Fast Guys, why do you run so little rear Sag?

I was pitted next to David Brown at Barber and my buddy asked if he could check his suspension. Rear was very stiff with almost no sag.

Walking through the paddock at Laguna and asked one of the MotoSport crew how much rear Sag they were running and he indicated 8mm.

Max McAllister recommends 30mm of rear sag.

I'm interested in learning what changes have occurred with tire / suspension technology resulting in such a significant change in rear sag. I'm also interested in learning how to determine when it is time to make the switch and what the drivers for that change are. Is it driven by lean angle or corner speed or ???

Thanks

 

are you talking rider sag, or free sag.. 8mm would sound about right for a free sag #, but 25mm is a pretty typical RIDER sag measurement.. (full extended to rider sitting on bike). But yea, it can vary ..

 

Polling the paddock and pushing on other riders' bikes will often get you headed in the wrong direction. Stick with commonly accepted numbers for sag and geometry or pay someone with experience to set your bike up with a good baseline.

I think the crew guy gave you the 1G sag number. (Bike as it sits with gravity.)

 

stactic

 

Some new set ups run very little sag #s. But don't concern yourself with that.

 

would that be the same as "free sag" Rick ?

 

Free sag, static sag, etc. On professional chassis setup logs, this is called 1G, for sag under one G of gravitational force, i.e. the weight of the bike.

8mm of rider sag would be nearly unrideable. You'd probably fall off leaving the pits.

 

I should have asked him to clarify.

So... if the stiffness felt when pushing on the rear of the bike isn't associated with Rider Sag (25mm vs 30mm) then it would have to be spring rate. If it's spring rate then I assume the answer is that spring rate is selected based on shock travel and the faster a rider goes the more cornering force is transferred to the spring thus requiring a higher spring rate.

I'm sure there are more factors involved than that but would that be an oversimplified explanation?


I'm not chasing set-up. Just trying to improve my understanding.

Thanks for the replies.

 

and understand that just pushing on the seat of most race bikes is gonna feel stiff. You can make it "bounce" much easier pushing on the tail section, there is much more leverage there. Also, the swingarm spools are usually in front of the axle, so you have reduced the leverage even further.. Off the stand, and really giving a good push is how i have seen the "stiff/bounce" test done. My 5'6" 165lb butt can't make even my bikes move much regardless.

 

Most important -Get the front end right first.

Sag is directly related to spring rate, but sag is only a starting point for spring rates, it's not a magic formula. You generally run the softest spring you can get away with - compliance and grip must be compromised. You might run an 8.5 kg/mm spring at one track and a 10.5 at another, and the amount of installed spring preload is also very important. Same rider, same bike, just different requirements for rear wheel behavior. It depends upon grip levels, tires, and the nature of the track.

Start with a spring that gives you 25-30mm of rear sag - make sure the shock is warm (do a few laps). Adjust from there.

 

with todays TTX, Vbleed Penske and DDS K-Tech the days of "seat bouncing" or even "25-30mm sag" are not absolute. like many said, there are way too many variables and shocks have drastically changed the last few years. 25-30 works for most guys/shocks but i know for a fact you dont set up the new Penske V-bleed that way.. there is no absolute number best is to find a local GOOD suspension tuner familiar with your exact shock and work with them to get it right. asking the internet not best imo.

 

100% with you on that one man, way to many fucktards that repeat bullshit read on another forum


Would anyone let a Street cleaner repair there rolex watch

 

my 2 cents
8mm rear is probably static...and CANNOT be rider sag.
seat bouncing has nothing to do with sag...it is to get a feel for compression/reb.

Sag- aka geometry needs to be set for front & rear
if you don't have your geometry right, you'll have trouble on the brakes/gas etc. and weight transfer trouble.

for example: lift your bike in the front with triple T stand
- measure fork extension
- if you let the bike sit, and measure same two points. difference is static sag or just sag.
- sit on the bike, measure same two points. difference is rider sag.

same concept applies to the rear.

Geometry depends on the bike, tires, rider weight.
usually, once set, you don't want to touch it. unless you get a different tire.

but 'feel' will change again depending on ambient temp, how hard the suspension is worked, level of grip available etc.

 

+1

 

No argument there. I understand the value of a relationship with a good suspension tech.

I'm not trying to get set-up advice via the interwebs. I'm simply trying to improve my overall understanding of the changes that are required as lap times drop. The stiffness of the rear suspension on a couple of fast guys bikes jumped out at me which led to the post.

 

Sag is not even rotely close to "AKA geometry".

 

Beat me to it. It affects geometry, but it's not how you go about setting it up. Kinda hard to change swingarm angle (for example) with preload adjustment.

 

:wow:

This is why taking advice from the interwebz can be dangerous to your health

 

Guys, i think you are being a bit over dramatic with a couple of the last few posts While sag is not exactly "AKA geometry".. sag IS part and parcel to geometry. Less or more sag, geometry changes. Is it where ride height is set with fork tube through triple clamps and shock length, no.. but it is at least a component of geometry.

i would agree. it affects geometry.

 

Well, I guess the larger point is that it shouldn't be. If you change sag and then don't change fork height or shock length to get the geometry back to where it was, then you don't have a clear idea of what's causing the resultant change in the bikes behavior. If you go from 30mm to 25mm of front sag and the bike works better, is it because of the sag change or because you now have ~2mm more trail?? This can really lead you down the wrong path.