Full Suspension Bikes - PSI in Front and Rear?

Shad6Bones · Jun 22, 2006

What formula do you guys use?

A friend of mine told me to put 75% of your body weight in the front and rear suspensions...

As a 175 pounder... That puts roughly 130-135 PSI in the front and rear suspension...

135 in the front seems like too much... And 135 in the back seems like not enough (for me).

Looking for your thoughts.

Thanks,
S6

Speedub.Nate · Jun 22, 2006

Ain't so easy!

Some fork manufacturers use high pressure / low volume air chambers. Others use the opposite. Some frames use high linkage ratios. Others low.

With few exceptions, any recommendations in the shock or fork or bike manual are ballpark figures at best, or otherwise pure rubbish.

Setup by sag instead. Rules of thumb are 25% sag for XC use, 33% or more for FR & DH applications. Again, rules of thumb -- frame manufacturer suggestions or recommendations from other owners are better.

Figure out what pressure you need to hit your desired sag, and make a note of it. That simple! From there, play with your pressure settings in 5 or 10 psi increments in either direction and try to make sense of the differences.

Good trails.

Shad6Bones · Jun 22, 2006

Thanks for your tips...

I also have a lefty...
(as pictured)

-S6

Psycho Mike · Jun 22, 2006

I agree with Speedub...

In setting up my FS, I used the recommendations as a "ballpark" to start the process, but it has been sag from there.

Just make sure that once you have "the" settings you like, write them down. You never know when something will happen and you'll need to set up a fork or shock again for whatever reason.

uktrailmonster · Jun 22, 2006

As others have said, relating shock pressure to your body weight is a waste of time. It will be different for every bike and shock combination. Work off suspension sag, starting with around 25% of travel at both ends.

Shad6Bones · Jul 17, 2006

Sag is?...

Dawg Fu · Jul 17, 2006

Sag is: put the bike next to a wall/shelf/railing/etc. so that you have some support. Get onto the bike gently without bouncing, get into your usual riding position and gently backpedal a few revolutions without any bouncing up and down. Stop and note (you can use a marking pen, or a zip tie or rubber band or string tied around your shocks) how far into their travel your front and rear shocks went. That percentage is the current "sag" in your shock setup.

Ricko · Jul 17, 2006

Shad6Bones said:
Sag is?...

...The amount that your bike will settle into it's travel when you sit on the bike suited up and ready to ride.

I usually put a zip-tie around a fork stantion to measure fork sag and measure a point between seatpost clamp and the ground to measure rear sag (have a friend help you to measure with and without you sitting on bike).

Shad6Bones · Jul 17, 2006

Will determining sag be a guestimate based upon ones visual assessment?

Everyone is recommending a 25 percent sag... so when i am seated on my full suspension bike i should only be showing 3/4 of the suspension that is left?

If this is the case...

I am going to have to let a lot of PSI out of my Lefty to have 25 percent sag when I sit on my bike... I've got it pretty firm (where I have to bounce on the bike to watch it move)... Also, my rear sus I will need to firm up pretty good to keep it at 25 percent sag...

Is this normal? A lot of PSI in rear sus and less in front??

S6

tonyjl · Jul 18, 2006

Psi?

Hi guys/girls.

Just joined the forum,a lot of great advice/tips in here,keep it up guys/girls.

My first post,i thought i'd kick-off by helping someone out.

Hi Shad6bones,the way i've always setup my sussers,is start with half my body weight up front,my full body weight at the rear. eg i weigh about 150 pounds,so up front i set to about 75psi and at the back about 150psi.
I then do some tweaking to get the sag,to setup the front,measure the actual travel of forks (manufacturers exaggerate) sit on ya bike and get into the attack position (eg out of saddle,chin close to stem etc) and adjust the pressure to acheive about 25% sag of actual travel when ya in this position. To set the rear,sit on ya bike,feet on pedals and sit bolt upright with ya arms hanging down by ya sides. (don't hold ya handle bars) and set sag to about 25% of actual travel.
When i've done all that,i go a good ride that includes lots of different elements,uphill,downhill,singeltrack,drop-offs etc, and just tweak the air pressures a little if needed,to make the bike feel good for ME.

Hope that helps mate,happy trails.

uktrailmonster · Jul 18, 2006

tonyjl said:
Hi guys/girls.

Just joined the forum,a lot of great advice/tips in here,keep it up guys/girls.

My first post,i thought i'd kick-off by helping someone out.

Hi Shad6bones,the way i've always setup my sussers,is start with half my body weight up front,my full body weight at the rear. eg i weigh about 150 pounds,so up front i set to about 75psi and at the back about 150psi.
I then do some tweaking to get the sag,to setup the front,measure the actual travel of forks (manufacturers exaggerate) sit on ya bike and get into the attack position (eg out of saddle,chin close to stem etc) and adjust the pressure to acheive about 25% sag of actual travel when ya in this position. To set the rear,sit on ya bike,feet on pedals and sit bolt upright with ya arms hanging down by ya sides. (don't hold ya handle bars) and set sag to about 25% of actual travel.
When i've done all that,i go a good ride that includes lots of different elements,uphill,downhill,singeltrack,drop-offs etc, and just tweak the air pressures a little if needed,to make the bike feel good for ME.

Hope that helps mate,happy trails.

Welcome to the forum!

The first bit about starting with half your bodyweight in the front and full bodyweight in the rear is irrelevant. For a start your bodyweight is not measured in psi (pounds per square inch). Secondly, different forks and shocks work with different pressures for the same weight acting on them. Thirdly, frames have different leverage ratios on the rear shock, which also affects shock pressure. So there is no fixed relationship between body weight and fork/shock pressure for every bike and shock.

The rest is fine, just measure the sag accurately and make a note of the shock pressures it took to achieve it. 25% is also a very rough figure, some bike designs work better with well over 30%, some as little as 15%. There's also a degree of personal preference involved. Start with 25% at both ends, ride it like that and see how it goes. If you're not getting close to full travel on a long ride, try it a little softer the next time. If it bottoms out too much, stiffen it up a little. It's not rocket science

fsrxc · Jul 18, 2006

Shad6Bones said:
Will determining sag be a guestimate based upon ones visual assessment?

Everyone is recommending a 25 percent sag... so when i am seated on my full suspension bike i should only be showing 3/4 of the suspension that is left?

If this is the case...

I am going to have to let a lot of PSI out of my Lefty to have 25 percent sag when I sit on my bike... I've got it pretty firm (where I have to bounce on the bike to watch it move)... Also, my rear sus I will need to firm up pretty good to keep it at 25 percent sag...

Is this normal? A lot of PSI in rear sus and less in front??

S6

Often yes, usually rear shocks run 100-200psi, but fork psi varies a lot, depending on the design. Get the sag around 25% (maybe slightly less sag in the front than the rear) and see how it rides (that's what really matters), adjust on the trail as needed. It helps to use the o-ring on the shock and a ziptie on the fork stanchion (not too tight so it doesn't scratch) so you know your max travel used, helps with tuning.

Full Suspension Bikes - PSI in Front and Rear?

Shad6Bones

Speedub.Nate

Shad6Bones

Psycho Mike

uktrailmonster

Shad6Bones

Dawg Fu

Ricko

Shad6Bones

tonyjl

uktrailmonster

fsrxc

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