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Discussion Starter #1
I'm having my K7 serviced and am considering new springs front and rear. Main use will be road and track (not racing), and I don't ride with passengers.

At 188cm (6'2") weighing in at 105 kg (230 lbs) without gear, I'm guessing I'm quite a bit outside the ideal range for the stock setup. Btw, are the stock springs progressive or linear?

Anyway, I want to go with linear springs and am wondering which spring rate would be correct for my weight and intended use?
 

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The OEM fork spring rate (0.976 kg/mm according to RaceTech) should be pretty good for your weight, generically speaking, so I assume the rear should be fine too. Do you have reason to believe it isn't?

Regarding spring linearity, I haven't seen any authoritative sources on the matter, but judging from appearance, I can't see how they can be anything other than linear. Common engineering sense would dictate that as well, as far as I can see. (I'm strictly talking about the spring itself here; the suspension's effective rate is quite nonlinear.)
 

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Discussion Starter #3
I talked to one of the mechanics at my local suzuki dealership and he said that the rear shock at least had a progressive spring. He also was of the opinion that I would benefit from different springs given my weight. I've never fiddled with the suspension so I'm letting a shop do the work and help me set the correct sag.
 

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Coil springs can be progressive, either by changing the diameter, or the pitch along the length of the spring, either continuously, or discretely, that is, in steps. I see no such features, at least not on any significant scale, on the OEM springs. The suspension itself, is nonlinear, due to its design, but you can't change that, at least not by changing springs.

I'd advise against changing springs, unless you have specific issues with the existing setup. That way, at the very least you have a certain idea of what to expect from the new parts. If all you expect is "a better suspension", then I would predict two possible outcomes: either you'll discover things don't work that way, which is, in my mind, the good alternative, or you'll be happy but misled, attributing to the new springs, changes to the bike's handling, which might well have nothing to do with them. (Instead, they might, for instance, be the result of attendant geometry changes which could have been achieved with the old springs just as well.)
 

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Discussion Starter #6
Coil springs can be progressive, either by changing the diameter, or the pitch along the length of the spring, either continuously, or discretely, that is, in steps. I see no such features, at least not on any significant scale, on the OEM springs.
I agree with your observation, that's why I posted the question here. I had some doubts when the mechanic said the springs were progressive. At least with the rear as the spring is visible.


I'd advise against changing springs, unless you have specific issues with the existing setup. That way, at the very least you have a certain idea of what to expect from the new parts. If all you expect is "a better suspension", then I would predict two possible outcomes: either you'll discover things don't work that way, which is, in my mind, the good alternative, or you'll be happy but misled, attributing to the new springs, changes to the bike's handling, which might well have nothing to do with them. (Instead, they might, for instance, be the result of attendant geometry changes which could have been achieved with the old springs just as well.)
I also checked out racetech's spring calculator and like you previously said the stock springs don't seem very far off for my weight. However it's also possible that racetech's information about the stock spring rates is incorrect, which is also why I wanted to check with you guys :)

My issues with the suspension as it is, are mostly with the rear shock. While cornering and accelerating out of corners the rear end feels a little unsettled. It's a bit difficult to explain, but it feels as if it's sort of "bouncy", for lack of a better word. I thought that it might be due to the spring being too soft.
 

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Discussion Starter #7
Btw, here are the recommended and stock spring rates from racetech's calculator, and available springs to compare:

FRONT FORK SPRINGS
Recommended Fork Spring Rate: 1.06 kg/mm (use closest available)
Stock Fork Spring Rate: 0.976 kg/mm (stock)

Available front fork springs (kg/mm):

RT FRK SPR 38x36x250 .95 kg
RT FRK SPR 38x36x250 1.00 kg
RT FRK SPR 38x36x250 1.10 kg

REAR SHOCK SPRINGS
Recommended Spring Rate: 11.7 kg/mm (use closest available)
Stock Spring Rate: 10.1 kg/mm (stock)

Available rear shock springs (kg/mm):
SK SPR 6x2.5" 9.8 kg
SK SPR 6x2.5" 10.7 kg
SK SPR 6x2.5" 11.6 kg

At first glance this appears fairly close to me. According to this I am about 1.5 "steps" off in the front, and about 2 steps off with regards to the rear spring.
So I guess the question is how much difference can be noticed from one or two steps? Is it too close to bother?
 

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I also checked out racetech's spring calculator and like you previously said the stock springs don't seem very far off for my weight. However it's also possible that racetech's information about the stock spring rates is incorrect, which is also why I wanted to check with you guys :)
Yes, unfortunately I don't know of any more authoritative source of OEM spring rates. There are a few other figures out there, at least for some of the older models, but I have no reason to assume they're not sourced from RaceTech as well. I can tell you though, for reference, that I'm 10kg heavier than you and about 10cm taller and I've recently swapped to 10N/mm (~1Kg/mm) front springs and a 90N/mm (~9.2Kg/mm) rear spring. My OEM rates were significantly softer, at 0.85Kg/mm and 7.3N/mm respectively, so the front value you have, is pretty close to what I would have chosen. I can't really be certain about the rear spring rate, as the effective rate there depends heavily on the linkage, but I see no reason to question the rate balance chosen by the OEM.

My issues with the suspension as it is, are mostly with the rear shock. While cornering and accelerating out of corners the rear end feels a little unsettled. It's a bit difficult to explain, but it feels as if it's sort of "bouncy", for lack of a better word. I thought that it might be due to the spring being too soft.
Others will probably be able to provide better advice, derived from track experience, but in general, it's good to know what one should expect from spring rate choice, which might potentially affect:


  1. The actual performance of the suspension as a suspension, in other words, its ability to absorb bumps of different sizes at different speeds. This depends on the road profile and vehicle speed, but in general, softer rates are better, as far as bottoming is avoided. Although this might seem only important for comfort, it's really just as important for roadholding as bumps cause variations in tire load and hence grip, which you'd prefer to avoid.

  2. The sag, hence chassis attitude and geometry under steady-state conditions, that is at constant speed. This affects many things, but it can and should be adjusted separately (through preload adjustment, shimming, sliding the forks through the tripple trees, etc.) so it shouldn't really concern us when considering the rate itself.

  3. The chassis attitude and geometry under transient load conditions, which can generally arise in two ways:

    One is due to acceleration or braking. This can cause the suspension to compress (usually referred to as "dive" for the front and "squat' for the rear) which again has two implications, one being that the geometry changes, potentially dramatically, the other that as the suspension travels towards the end of its stroke, it effectively becomes stiffer and also increases its chance of bottoming when trying to absorb even moderate bumps.

    Another case where the load on the wheels might change, is during a turn, where the effective force of gravity increases, which increases the load on both wheels. The choice of spring rate here affects both how much each suspension will compress further with the previous implications of increased rate and reduced bottoming resistance, but also the relative front and rear stiffness will now also affect chassis attitude: If the front effective spring rate is lower than the rear for instance, the bike's nose will dive, with the previously mentioned implications in geometry. This effect is generally small at lower lean angles, but can become quite significant. For instance during a 45° turn, the effective force of gravity is about 40% larger than normal.
The upshot of all the above is that, unless you know what you're looking for, the best you could do, as far as I can tell, would be to choose springs that:


  1. Make the best possible use of the suspension stroke, meaning using most of it during the course of the average trip, but also leaving enough reserve for special cases, such as hitting a pothole during a panic-brake maneuver.

  2. Result in sag and squat, these being the chassis effects that are easiest to assess, that are to the rider's liking. Note here that one can and should control these with slow-speed compression damping, but ultimately, the damping force can only affect the rate of the dive, that is, how quickly/violently the nose will dive after applying the brakes. The actual magnitude of the dive, given enough time, will be the same and can only be controlled through the spring rate.
 

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Discussion Starter #9
I appreciate you taking the time to reply.

I imagine there are guidelines on how to set sag in such a way that one can determine if the springs will perform sufficiently for certain types of riding.

I.e. the sag should be x mm without rider, and y times x with rider. Perhaps this is how one determines if the preload can be set in the desired range?

I'd be grateful if someone could provide basic recommended sag settings for street and track use :)
 

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The practical use of sag, is to make sure that the suspension can both compress and extend to absorb bumps and dips. Say you have a front suspension stroke of 120mm and a sag of 40mm including rider in riding position. Roughly that means, that when you're going at constant speed, the suspension can absorb bumps of about 80mm and potholes of about 40mm before bottoming or topping out respectively. Of course if you're accelerating or braking the suspension will travel away from that static position, its sag will change and so will its ability to absorb bumps and dips and there are some more details, such as change of effective rate along the travel, etc., but that's what sag does basically.

It also affects geometry, but one can and, I think, should compensate for that otherwise, as I've mentioned before, so that sag is used mainly for the previously described purpose (and perhaps for geometry finetuning). So, as far as I can tell, one should set sag based on street conditions and use. The conventional wisdom is somewhere around 30-40mm for the street and less than that on the track, where you can expect dips to be small and you need more travel in compression, to help with dive during heavy braking.

You're right in that the relation to rider and free sag only depends on, and therefore determines spring rate, so that one could in theory use it to select springs. If there's a source of credible information on that (i.e. not just spouting numbers without justification, in some forum thread, or blog post), I'm not aware of it.
 
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