Damper rod forks, emulators, intminators and progressive springs

[Robert Taylor]

This is a first instalment and by no means complete as its a time consuming subject. We in the 21st century still have very many bikes with damper rod forks, technology from around the middle part of last century, terrible then and terrible now.
Damper rods are by definition ''fixed orifice / velocity squared damping'' Forcing oil through a fixed size hole means that the damping will only be roughly correct at one particular fork velocity. But forks dont operate at one particular velocity!
In real terms the ''character'' of a damper rod fork is that initially they will be very sloppy / uncontrolled and have a propensity for ''blowing through their stroke'' under brakes. In this respect a number of bikes spring to mind as being dangerous. But also when you ride over something abrupt such as a badly installed manhole cover ( councils are very poor at rejecting poor quality work from contractors ), motorway expansion joints or a sudden seal lip change it then feels like youre on top of a jackhammer! Many riders will either subconciously or even conciously avoid such bumps, so the bumps are to a large degree having first say over your line choices. It shouldnt be that way!!!!
That we still have ''modern'' bikes with abysmally bad forks is to the blame of many. Consumers are to blame for demanding that bikes are as cheap as possible and manufacturers are also to blame in concentrating on areas of the bike i.e engines etc that deliver a performance and marketing improvement. But there is a more real world improvement in actually getting bikes to ride the bumps better but also with much better chassis control. Given we are seeing more and more bikes from mainland Asia the next few years or even decades dont bode well for quality or chassis performance from such machines. Hopefully I will be incorrect but the current offerings really leave me cold.
A 70s ''solution'' for better control of damper rod forks is to instal progressively wound springs, its actually not the correct way of skinning the cat and sorry, it should have remained in the 70s!!!!!!! Although it varies a little its fair to say that only 50% of the issue is in fact springing, the other 50% is very poor hydraulic control

More to follow.....

[SWERVE]

Dont tease Robert...... more..more

[SPman]

Yes please - I've got an XJR that needs it's front end done next.......

[AllanB]

Yes please - I've got an XJR that needs it's front end done next.......

I've recently popped a set of Roberts fork springs in my Hornet 900. I wish I'd done it 3 years ago when I purchased the bike. Huge returns for small $.

[hayd3n]

the trouble i have is getting something within my budget

[riffer]

Keen to hear more.

What in the heck is an intminator?

[Woodman]

Will be interested to see what RT's opinion of intiminators is.

All i know is that I put some in my bike and it made an instant improvement. Very planted front end, dealt with corrugations under brakes, stopped the front from slipping out etc. This is on gravel/offroad .

[Brian d marge]

Sorry , have to disagree my Enfield has fixed velocity front suspension , Solid as a rock , never seen them move , Coupled with a well made secondary spring , "the Avon Speedmaster " ... The ride is as firm and as dependable a teenagers backside

On a lighter note, Robert , you deal with fluids have a look at openFoam CFD bit of a learning curve ( not hard but different ) but you can model fluid flow through Damper rods , its free and its growing on me day by day !

and with rapid prototyping as cheap as it is , beats machining the thing ....( test only)

ok carry on as you were

Stephen

[Robert Taylor]

Aside from abysmal compression damping control that may as well have come out of the stone age the major achilles heel of damper rod forks is ''uncontrolled bleed''
If you remove the spring, drain the oil and then shine a torch down the bore of the inner fork tube you will find more often than not that the damper rod is not concentric with the centreline of the fork tube! Most often it will be noticeably cocked to one side. That means that its ''top hat'' piston ring will not be sealing concentrically and will wear unevenly. This is but one cause of uncontrolled bleed. Reasons for this include;
1) The floor of the lower fork casting that it bolts onto is not perpendicular to the centreline of the fork ( not easy and expensive to correct )
2) The lower seating surface of the damper rod itself is also not square ( machinable ) and is so tenuously narrow that there is a very narrow surface area of seating contact. Any slight misalignment here magnifies by the time you are at the top hat end of the damper rod. Also that small surface contact area of the rod can chew into the relatively soft lower fork casting and exacerbate the misalignment issue. Especially if the guy doing up the lock bolt has the finesse of a rhinocerous.
3) The tolerancing of the female thread form in the damper rod and the lockbolt itself can be ''sloppy''. And our mainland Asian friends can be very laxadaisical in that respect
4) DTI a damper rod on a pair of v blocks and you will often find they are out of round and also not straight! The nastiest ones are pressed steel and that really is a bridge too far in the continual degradation of standards that we see with many products. But as I have said previously consumers are just as guilty in demanding lower prices for products. For those of us that were born at a time when quality was still a paramount consideration in manufacture of everyday goods its sad to see where the world is at now!

Misalignment and untruth of that damper rod sets off a whole load of other issues. The top hat seating surface of the damper rod is then not perpendicular to the centreline of the fork tube and therefore by implication also not perpendicular to the spring that seats on it, otherwise centralised by the internal bore of the fork tube. Or indeed a Race Tech Emulator or Intiminator that seats directly upon it. With respect to the spring it creates an unwanted side thrust that in turn magnifies spring rubbing and friction against the inner walls of the fork tube. The fork action is compromised and the internal rubbing shaves small chards of metal off the inner fork walls, contaminating the oil and embedding particles in the teflon coated fork glide bushings. Not nice. Its also fair to point out that we often see fork springs with end conditions that are also not square. All springs are not equal and a number of the cheaper aftermarket spring kits are afflicted with nasty cost reducing issues.

A Race Tech Emulator or Intiminator will also not seat properly and therefore not seal properly against the top of an unsquare damper rod. More uncontrolled bleed. Approximately 7% of the worlds economy is piracy or to put it another way blatantly and unethically copying another companies product and therefore short circuiting the not inconsiderable development costs incurred by the original company. One example is the nasty Asian made copy of the Race Tech emulators on the market. These have a tenuously thin hard surface coating that wears in short order through to the soft alloy underbelly. Given that a lot of these will be self instal and that many installers will not recognise what I have detailed above, accelerated wear of the hard coating due to inconsistent seating is a certainty. We have evidenced this issue once again among other blatant shortcomings, this past week. But more on Emulators, how they work, variations on the theme and the shortcomings they are all afflicted with in the next post.

In the lower reach of the inner fork tube is a one way check plate that ensures flow is correctly directed and ''sealed off'' in and around the damper rod as it cycles between compression and rebound cycles. The inner diameter of this check washer ( or sleeve ) is supposed to seal as effectively as possible aginst the od of the damper rod tube. But we evidence all the time that the disparity in diameter between check plate id and damper rod od can be HUGE. Exacerbating that are the aforementioned tolerance issue with the rods themselves. This is another huge area of uncontrolled bleed.

This whole uncontrolled bleed issue affects compression damping but sometimes in an ''unimproved'' fork thats just as well( ! ) because the fixed orifice damping holes drilled into the damper rods are so tiny they will not pass enough oil at high deflection velocities. The fork hydraulically locks and then the poor tyre is overstressed (and wear accelerated ), as is the rider! But mostly it will reflect in very very poor rebound damping control.

The time honoured ''fix'' / BAND AID is to fill with a heavier oil to get some vestige of rebound control. But the problem is that the thicker the viscosity of the oil the more sensitive it is to a change in ambient temperature. Except in racing situations with sophisticated cartridges and aggressive settings fork oil doesnt run at that much above ambient temperature. Forks are always in the cooling airstream and the mass of metal is a heat dispersant. So, the forks may have a ''correct'' rebound speed at 12 degrees celsius, but in the morning when its frosty their rebound return speed is very lethargic, thats crash material.

So given that mentality the forks should be engineered with precise alignment and tolerances to run as thin an oil as possible. And to not put too fine a point on it with damper rod forks oil viscosity first, foremost and ONLY should be selected to attain the correct rebound speed. It should absolutely NOT be used to try and tune compression feel / speed.

Pro Twins racing SV650s are a good example and we have over the years modified the lions share of these by fitting Ohlins linear wind springs and Race Tech emulators ( that are also modified for race spec, not just a straight drop in )
If we use the standard damper rods and correct the alignment issue as much as possible we have to fill with Ohlins 98 centistoke oil ( broadly similiar to the shonky and highly inaccurate SAE rating of suspension fluids, 20-25 weight. But thats another story ) If we fit Traxxion Dynamics damper rods ( that are actually straight and tighter toleranced ) we can fill with Ohlins 40 centistoke oil, broadly similiar to 10 weight. This spec Johny Small used as did Geoff Booth one year preceding to win their repective Pro Twins NZ no 1 plates

A set of forks that we have re-engineered and made our own CKT emulators for ( and these are totally different to anything else on the market ) we are able to run with Ohlins 19 centistoke oil, broadly similiar to 5 weight. These forks are highly insensitive to ambient temperature shift and while there is more work to be done they have much better control and bump compliance than anything else we have tested.

Selecting fork oils is also a little different to selecting shock oils. Aside from viscosity and anti foaming etc the major preoccupation is selecting an oil that has consistent flow properties at closer to ambient temperature, as eluded to above. Rear shock absorbers will run at 40 to 80 degrees celsius according to how aggressive the setting is and proximity to hot engines, exhaust headers and mufflers. In one Formula car installation we evidenced 120 degrees celsius, but remember that pressurisation raises the boiling point. But forks need to be responsive at ambient and they are running in a cooling airstream! The ambient may be 5 degrees celsius in a winter race meeting, fork oil temperature the same or a couple of degrees higher and rear shock temperature 50 degrees celsius...

In an unashamed plug for the Ohlins fork ( and shock ) oils we distribute Ohlins are very attentive to the need for excellent cold flow properties in fork oils, having for many years built snowmobile suspension. And we do race rather a lot in cold winter temperature here compared to first world Northern European and American countries where it pretty much shuts down until the seasonal shift back to warmer temperatures. Ohlins oil is the most expensive suspension oil on the NZ market but you get what you pay for!

Next post we will get into the Emulators themselves and issues

[Robert Taylor]

the trouble i have is getting something within my budget

If you end up spending a little more at the outset youll end up subsequently spending a lot less. We have quite a tally now of cheaper shocks and rip off copies of emulators etc that we have removed because they didnt cut the mustard.

[Robert Taylor]

Keen to hear more.

What in the heck is an intminator?

To follow. Like an Emulator they are another form of ''band aid''

[tri boy]

"laxadaisical"
Show off
(think it is lackadaisical.Hitcher, help................)

America-First world country?
Not for much longer.

Besides those two little points, loving the lesson

[Woodman]

Very good, keep it coming.

[riffer]

To follow. Like an Emulator they are another form of ''band aid''

I think I get it. Its an emulator works on compression, intiminator works on rebound kind of thing?

[hayd3n]

If you end up spending a little more at the outset youll end up subsequently spending a lot less. We have quite a tally now of cheaper shocks and rip off copies of emulators etc that we have removed because they didnt cut the mustard.

well what ever i put in will better then the shit factory ones