the next is the spring rate in N/mm. So, 1093-26/85 is a 150 mm spring with an 85N/mm spring rate.

The following table lists the Ducati standard Öhlins rear springs:



Notes:

• 160mm (6.3 in) 57mm I.D. springs have the Ohlins’ length code 1091

• 64N/mm (365 lbs/in) springs have the Ohlins’ stiffness code -16

• 66N/mm (377 lbs/in) springs have the Ohlins’ stiffness code -17

• 68N/mm (388 lbs/in) springs have the Ohlins’ stiffness code -18

• 70N/mm (400 lbs/in) springs have the Ohlins’ stiffness code -19

• 75N/mm (428 lbs/in) springs have the Ohlins’ stiffness code -21

• 80N/mm (457 lbs/in) springs have the Ohlins’ stiffness code -24

• 85N/mm (485 lbs/in) springs have the Ohlins’ stiffness code -26

• 90N/mm (514 lbs/in) springs have the Ohlins’ stiffness code -29

• 95N/mm (542 lbs/in) springs have the Ohlins’ stiffness code -31

• 100N/mm (571 lbs/in) springs have the Ohlins’ stiffness code -34

• 105N/mm (600 lbs/in) springs have the Ohlins’ stiffness code -36

• 110N/mm (628 lbs/in) springs have the Ohlins’ stiffness code -39

As far as I know, Ohlins doesn’t make a stiffer spring than 1091-39








[Edited on 6-5-2005 by Shazaam!]

Guys

thanks for all your input, I called Steve Jordan today and have spoken to him about getting the bike looked over and then possibly booked in.

Shazaam, the spring I am using at the moment is an ohlins 62N/mm, code 15. With the rider sag correct the free sag isn't much differance which made me believe that the spring was still to hard, however I am getting full deflection with the shock just touching the bump stop on large deflections, and K-tech told me that they would be very surprised if I needed anything softer. Any thoughts? Do Ohlins go much lower than code 15?

They were also able to set me straight on the internals of my shock and confirm that the shim stacking was no differant than on the original unit for my bike. I have managed to get a better road ride by removing all compression damping and backing off the rebound by 30 clicks. I will try and get a good track setting next week at silverstone.

Finally I did manage to weigh myself accuratly at 59kg, the MOT tester at work is definatly out.

thanks again for all your help.

Lighter Springs:

• 58N/mm (331 lbs/in) springs have the Ohlins’ stiffness code -13, rider weight 55-60Kg

• 1091 length spring is not available with Ohlins’ stiffness code -14, rider weight 60-65Kg

• 62N/mm (354 lbs/in) springs have the Ohlins’ stiffness code -15, rider weight 65-70Kg

• 64N/mm (365 lbs/in) springs have the Ohlins’ stiffness code -16, rider weight 70-75Kg


So, Ohlins doesn't make a -14 spring in the 1091mm length. They do make a -13, 58N/mm spring for a rider weight of 55-60 Kg which is too light for you assuming you're wearing 4Kg of gear.

You haven't mentioned the front forks. Have you re-sprung these also, and are you getting full travel? Also remember to set your sag measurements with a half tank of petrol.

Shazaam, I have ohlins on the front as well and have never had any problems other than it dives on the brakes a little and will therefore then to sit up if running into a corner. They still have the original springs in.

static sag on the front without rider is 26mm, I will check later the value with myself on it when I get some help to measure.

Looks like I have the best option for rear spring as I am just about getting full deflection on the shock. Again the front just about gets full travel to.

thanks again.

The front static sag should be 30mm, so the front needs softer springs as well.

A mismatch in spring rates front to rear is likely the main cause of your handling difficulties. Every time you hit a bump the bike's fore-aft geometry changes (i.e. CG, steering head inclination, trail, pitch.)

Once you've got the proper springs, then if it bottoms too easily in long dips increase your compression damping 2-4 clicks until it begins to feel harsh over changes in road paving.

Shazaam, what is the source of that table showing recommended rider and free sag? Not doubting it or you (hey, you're the resident expert and I'm a newb), but that is a higher front rider sag number than I have normally seen when working with other bikes. I have always been told by people in the know to set front rider sag between 30-40mm. That is based on the calculation of using roughly 1/4 to 1/3 of the available travel. Most Japanese sportbikes run between 110 and 125mm of front travel. Therefore 30-40mm of rider sag. What is the available travel on Ducati forks? Well, I realize that you'll have several variations for Showas, Ohlins, etc. for the various years, but do they have more than 125mm of travel as a rule? If not, what is the reasoning for having 45-50mm of rider sag?

add-on: Found the available travel in my handy dandy owner's manual. 127mm of available travel for my 2001 996 bp with the Showas. So, using my experience, I would expect front rider sag of roughly 40mm rather than the listed 45-50. Thoughts?

The table was taken from an Ohlins Spring Manual.

You’re correct though, the accepted range of values is typically 1/4 to 1/3 of total travel so 32mm to 42mm would be correct for your Showas for the street.

If you preload to get 32mm of sag then the rider’s neutral position is closer to the fully-extended position so you increase the tendency to hit the top bump-stop but the suspension moves less, resists the larger forces generated by the higher speeds, and giving better handling. That’s why this is a good track setting.

With less preload you’ll get 42mm, which softens the ride considerably for rider comfort but increases the tendency to bottom-out on severe bumps. If you want a plush ride without bottoming-out then you shouldn’t buy a sportbike.

In short, when you set sag, you are determining how much suspension travel will be available for bump absorption. The maximum travel is decided by the bike designer, i.e. a lot is needed for off-road bikes, very little for track bikes because of the smoother surfaces they encounter.

So a 996’s stock suspension would bottom-out regularly off-road, but allow too much squat and dive on a typical track - which is why recommended sag values for the track are lower than street values.

That means that a sag value for a given suspension travel can be selected to maximize comfort at the sacrifice of larger suspension movement that limits handling performance. Bryan’s original concern arose from a question of comfort to avoid “kidney shake” so it would be better to err with a higher sag value than a lower one.

Great info! I'm sort of the go-to guy in my group when it comes to suspension, so I wanted to be sure my assumptions were correct. Interesting that Ohlins (who one would assume would make outstanding recommendations) would list rider sag on their chart a bit higher. Wonder what their reasoning is??

To contribute to the original question more, Shazaam has supplied fantastic info above. And there is a lot of it! So, Bryan if you're still seeking a remedy yourself, keep the simple ideas in mind first. Sag is the most important because everything else is affected by spring preload and chassis attitude. While many of the numbers listed above are very specific, you don't have to hit those exact numbers. They are a target and getting in the right ballpark will be good enough for most of us mortals. From there, you can fine tune for specific uses, styles, and preferences. But don't sweat it if you are one spring rate away from what the Ohlins chart says is optimum. Take some good sag measurements, and try to keep a balance between front and rear (if you're a little stiff out back, don't let the front be a little soft). But, like many have recommended, at your weight, the only real solution may be a professional rebuild. Too many things start needing to change (when you mess with one thing, another gets thrown out of whack) and the only real way to fix it all is to have someone who really knows what they are doing attack the entire setup.

How much travel should be left
 

After the suspension has been setup front and rear (sag set etc)
How much travel should there be left.

After fitting a cable-tie round the front fork and rear shock rod i had 13mm of unused travel on the front and the rear was showing that all the travel had been used up.

how much should i be using.

:puzzled: