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DaveJ

New theories into setting rear sag

8 posts in this topic

I just wanted to post some thoughts and recommendations around setting rear sag. It’s bit long, so grab a snack and sit down.

It seems to me that there is still too much mystery or experimentation into the process of setting rear sag, with little or no consideration to the overall suspension setup on the bike.

My thoughts are that setting rear sag should be a means of achieving what I will call “suspension balance”. That is, with the weight of the bike and rider centered over the pegs, you’re suspension should be working evenly between the front and rear. Of course your weight shifts, but we’ll use this as a marking point.

Rear sag is the measurement of how much the rear suspension compresses when the weight of the bike and rider is applied. It is a marker or indicator of real spring pre-load.

And of course real pre-load is the difference in spring height between a free standing spring and one that is installed on a shock, (or put into a fork).

So my concerns with setting correct rear sag is the lack of consideration to front end spring rates and spring pre-load. Meaning that in order to achieve a properly balanced bike, one would have to consider front sag as much as it considers rear sag. Obviously, if I change my front springs from a .46 to a .47, my rear sag is going to change, as will the balance between the two systems. Specially, the rear would then get more work. Which also means that one setting that works for one rider may not work for another. So much for using the settings of the pros.

For most of us, it’s a guessing game. If we can stand it, we’ll stop riding long enough to try a collection of different rear sag settings to see what feels best. If we get close enough, we can only assume that perhaps there is a fine adjustment that would take us another 10% and perhaps scrub off another second or two per lap. But can we feel the performance difference between 2 to 5mm? If ideal rear sag was off by just 2mm, how would we know? The bike would certainly still feel reasonably well. Then what happens when we make a modification somewhere else? Front end springs, triple clamps, fork height, front pre-load, or the change due to spring fatigue. Do we then have to start over with the same guessing procedure?

Well back to “suspension balance”.

I spend most of time on the pegs. It lowers the center of gravity and keeps me off the seat. I shift my weight around, but on average I think my head and torso seems to spend most of the time over my feet. This means that if I’m bumping along, I would like the rear end taking an equal amount of available movement as is the front. Meaning that an equal amount of suspension usage is applied to both the front and rear. This is also a good time to begin thinking about exactly what happens when a bike goes over a series of bumps, but we’ll chat about that later.

If I could measure average suspension cycles during a ride around the track, I would hope that the front and rear are getting used equally. This may be the only flaw with what I’m about to proposed, but I have yet to figure out a reason why a rider would want anything other than even usage.

So what does that mean – “even usage”? It means the front and rear suspension get used in proportion to the overall effective suspension movement. Meaning, you use the same percentage of your suspension on the front as you do in the rear. Both in sag and usage.

Here’s how it works.

On my 00 426 I measured the distance from the lower edge of the fork seal dust cap, to the upper edge of the bottom casting with the bike on the stand. Basically the distance of the shiny part. This is about 30.5 cm. I also measured the height from the top of the rear axle to the centerline of the seat bolt. This was about 64cm. When I sat on the bike, both front and rear compressed. The front end measurement became 24.5cm, and the rear became 52.5cm. This translates into 115mm of rear sag and about 60mm of front sag. This means that 19% of the total front travel goes to sag from the weight of the rider and bike, and the same for the rear at 18%. Hence my theory of balance. The bike feels great, handles very well, and most importantly, launches off of the double and triples with little need for mid-air trouble-shooting.

So what happens when I change the rear from just 18% to 16% - a difference of 5mm in rear sag? The rear begins to feels high and I begin to get a slight rear kick. If I fall below 18%, the front end feels high, and front end traction is compromised. I may have myself fooled, but perhaps I’m now a little sensitive to this coming back from nirvana.

Essentially, you could also make the same calculations from the before and after measurements taken from the floor up to the tip of the handlebars, and to the top of the rear seat.

So that’s it.

I’m not concluding anything other then perhaps a more sophisticated starting point, or at least a marker from which our unique bikes and weights can begin. Is this ideal? I don’t know. Is it better then assuming that one rider’s 115mm of sag will work for everyone? It just may. My best recommendation would be to try it and see if you like your results.

Lastly, I think there’s an argument to be made to make a balancing measurement in the difference between bike sag and rider-with-bike sag. But I’ll leave that for another posting.

Please share your thoughts.

DaveJ

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Whew!!

Seriously, that does make sense.

One comment I’ll make is that the preload or spring rate up front should affect your rear sag. If your forks were immovable objects more of your weight would be borne by the shock and your sag would increase, ceterus peribus.

Okay, I agree that there is no call to use Latin here.

If you changed to stiffer springs up front I would imagine that your rear sag would then be greater if you didn’t change anything else.

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Just when I think I'm starting to understand something like rear sag , DaveJ has to go put more thoughts and ideas in my head. Is that how fire was invented? :)

------------------

00 YZ426F

01 TT-R125L (my son's)

91 CR125

83 YZ490

74 Hodaka Super Combat(gone but not forgotten!)

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Hick - I agree (don't know how to say that in Latin). Hope I didn't imply something other in the original post.

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DaveJ,

I am under the impression that 'race sag ' is a matter of proper frame angle just as much as fork height in the clamps. This is of course something that is set in a static state but is used in a dynamic state. Spring rates, inertia, shock/fork valving, accelleration/decelleration, suspension travel angles, track conditions and of course the riders weight are all significant factors.

Changing any single one of these factors will have to make a difference in how a bike handles.

Softer fork compression will allow the front to dive more quickly into a turn, thus creating a higher chassis angle and perhaps allowing the rider to shift more weight to the front of the bike. increasing preload in the rear would also do this but of course there are side affect to both of these. Increasing rear preload is going to affect chassis angle under all conditions both static and dynamic whereas fork valving is only a dynamic factor and will not have an equal affect on handling on all parts of the track.

I think that more people should think and experiment with more of the tuning options we have available to us (myself included!!) and really get a working practicle understanding of how everything works. I personally have too much fun just riding to spend lots and lots of time fiddling with different settings but I know if I did, I would be able to go faster with less effort under more conditions. On practice days at the track, I try to make a point of spending a certain amount of time playing with suspension settings and soon jetting and gearing as well.

I usually find that even if I make a change that I dont like and then go back, I have learned something. Maybe it didnt help right then and there but it will pay off in the long run.

I am always tickled that the human element seems to be able to make up for all but the worst of suspension/handling problems given enough time and practice. Have you ever gotton on your old bike and nearly killed yourself doing things on it that you used to before you got your new bike? Somehow you adapted, albiet with more physical effort and less confidence, but you could ride it.

Seems like nearly any new change I make feels unbalanced and unnatural for a couple of laps and I try to ride through that period with that in mind before I decide that I dont like what I just did. I may just need a chance to adapt to a better way!

What was I saying again? Oh yeah, I really dont know what the hell I am talking about here, just thinking out load.

Dave S

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Dave - you're cracking me up!!!

Seems like the only time we can stop long enough to try new setups is when we are completely worn out from riding.

DaveJ

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Yea, I think it is a zen thing.

One can decide for ones self how well ones bike handles.

Whatever that means!

Its alot more like work to tune than it is to ride!

Dave S

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