What's the problem here?

[idl1975]

'94 YZF750R (preload only) forks - using only 60-65% of their travel even 'smokin' the anchs'.

Low speed and high speed compression damping seem fine, much less harsh than the TRX. Feels bouncy on the rebound. Feels completely out of sync with the Ohlins shock (shock feels about 50% "softer" i.e. moves much more). I haven't even checked static sag simply because they're so far out on travel there doesn't seem a point.

This sounds like the front spring rate is way too high, right?

Think the forks must have been used by an extra-fat b'stad on a race bike. With slicks.

On a related note, does anyone have the dimensions for YZF springs? I would need to know free length, minimum spring travel and internal diameter I guess - I have a local spring manufacturer near me, but they need the measurements. If anyone has that info, it will save pulling the existing springs and having the bike off the road while the new ones are made up.

[Kayla]

Daft question- I'm guessing the preload adjusters don't make any (positive) difference?

Try giving Hagon a ring about the dimensions of the springs, they might be able to help and it's only a phone call

[idl1975]

Daft question- I'm guessing the preload adjusters don't make any (positive) difference?

Try giving Hagon a ring about the dimensions of the springs, they might be able to help and it's only a phone call

As I understand it, preload alters the amount of fork you need to put on the spring before it moves. Surely if it's not using all the travel that wouldn't be the problem? But I'm obviously no expert.

Suppose I could try winding them out and see what happens...

[Kayla]

Preload alters the force needed to compress the spring. If it's a 100N/mm spring then it'll need 100N (or roughly 10kg) to move it 1mm, then another 100N to move it a further 1mm, assuming the spring is linear. If you have, say, 10mm of preload already wound onto the spring it's already had the first 1000N (or 100kg) applied to it via the threaded adjuster before taking into account the weight* of the bike and/or rider, so any weight up to 1000N (100kg mass) that is applied to the spring will not compress it.

Try backing the preload off a touch and see what happens. It's a free mod and easily reversible

* don't confuse 'weight' with 'mass'.

[idl1975]

What happens if I flip my carbon fibre toilet seat upside down?

Will that help?

Preload alters the force needed to compress the spring. If it's a 100N/mm spring then it'll need 100N (or roughly 10kg) to move it 1mm, then another 100N to move it a further 1mm, assuming the spring is linear. If you have, say, 10mm of preload already wound onto the spring it's already had the first 1000N (or 100kg) applied to it via the threaded adjuster before taking into account the weight* of the bike and/or rider, so any weight up to 1000N (100kg mass) that is applied to the spring will not compress it.

Try backing the preload off a touch and see what happens. It's a free mod and easily reversible

* don't confuse 'weight' with 'mass'.

[Kayla]

What happens if I flip my carbon fibre toilet seat upside down?

Will that help?

It could get messy. I don't recommend it. Did my post make sense? Basically you need to reduce the preload in order to use more travel.

Edited for stupidness.

[idl1975]

What happens if I flip my carbon fibre toilet seat upside down?

Will that help?

It could get messy. I don't recommend it. Did my post make sense? Basically you need to reduce the preload in order to use more travel.

Edited for stupidness.

I guess I'm having trouble with the concept.

If we assume two sets of forks with straight-rate springs, one preloaded with 50% of the load needed to compress it completely and one with zero preload, and then subject them over a few seconds to an increasing load up to 100% of their maximum, I thought the only difference would be that the preloaded fork would not appear to move until the load reached more than 50% of maximum. Both forks should end up with the springs fully compressed (other otherwise run out of travel).

So if I can't get anywhere near the bottom 1/3 of travel on the fork but there's no indication of hydraulic lock (no harsh damping), wouldn't that mean it has the wrong springs (e.g. coilbound, but I bet you'd notice that!) or that the rate is simply too high and the TRX can't exert enough force to compress them.

I assumed that the function of preload is to prevent unwanted movement when the fork is lightly loaded and/or to stop the fork topping out under power. So that's why I'm not getting the thing about reducing preload to use more fork travel - could the extra preload actually stop the springs compressing into the bottom third of travel?

Still scratching my head, but that's probably why I'm not an engineer.

[Kayla]

I guess I'm having trouble with the concept.

If we assume two sets of forks with straight-rate springs, one preloaded with 50% of the load needed to compress it completely and one with zero preload, and then subject them over a few seconds to an increasing load up to 100% of their maximum, I thought the only difference would be that the preloaded fork would not appear to move until the load reached more than 50% of maximum. Both forks should end up with the springs fully compressed (other otherwise run out of travel).

I think the confusion may be that you're assuming the small amount of linear compression applied by the adjusters is enough to coil bind the springs or that the adjuster uses up available travel. Preloading the spring just makes it 'need' more force initially to get things moving. Once that initial force is applied it acts as it would normally under load.

Say it's preloaded by 10N and it's a 10N/mm spring. It will have been compressed by 1mm. You need to apply a 20N load to get it to move a further 1mm, as the 'first' 10N of that 20N is taken up by the preload. Any load applied after that will have the spring behaving as it would normally, ie compressing by 1mm for every 10N applied.

It depends how far the adjusters are wound in on your bike and of course on the spring rates, but I'd be surprised if just backing them right off didn't make a big difference. It's just physics, innit?

[Kayla]

...could the extra preload actually stop the springs compressing into the bottom third of travel?

That's it exactly! If the combination of the mass of the bike, you and the extra forces provided by braking aren't enough to 'overcome' the intitial preload then the forks won't compress any further. There are a few answers to this-

1. Eat more pies.
2. Ride faster and brake harder.
3. Reduce the preload.

Out of the three, I'd probably recommend no. 3 as the safest option

[idl1975]

...could the extra preload actually stop the springs compressing into the bottom third of travel?

That's it exactly! If the combination of the mass of the bike, you and the extra forces provided by braking aren't enough to 'overcome' the intitial preload then the forks won't compress any further. There are a few answers to this-

1. Eat more pies.
2. Ride faster and brake harder.
3. Reduce the preload.

Out of the three, I'd probably recommend no. 3 as the safest option

Ah, so it will travel but then will stop before the springs are fully compressed? I admit I still can't visualise this, but I'll take your word for it.

Braking harder wouldn't be safe. It's basically out of braking traction as far as I can tell. If I pulled much harder it would just lock the front. That's just one reason why I need to sort it before the TRX does another trackday.

I shall have a go at winding the preload adjuster all the way off.

[Greg]

Have you checked there's not too much oil in the tubes... even in just one leg..?

When I did my R1 USD conversion, I was surprised at how harsh and unyielding they felt compared to the STD TRX jobbies. I wasn't convinced they were working at all until I used the cable tie to see how much travel I was using .. and yup, it was just about right. I backed the pre-load right off and set the damping rates to middle clicks on both C & R ..

One thing I was alway foxed by was the sag thing... when I sat on the bike, the forks didn't compress at all (beyond the static sag limit anyway) - if i just bounced on the seat the rear would compress but the front would not. However, sitting on the bike with the front brake on and bouncing gave me a lovely equal sag at both ends.... I never got round to measuring the weight bias of the TRX...

It's a lot easier to look at the pre-load adjuster more as a ride-height adjuster

idl1975 wrote:
I guess I'm having trouble with the concept.

If we assume two sets of forks with straight-rate springs, one preloaded with 50% of the load needed to compress it completely and one with zero preload, and then subject them over a few seconds to an increasing load up to 100% of their maximum, I thought the only difference would be that the preloaded fork would not appear to move until the load reached more than 50% of maximum. Both forks should end up with the springs fully compressed (other otherwise run out of travel).

That's exactly correct ... even easier is just to imagine two plain old springs - they both have the same spring rate, in other words they both require the same load to deflect the same distance. Pushing one through half it's deflection pre-loads it and, as Kayla rightly says, doing so requires a certain effort and the other spring will behave in exactly the same way once it's been applied with the same force.
A spring with a higher spring-rate requires more force to get it to deflect the same distance - changing the pre-load doesn't change the spring's rate.

My thoughts --- Unless you've got an absolute Frankenstein fork, it's unlikely that it's going to get coil-bound before the full travel is used up - even if full pre-load is applied (the manufacturers will have built that into their calculations...) . Even with higher rate springs you'll be able to get the fork to compress to max travel, you just have to apply more force, which is why I'm thinking you've too much oil. Air compresses exponentially (I think that's correct) in that each time you halve its volume you double the previously required level of force (PSI or pascal/mm2 if you want to split hairs).. So in your forks, there's a volume of air above to oil which when compressed will also act as a spring (but not in a linear fashion..) If the forkleg is correctly filled, then that air spring effect is less than the spring's spring effect so the metal spring becomes the principal resistance to overcome. If however the fork is overfilled, the force required to compress that air can be so great that you'll get to a point where you simply cannot compress the air any further - and travel appears to be all used up even though the metal spring is not at full deflection.... Geddit?

Usual disclaimer applies here - I'm probably talking complete bollocks YMMV etc ...

Greg

[Kayla]

Have you checked there's not too much oil in the tubes... even in just one leg..?

...Air compresses exponentially (I think that's correct) in that each time you halve its volume you double the previously required level of force (PSI or pascal/mm2 if you want to split hairs).. So in your forks, there's a volume of air above to oil which when compressed will also act as a spring (but not in a linear fashion..) If the forkleg is correctly filled, then that air spring effect is less than the spring's spring effect so the metal spring becomes the principal resistance to overcome. If however the fork is overfilled, the force required to compress that air can be so great that you'll get to a point where you simply cannot compress the air any further - and travel appears to be all used up even though the metal spring is not at full deflection.... Geddit?

Usual disclaimer applies here - I'm probably talking complete bollocks YMMV etc ...

Greg

That's a 'fix' for TRX forks isn't it? Thinner oil but slightly more of it, which reduces the harshness of the damping but increases the 'spring'. It's certainly something else that's free to look at before buying new springs.

[idl1975]

Stock TRXs are supposed to be 53% rear IIRC.

I can see what you're saying about the air gap / excess oil. It might effectively be multiplying the spring rate once you get to 50% of the travel, so it never goes much further. It's not locking up and not turning into a rigid fork, so I can't believe the springs are actually coil-bound. Thing is, since the effect is noticeable even without lots of weight on the front, is that likely to be the problem? It sounds like with too little air gap you get effectively a very harsh rising rate effect, so you wouldn't expect it to feature until you really load up the forks.

It basically still feels "bouncy" under full braking and soaks up bumps well, but the front just doesn't get enough weight on it to brake or turn in hard. On the throttle, it feels like the front almost tops out, so there can be a bit of a hinge-in-the-middle feel if you hit a mid-corner bump, i.e. it bump-steers more at the front than the back.

It could of course be both. Maybe the preload is wound right up and it's had stiffer springs or too little air gap.

PS - my CBR4 actually DOES have air forks, complete with valves in the fork tops. Relies on exactly the effect you're describing I should imagine. Hey, since they're air forks, does that mean I should put them on my mountain bike? Rock Shox charge a lot of money for that shit you know.

ANYWAY, I shall turn down the preload, and if that doesn't work I'll be pulling the springs and draining the forks anyway.

Have you checked there's not too much oil in the tubes... even in just one leg..?

When I did my R1 USD conversion, I was surprised at how harsh and unyielding they felt compared to the STD TRX jobbies. I wasn't convinced they were working at all until I used the cable tie to see how much travel I was using .. and yup, it was just about right. I backed the pre-load right off and set the damping rates to middle clicks on both C & R ..

One thing I was alway foxed by was the sag thing... when I sat on the bike, the forks didn't compress at all (beyond the static sag limit anyway) - if i just bounced on the seat the rear would compress but the front would not. However, sitting on the bike with the front brake on and bouncing gave me a lovely equal sag at both ends.... I never got round to measuring the weight bias of the TRX...

It's a lot easier to look at the pre-load adjuster more as a ride-height adjuster

idl1975 wrote:
I guess I'm having trouble with the concept.

If we assume two sets of forks with straight-rate springs, one preloaded with 50% of the load needed to compress it completely and one with zero preload, and then subject them over a few seconds to an increasing load up to 100% of their maximum, I thought the only difference would be that the preloaded fork would not appear to move until the load reached more than 50% of maximum. Both forks should end up with the springs fully compressed (other otherwise run out of travel).

That's exactly correct ... even easier is just to imagine two plain old springs - they both have the same spring rate, in other words they both require the same load to deflect the same distance. Pushing one through half it's deflection pre-loads it and, as Kayla rightly says, doing so requires a certain effort and the other spring will behave in exactly the same way once it's been applied with the same force.
A spring with a higher spring-rate requires more force to get it to deflect the same distance - changing the pre-load doesn't change the spring's rate.

My thoughts --- Unless you've got an absolute Frankenstein fork, it's unlikely that it's going to get coil-bound before the full travel is used up - even if full pre-load is applied (the manufacturers will have built that into their calculations...) . Even with higher rate springs you'll be able to get the fork to compress to max travel, you just have to apply more force, which is why I'm thinking you've too much oil. Air compresses exponentially (I think that's correct) in that each time you halve its volume you double the previously required level of force (PSI or pascal/mm2 if you want to split hairs).. So in your forks, there's a volume of air above to oil which when compressed will also act as a spring (but not in a linear fashion..) If the forkleg is correctly filled, then that air spring effect is less than the spring's spring effect so the metal spring becomes the principal resistance to overcome. If however the fork is overfilled, the force required to compress that air can be so great that you'll get to a point where you simply cannot compress the air any further - and travel appears to be all used up even though the metal spring is not at full deflection.... Geddit?

Usual disclaimer applies here - I'm probably talking complete bollocks YMMV etc ...

Greg

[Greg]

it could of course be both. Maybe the preload is wound right up and it's had stiffer springs or too little air gap.

PS - my CBR4 actually DOES have air forks, complete with valves in the fork tops. Relies on exactly the effect you're describing I should imagine. Hey, since they're air forks, does that mean I should put them on my mountain bike? Rock Shox charge a lot of money for that shit you know.

Eggsackerly...

You could just be at the point where everything starts to 'work' at the same time, effectively having 2 or three forces applied at once..

The valves on top of your Honda ( Is there another Honda owner apart from me...?) forks are there to give supplemental springing I think, certainly not entirely air sprung like the old Kawa rear units used to be ... IIRC, there was a max of 6psi before you started to do some serious seal blowing (not in any sea mammal pleasuring sense you understand....)

[idl1975]

Eggsackerly...

You could just be at the point where everything starts to 'work' at the same time, effectively having 2 or three forces applied at once..

The valves on top of your Honda ( Is there another Honda owner apart from me...?) forks are there to give supplemental springing I think, certainly not entirely air sprung like the old Kawa rear units used to be ... IIRC, there was a max of 6psi before you started to do some serious seal blowing (not in any sea mammal pleasuring sense you understand....)

Yes, they're essentially helper springs but using air - like a number of MTB forks which are "air assisted". And I read that you basically shouldn't change the pressures as aside from being stampeded by marine mammals, the effect is very harsh. The last survivors from the days of anti-dive mania? (Well, my TRX has anti-dive, but I don't like it, as we've established...)

The CBR's forks work fine, surprisingly for 1989 equipment, although there was a tiny bit of chatter into Gerrards at Mallory last week on turn in. Wasn't a huge problem seeing as I had no idea where to apex and went in rather slow. I did wonder what all those sea lions were doing watching from behind the fencing - and now I know.