Robert Taylor
15th May 2011, 18:16
In another thread I drew attention to the need for ''rider feel and confidence'' and Ryan Withers ( Cowpoos ) picked up on that very real issue that suspension tuners battle with worldwide, irrespective of what box of toys they have got to play with.
I guess there are several ways of describing ''rider feel'' but I understand it as the rider having great ''connection'' or contact feel to the track through the suspension and tyres. Its important to have great feel at both ends but especially so in the front end as if youve got no confidence in the front end you cannot go as fast as youd like. That especially applies as you near the limits of adhesion, the chassis, suspension and tyre package should give you ample warning of that.
Factors that will affect feel ( and there will be a few more ) include;
Chassis rake and trail, this one is a biggie and bikes like the BMWS1000RR suffer with front end geometry that is a little off. Everytime you alter fork position in the clamps, rear ride height and even rear axle position you are manipulating those figures
Rear axle position, this is the biggest single thing that alters forwards / rearwards weight distribution as it is sort of like moving the motor backwards and forwards. Often it can have a major influence on front end feel
Footpeg and handlebar position, altering rider position and therefore overall static weight distribution
Chassis height, if you move the fork position in the clamps and rear ride height you are moving that big lump of metal called the motor upwards or downwards. That manipulates forwards and backwards pitch under braking / acceleration and affects feel
Tyre choice and compound. In the most simplistic terms the stickiness of the tyre and the frequencies it imparts into the suspension and chassis can have quite a marked affect. Tyres like Pirellis have soft sidewalls and ( for example ) their front tyres need a reasonable amount of pressure and a linear fork action to make them work properly. If youve got great settings for Pirellis / Metzelers expect to have to markedly change your internal suspension valving, spring rates and preload if you change to ( for example ) Dunlop N-Tecs. If you go tyre testing with a different brand and therefore construction of tyre its not a foolproof test to back to back unless you are able to make those suspension changes to compliment the tyres!
Tyre pressure and therefore its affect on surface area of contact patch.
Controlled chassis flex when the bike is on its side at more and more extreme bank angles. A hot topic in MotoGP at present and arguably a major issue with the Ducati MotoGP bike.
SUSPENSION! The above should illustrate that the suspension units are a part of the equation, albeit quite a major part. I get the feeling that a lot of people may think that the suspension needs to be ''firm'' to impart maximum feel. That is something that can often be far from the truth and the issue is in fact quite complex. Certainly the suspension needs to be firm enough to give great chassis control and so that the bike doesnt wobble like a jelly or pitch backwards and forwards like a rocking horse! The front end needs to have great brake dive resistance for racetrack situations but then you must also have enough initial forward pitch to help initiate turn in. In the rear end there must be enough squat control so that the bike doesnt understeer off the turn when under power. But it also must have enough pitch that the weight transfer assists in placing load on the rear tyre to minimise wheelspin. This is especially true of Superbikes, such bikes we often run the rider sag at bigger numbers than 600s because they need more weight transfer to load the rear tyre.
Now heres the biggie when talking about the forks. Suspension is always reverberating at various frequencies and amplitudes between compression and rebound strokes. Anyone who has ever viewed a datalogging trace with a minimum sampling rate of 1000 hertz will understand this, suspension never ''stands still'' Apart from high priced gas charged cartridges most of the forks we as mere mortals run are non pressurised and pretty much are ''open'' units. This of course is not completely true as under fork compression the air cavity above the set oil level rises in pressure. But that is a pressure that is position sensitive and there is never consistent and enough pressure to prevent cavitation.
As a fork reverses direction ( constantly ) damping fluid must flow back and forth between one side of the pistons and the other. Picture that even under a brake dive scenario on an even slightly bumpy track it will not be one big compression movement, there will be slight reverberations back and forth. That oil has got to find its way very quicly from one side to the other otherwise the amount of ''froth'' or ''milkshake'' the pistons are running in becomes extreme. That really hurts damping response and in turn ''rider contact feel''. There is ( simplistically ) not enough damping response to assist the spring in keeping the tyre as loaded as possible. When you are getting to the limits of adhesion it becomes of huge importance that the suspension units must as quickly as possible ''pressure balance'' between both sides of the pistons.
If the rider is right on the edge of adhesion and has a slight tuck / lose its important that the damping resonse is as instantaneous as possible. We have noticed that the ability to constantly save such ''tuck'' situations is a very positive feature of fork cartridges that are totally seperated in fuction, i.e compression function totally in one leg and rebound function in the other. In not having to share such duties the damping response on stroke reversal is for ''refill'' to the other side of the chamber, not also to porvide damping as part of that same refill function.
4 or so years ago I was contracted to work for Ohlins at World Superbike pre season tests and found myself revalving fork cartridges and shocks for Fujiwara and Broc Parkes. One of the interesting things that I bought back from those tests was a valving techique for forks where you run the compression clickers a long way open with an otherwise more aggressive main valving stack. While on the face of it there is less initial damping because the bleed bypass passage is a whole lot free'r flowing there is in fact much much less damping response lag because the system is able to equalise with a significant amount less cavitation. The stronger valving stack compensates and helps to shape the overall damping force curve required. Armed with that info I returned to NZ and we applied the technique into Jay Lawrences 07 ZX6 that he was campaigning that year. That model wasnt the sharpest tool in the shed and with all respect to Jay there were 2 or 3 riders personally faster than him. But with that setting he was able to stick the bike on pole at Timaru ( unless an alzheimers moment is fooling me ) and he was very very happy with the feel and feedback of those forks. This setting worked very well at all circuits except Pukekohe where the bumps required a softer stack, and in turn less bypass bleed. We also applied exactly that same technique to Andrew Strouds forks and he too reported a huge improvement in feel, especially approaching the limits of traction.
Latterly this same technique was used in the Superstock forks of Midge Smarts and Jaden Hassans R6s, but a little more involved here as there is a long and restrictive lead in passage we rework. This kind of work often but not always results in faster lap times BUT can go a long way towards reducing the chances of crashing as there is a huge amount more feedback to the handlebars as the limits of adhesion are reached.
Interestingly this same line of thinking has now been applied in the second generation of Ohlins NIX30 cartridges and part of the explanation from Ohlins for these new pistons is ''improved rider feel''. In NZ these have thus far been used once by Craig Shirriffs to take his maiden Superbike win at Manfield (As has been discovered only last Friday a coolant leak caused his crash in race two ) While thats only one sample it was a positive result and Craig certainly reported great feel.
Which leads to another issue that has been aired elsewhere, aftermarket handlebar clamps that are not toleranced so well can under delective loads bind up fork bushing clearances and totally destroy rider feel. BPF forks with sliding top bushings are very suspectible to such an issue, as at the lower reaches of stroke compression the bushing is getting slightly squeezed by a pinched fork.
Lets take anothre common example. Pre year 11 GSXR600 / 750s have particularly weak high speed rebound damping that causes the bike to ''stand up'' mid turn after youve released the brakes after using up most of the fork travel. The absolutely correct way of solving that issue is to have the rebound pistons internally revalved. The absolutely INCORRECt way of ''solving'' the issue is to close in the rebound adjusters a long way. It may ''kind of fix'' the issue but it creates a few more besides;
The forks then have too much low speed rebound damping so at the last stages of fork re-extension the fork action is lazy / lethagric. That can really hurt edge grip as you are accelerating off turns
As the rebound adjuster simply moves the position of a tapered needle in an orifice and that orifice is in the end of the rebound shaft it ''cross talks'' I.e it not only alters the flow in rebound direction but also in compression direction. So if your compression damping was feeling about right before your rebound adjustment it will now feel different after making that adjustment
The needle now closes that orifice off so much that the cartridge cannot pressure balance so quicly on stroke reversal movements, that really hurts damping response. Generally the forks now feel ''wooden'', there is poor rider feel.
There is an interesting aside to this that really illustates the ''cross talking'' issue and how it can mess up your understanding of what the damping adjusters actually do. This last Friday we ran for the first time Glenn Skachills new year 11 GSXR600 at Manfield VMCC. New Ohlins NIX30 cartridges were still in the air halfway between Stockholm and New Plymouth so we ran with the stock forks. This is what we did;
Fitted a TTX36 shock and refined the internal valving, spring rate and preload through the weekend
Ditched the standard electronic steering damper which for god only knows what reason is forward speed sensitive rather than handlebar arc velocity sensitive. Steering dampers that work properly should a) like a fire door closing damper be progressive in action , the harder you deflect the door damper the more resistance there is, exactly how a steering damper should work. b) the damper should provide very little resistance at low velocity stroke movements and the seals in that damper should be very low drag as that destroys rider feel. c) the mounting position affects damper shaft velocity and therefore damping, this is more critical than people realise.d) The damper should not cavitate and suffer from hysterisis on stroke reversal, sadly most of the budget steering dampers and all of the oem dampers are atrocious in this respect. Well designed steering dampers aid rider feel.
We played externally with the BPF forks to see how well we could make them work. The dust seals had a slice clipped into them to reduce their tension on the lower legs and therefore reduce drag, also the seals werre lubricated with a slippery grease. This leads to a pet subject/ hate of people installing ''long life multi lip seals'' that inevitably have much more drag than oem seals which are principally supplied by manufacturer NOK. Manufacturers go to great lengths to reduce seal drag and friction in general, why would you in no small way inhbit ''rider contact feel'' by installing cheap high drag aftermarket fork seals?
BPF forks have a rebound adjuster that again is an open orifice and bleed jet, so it cross talks. But as the mass flow in such big piston forks is significantly greater than in the ''conventional'' cartridges we have been more used to and adjustment of rebound ( which you should always do first ! ) massively changes the compression feel. So with every rebound adjustment we effected through the weekend we had to change the compression adjuster to bring back the same ''feel'' we had. The compression adjuster is not a bypass bleed with BPF forks, its a preloadable hat ring on the compression stack so the effect is a little different. Such forks present more of a challenge than in earlier designs.
The forks didnt work too bad and indeed were quite raceable to a certain point but dont provide either the ''edge of adhesion rider contact feel'' or ease of tunability of thoroughbred racing cartridges. What must also be realised here is that its not straight ''plug and play'' stuff. All of the aftermarket stuff on the markert has to be optimised for rider height / weight / riding style / circuit conditions / track temp / tyres used etc etc. That requires feet on the ground and enthusiasm!
In Glenns case because of his light body mass we ran the compression adjuster considerbaly ''weak'' to try and maximise what feel was available. Stiffer is not always faster.
I guess there are several ways of describing ''rider feel'' but I understand it as the rider having great ''connection'' or contact feel to the track through the suspension and tyres. Its important to have great feel at both ends but especially so in the front end as if youve got no confidence in the front end you cannot go as fast as youd like. That especially applies as you near the limits of adhesion, the chassis, suspension and tyre package should give you ample warning of that.
Factors that will affect feel ( and there will be a few more ) include;
Chassis rake and trail, this one is a biggie and bikes like the BMWS1000RR suffer with front end geometry that is a little off. Everytime you alter fork position in the clamps, rear ride height and even rear axle position you are manipulating those figures
Rear axle position, this is the biggest single thing that alters forwards / rearwards weight distribution as it is sort of like moving the motor backwards and forwards. Often it can have a major influence on front end feel
Footpeg and handlebar position, altering rider position and therefore overall static weight distribution
Chassis height, if you move the fork position in the clamps and rear ride height you are moving that big lump of metal called the motor upwards or downwards. That manipulates forwards and backwards pitch under braking / acceleration and affects feel
Tyre choice and compound. In the most simplistic terms the stickiness of the tyre and the frequencies it imparts into the suspension and chassis can have quite a marked affect. Tyres like Pirellis have soft sidewalls and ( for example ) their front tyres need a reasonable amount of pressure and a linear fork action to make them work properly. If youve got great settings for Pirellis / Metzelers expect to have to markedly change your internal suspension valving, spring rates and preload if you change to ( for example ) Dunlop N-Tecs. If you go tyre testing with a different brand and therefore construction of tyre its not a foolproof test to back to back unless you are able to make those suspension changes to compliment the tyres!
Tyre pressure and therefore its affect on surface area of contact patch.
Controlled chassis flex when the bike is on its side at more and more extreme bank angles. A hot topic in MotoGP at present and arguably a major issue with the Ducati MotoGP bike.
SUSPENSION! The above should illustrate that the suspension units are a part of the equation, albeit quite a major part. I get the feeling that a lot of people may think that the suspension needs to be ''firm'' to impart maximum feel. That is something that can often be far from the truth and the issue is in fact quite complex. Certainly the suspension needs to be firm enough to give great chassis control and so that the bike doesnt wobble like a jelly or pitch backwards and forwards like a rocking horse! The front end needs to have great brake dive resistance for racetrack situations but then you must also have enough initial forward pitch to help initiate turn in. In the rear end there must be enough squat control so that the bike doesnt understeer off the turn when under power. But it also must have enough pitch that the weight transfer assists in placing load on the rear tyre to minimise wheelspin. This is especially true of Superbikes, such bikes we often run the rider sag at bigger numbers than 600s because they need more weight transfer to load the rear tyre.
Now heres the biggie when talking about the forks. Suspension is always reverberating at various frequencies and amplitudes between compression and rebound strokes. Anyone who has ever viewed a datalogging trace with a minimum sampling rate of 1000 hertz will understand this, suspension never ''stands still'' Apart from high priced gas charged cartridges most of the forks we as mere mortals run are non pressurised and pretty much are ''open'' units. This of course is not completely true as under fork compression the air cavity above the set oil level rises in pressure. But that is a pressure that is position sensitive and there is never consistent and enough pressure to prevent cavitation.
As a fork reverses direction ( constantly ) damping fluid must flow back and forth between one side of the pistons and the other. Picture that even under a brake dive scenario on an even slightly bumpy track it will not be one big compression movement, there will be slight reverberations back and forth. That oil has got to find its way very quicly from one side to the other otherwise the amount of ''froth'' or ''milkshake'' the pistons are running in becomes extreme. That really hurts damping response and in turn ''rider contact feel''. There is ( simplistically ) not enough damping response to assist the spring in keeping the tyre as loaded as possible. When you are getting to the limits of adhesion it becomes of huge importance that the suspension units must as quickly as possible ''pressure balance'' between both sides of the pistons.
If the rider is right on the edge of adhesion and has a slight tuck / lose its important that the damping resonse is as instantaneous as possible. We have noticed that the ability to constantly save such ''tuck'' situations is a very positive feature of fork cartridges that are totally seperated in fuction, i.e compression function totally in one leg and rebound function in the other. In not having to share such duties the damping response on stroke reversal is for ''refill'' to the other side of the chamber, not also to porvide damping as part of that same refill function.
4 or so years ago I was contracted to work for Ohlins at World Superbike pre season tests and found myself revalving fork cartridges and shocks for Fujiwara and Broc Parkes. One of the interesting things that I bought back from those tests was a valving techique for forks where you run the compression clickers a long way open with an otherwise more aggressive main valving stack. While on the face of it there is less initial damping because the bleed bypass passage is a whole lot free'r flowing there is in fact much much less damping response lag because the system is able to equalise with a significant amount less cavitation. The stronger valving stack compensates and helps to shape the overall damping force curve required. Armed with that info I returned to NZ and we applied the technique into Jay Lawrences 07 ZX6 that he was campaigning that year. That model wasnt the sharpest tool in the shed and with all respect to Jay there were 2 or 3 riders personally faster than him. But with that setting he was able to stick the bike on pole at Timaru ( unless an alzheimers moment is fooling me ) and he was very very happy with the feel and feedback of those forks. This setting worked very well at all circuits except Pukekohe where the bumps required a softer stack, and in turn less bypass bleed. We also applied exactly that same technique to Andrew Strouds forks and he too reported a huge improvement in feel, especially approaching the limits of traction.
Latterly this same technique was used in the Superstock forks of Midge Smarts and Jaden Hassans R6s, but a little more involved here as there is a long and restrictive lead in passage we rework. This kind of work often but not always results in faster lap times BUT can go a long way towards reducing the chances of crashing as there is a huge amount more feedback to the handlebars as the limits of adhesion are reached.
Interestingly this same line of thinking has now been applied in the second generation of Ohlins NIX30 cartridges and part of the explanation from Ohlins for these new pistons is ''improved rider feel''. In NZ these have thus far been used once by Craig Shirriffs to take his maiden Superbike win at Manfield (As has been discovered only last Friday a coolant leak caused his crash in race two ) While thats only one sample it was a positive result and Craig certainly reported great feel.
Which leads to another issue that has been aired elsewhere, aftermarket handlebar clamps that are not toleranced so well can under delective loads bind up fork bushing clearances and totally destroy rider feel. BPF forks with sliding top bushings are very suspectible to such an issue, as at the lower reaches of stroke compression the bushing is getting slightly squeezed by a pinched fork.
Lets take anothre common example. Pre year 11 GSXR600 / 750s have particularly weak high speed rebound damping that causes the bike to ''stand up'' mid turn after youve released the brakes after using up most of the fork travel. The absolutely correct way of solving that issue is to have the rebound pistons internally revalved. The absolutely INCORRECt way of ''solving'' the issue is to close in the rebound adjusters a long way. It may ''kind of fix'' the issue but it creates a few more besides;
The forks then have too much low speed rebound damping so at the last stages of fork re-extension the fork action is lazy / lethagric. That can really hurt edge grip as you are accelerating off turns
As the rebound adjuster simply moves the position of a tapered needle in an orifice and that orifice is in the end of the rebound shaft it ''cross talks'' I.e it not only alters the flow in rebound direction but also in compression direction. So if your compression damping was feeling about right before your rebound adjustment it will now feel different after making that adjustment
The needle now closes that orifice off so much that the cartridge cannot pressure balance so quicly on stroke reversal movements, that really hurts damping response. Generally the forks now feel ''wooden'', there is poor rider feel.
There is an interesting aside to this that really illustates the ''cross talking'' issue and how it can mess up your understanding of what the damping adjusters actually do. This last Friday we ran for the first time Glenn Skachills new year 11 GSXR600 at Manfield VMCC. New Ohlins NIX30 cartridges were still in the air halfway between Stockholm and New Plymouth so we ran with the stock forks. This is what we did;
Fitted a TTX36 shock and refined the internal valving, spring rate and preload through the weekend
Ditched the standard electronic steering damper which for god only knows what reason is forward speed sensitive rather than handlebar arc velocity sensitive. Steering dampers that work properly should a) like a fire door closing damper be progressive in action , the harder you deflect the door damper the more resistance there is, exactly how a steering damper should work. b) the damper should provide very little resistance at low velocity stroke movements and the seals in that damper should be very low drag as that destroys rider feel. c) the mounting position affects damper shaft velocity and therefore damping, this is more critical than people realise.d) The damper should not cavitate and suffer from hysterisis on stroke reversal, sadly most of the budget steering dampers and all of the oem dampers are atrocious in this respect. Well designed steering dampers aid rider feel.
We played externally with the BPF forks to see how well we could make them work. The dust seals had a slice clipped into them to reduce their tension on the lower legs and therefore reduce drag, also the seals werre lubricated with a slippery grease. This leads to a pet subject/ hate of people installing ''long life multi lip seals'' that inevitably have much more drag than oem seals which are principally supplied by manufacturer NOK. Manufacturers go to great lengths to reduce seal drag and friction in general, why would you in no small way inhbit ''rider contact feel'' by installing cheap high drag aftermarket fork seals?
BPF forks have a rebound adjuster that again is an open orifice and bleed jet, so it cross talks. But as the mass flow in such big piston forks is significantly greater than in the ''conventional'' cartridges we have been more used to and adjustment of rebound ( which you should always do first ! ) massively changes the compression feel. So with every rebound adjustment we effected through the weekend we had to change the compression adjuster to bring back the same ''feel'' we had. The compression adjuster is not a bypass bleed with BPF forks, its a preloadable hat ring on the compression stack so the effect is a little different. Such forks present more of a challenge than in earlier designs.
The forks didnt work too bad and indeed were quite raceable to a certain point but dont provide either the ''edge of adhesion rider contact feel'' or ease of tunability of thoroughbred racing cartridges. What must also be realised here is that its not straight ''plug and play'' stuff. All of the aftermarket stuff on the markert has to be optimised for rider height / weight / riding style / circuit conditions / track temp / tyres used etc etc. That requires feet on the ground and enthusiasm!
In Glenns case because of his light body mass we ran the compression adjuster considerbaly ''weak'' to try and maximise what feel was available. Stiffer is not always faster.