[jscottyk]

@jcmonty has discussed his impressions running both of these on the Push HC97 thread and the DSD Runt thread. In the spirit of good forum etiquette and not duplicating a question on two different threads, I thought I would start a place for discussing tuning setups using both.

Seem like these two pieces will play together nicely but I'm curious about rebound damping. Both come with rebound damping recommendations and I'm sure they were not made considering the other.

Anyone else considering using both the HC97 and Runt?

And thanks @jcmonty for kicking us off!

 

[294037]

The push rebound tunes are almost identical to what I've experienced so thats what I would recommend, the dsd tunes are way way harder and I can't think of any good reason why they would work better

 

[gregnash]

I agree with JohnnyC7 and would go with the PUSH rebound tune as it is specific to their product that is modifying the damper side. From there, for actual tuning of the rebound I would go through Dougal's frequency tuning guide to get the tune for the rebound more dialed. Remember that tuning of all things is going to be very specific to your riding style, terrain, weight, bike, etc. so these are nothing more than guidelines to start with.

 

[jscottyk]

Good stuff. Thanks for the input folks. I'm starting to realize I need to educate myself more on the two schools of thought on rebound valving. It seems one side comes at it more from a moto perspective and the other from a more pure bicycle perspective.

 

[jscottyk]

I agree with JohnnyC7 and would go with the PUSH rebound tune as it is specific to their product that is modifying the damper side...

Thinking about this more, it's my understanding (that could be totally wrong) the rebound damping rates are most directly related to spring rates. Considering that, wouldn't DSD rebound recommendations be more applicable since the RUNT is impacting spring rate?

 

[gregnash]

Thinking about this more, it's my understanding (that could be totally wrong) the rebound damping rates are most directly related to spring rates. Considering that, wouldn't DSD rebound recommendations be more applicable since the RUNT is impacting spring rate?

See my understanding is the opposite, but I could be wrong.
In my mind the easiest way to explain this is by turning the rebound all the way up (slowest) and then pushing on the fork. If what you were talking about was true, then doing so in the damper would cause the rebound to slow but not to the significant amount that is exhibited by the change you instituted. While the progressiveness or linearity of the air side does play a role in it, the main affect is derived from the flow rate associated with the rebound tune.

 

[jscottyk]

I'm a completely noob about this topic but my understanding is coming from a couple recent posts.

@Dougal mentions "Rebound damping depends on mass and spring rate but more heavily on spring rate. So two riders the same weight but one running more air pressure, expect him to need more rebound damping." in this post.

Also this video was recently referenced.

Especially 5:10 to the end.

 

[mullen119]

See my understanding is the opposite, but I could be wrong.
In my mind the easiest way to explain this is by turning the rebound all the way up (slowest) and then pushing on the fork. If what you were talking about was true, then doing so in the damper would cause the rebound to slow but not to the significant amount that is exhibited by the change you instituted. While the progressiveness or linearity of the air side does play a role in it, the main affect is derived from the flow rate associated with the rebound tune.

Rebound is directly related to spring rate.

Unlike compression, rebound force is fixed. The maximum shaft pressure a fork/shock can have on the rebound stroke is the whatever the spring can produce. Rebound tunes recommendations(like from push) are based on rider weight because heavier riders have stiffer springs to accommodate their weight. Heavier rider, stiffer spring, more damping needed to control the rebound action.

Spring curves play a factor as well. Extremely progressive set ups have a lot of spring energy store up near full compression, but not a lot of spring energy stored in the first half of the stroke.. These set ups can be hard to tune for as they can be hard to find a good balance between low/mid speed rebound damping and high speed rebound damping.

Speaking of which, the rebound speeds being sensitive to the spring makes them position sensitive. This is why Rock Shoxs refers to HSR as ending stroke rebound and LSR and beginning stroke (in products that have external HSR adjustments). HSR requires deeper stroke impacts to build enough spring pressure to achieve HSR shaft speeds. (LSR and MSR can happen at any point in the stroke as rider weight bias and G- loads can cause lsr events deep in the stroke, but you won't get HSR at the beginning of the stroke as the spring force isn't there)

 

[Dougal]

I'm a completely noob about this topic but my understanding is coming from a couple recent posts.

@Dougal mentions "Rebound damping depends on mass and spring rate but more heavily on spring rate. So two riders the same weight but one running more air pressure, expect him to need more rebound damping." in this post.

Also this video was recently referenced.

Especially 5:10 to the end.

So yeah, it gets more complicated before it becomes clearer.

You have low speed rebound which controls body and chassis weight at the frequency dictated by the spring-rate and sprung mass.
You have high speed rebound which controls wheel motion at the frequency dictated by the spring-rate, unsprung mass and damping.

Rebound damping is doing two very different jobs with rapid switch between the two.

That video has made everyone a lot more sympathetic towards suspension tuners. Great job by Jordi.

 

[jscottyk]

So yeah, it gets more complicated before it becomes clearer.

You have low speed rebound which controls body and chassis weight at the frequency dictated by the spring-rate and sprung mass.
You have high speed rebound which controls wheel motion at the frequency dictated by the spring-rate, unsprung mass and damping.

Rebound damping is doing two very different jobs with rapid switch between the two.

That video has made everyone a lot more sympathetic towards suspension tuners. Great job by Jordi.

Thanks for that Dougal. Any opinion on the original question? Set rebound damping based on the Push HC97 recommendations? Or on DSD RUNT recommendations?

 

[Dougal]

Thanks for that Dougal. Any opinion on the original question? Set rebound damping based on the Push HC97 recommendations? Or on DSD RUNT recommendations?

Neither. I don't know what DSD recommend for rebound but my rebound tuning scheme and results are very different to Push.

 

[PUSHIND]

When it comes to suspension tuning you have two choices:

1. Trial and error. Ultimately not having an understanding of exactly what changes resulted in where you ended up.

2. Suspension Engineering. This involves on-board data loggers and suspension dynamometers providing precise data to even the most minute detail to the changes you made.

We choose number 2 because we don't like to guess:

Darren

 

[gregnash]

Ok that makes sense now.. because ultimately if the "spring side" did not have any air in it then the rebound would only travel as far as it physically was allow, if at all.

One of these days I will be able to wrap my head around it..

 

[jscottyk]

Thanks for the discussion folks. This really help me get my brain around this.

You have low speed rebound which controls body and chassis weight at the frequency dictated by the spring-rate and sprung mass.
You have high speed rebound which controls wheel motion at the frequency dictated by the spring-rate, unsprung mass and damping.

Considering this, in most MTB front suspension are both the LSR and HSR controlled with a single rebound valve stack?

Sounds like the HSR approach would be very different for bicycles considers how much lighter the unsprung mass is compared to moto.

 

[Dougal]

Thanks for the discussion folks. This really help me get my brain around this.

Considering this, in most MTB front suspension are both the LSR and HSR controlled with a single rebound valve stack?

Sounds like the HSR approach would be very different for bicycles considers how much lighter the unsprung mass is compared to moto.

Yes you need to have one stack and bypass controlling both.

It's the ratio of sprung to unsprung that messes with things. We don't have a rigid mass of sprung weight like moto do. Yet moto tuning schemes are throughout mtb with results I don't like.

 

[gregnash]

So Dougal et al..
What would a process look like for getting this setup? Aside from your frequency tune article.

 

[Dougal]

So Dougal et al..
What would a process look like for getting this setup? Aside from your frequency tune article.

The low speed and high speed rebound basics are covered in that same article: https://www.shockcraft.co.nz/technical-support/setup-guide
It was difficult to condense it enough to fit on one sheet.
Beyond that it gets quite involved very quickly. You've got several spring-mass-damper systems all trying to work together and finding one rebound damping curve that will give you the best result for all.

The moto and push settings give much slower rebound that will limit overshoot to one small cycle while you are sitting on the seat and holding the bars with your arms locked. Treating your body weight the same as a solid motorbike chassis.

The downside to that tuning scheme is it overdamps the response of the unsprung wheels, slows the suspension frequency and limits the ability to follow rough terrain at speed. It feels nicely smooth and controlled on rolling trails with G-outs where the chassis is thrashing up and down. Especially when seated.

 

[jscottyk]

Reporting back with something I heard from DSD today.

"I haven't had anyone running the push conversion yet but it should mesh fine with the runt reducing compression damping in the charger 2. We typically run more high speed rebound than push because we did not come from the moto side of tuning. we have a more bike specific tune."

"The (DSD) rebound re valve is increasing the Rebound damping per your body weight / air pressure. The HC97 Push has a rebound tune / shims but it is still on the light side. use our rebound tune and bolt on their upper half."

So, there's that. I'll talk through things with the shop that will do my install (Canyon Cycles in Dripping Springs) and report back.

 

[Dougal]

Reporting back with something I heard from DSD today.

"I haven't had anyone running the push conversion yet but it should mesh fine with the runt reducing compression damping in the charger 2. We typically run more high speed rebound than push because we did not come from the moto side of tuning. we have a more bike specific tune."

"The (DSD) rebound re valve is increasing the Rebound damping per your body weight / air pressure. The HC97 Push has a rebound tune / shims but it is still on the light side. use our rebound tune and bolt on their upper half."

So, there's that. I'll talk through things with the shop that will do my install (Canyon Cycles in Dripping Springs) and report back.

So is DSD is running more HSR damping than push or more HSR speed? Sounds like they are increasing damping even more than Push do. Which is getting really interesting.

 

[Jesse Hill]

So is DSD is running more HSR damping than push or more HSR speed? Sounds like they are increasing damping even more than Push do. Which is getting really interesting.

So would this mean so far only you and Rockshox seem to think they didn't have a fast enough HSR in the Charger 2 (since the ports got enlarged in 2.1). Seems odd how if spring rate is the only factor for rebound how can everyone be so far off from each other. According to multiple articles RS did a lot of actual data analysis and blind tested changes when retuning the 2.1 so unless they're lying then they didn't just do guess work on the changes either. So far still no review of both a HC97 and 2.1