Custom-Tuned Versus Stock Mountain Bike Suspension - Vital MTB Advanced Class 21

Have you ever wondered what's possible when it comes to custom mountain bike suspension tuning? Can they really dial things in for you more than stock? Is a fully tuned setup faster? Vital MTB's Brandon Turman dives into the world of custom-tuned suspension, doing multiple back-to-back runs with side-by-side video analysis. He also did mock race runs on both a stock and tuned setup. Dig in!

Key Timestamps

  • 0:34 - Common things you can address with a custom tune
  • 1:41 - Main areas of focus while tuning (compression, rebound, air spring)
  • 4:32 - Back-to-back test setup
  • 5:13 - Slow-motion comparison
  • 8:06 - POV fork comparison
  • 9:39 - POV rear shock comparison
  • 10:52 - THE RACE
  • 12:40 - Race/ride analysis and observations
  • 19:44 - What's the bottom line?

Fork Tuning Notes

  • Added the DSD RUNT
  • Due to added support from the air spring, compression damping was lessened at the mid-valve
  • Added high-speed rebound damping for control on big impacts
  • Lighter low-speed / off-the-top rebound damping
  • Refined rebound range
  • Stock oil

Rear Shock Tuning Notes

  • Tuned to rider weight, riding style, and to balance with the fork
  • 3 vs 5 weight oil

Visit www.diazsuspensiondesign.com for additional details.


About The Tester

Brandon Turman - Age: 33 // Years Riding: 18 // Height: 5'10" (1.78m) // Weight: 165-pounds (74.8kg)

"I like to have fun, pop off the bonus lines on the sides of the trail, get aggressive when I feel in tune with a bike, and really mash on the pedals and open it up when pointed downhill." Formerly a mechanical engineer and Pro downhill racer, Brandon brings a unique perspective to the testing game as Vital MTB's resident product guy. He has on-trail familiarity with nearly every innovation in our sport from the past several years and a really good feel for what’s what.

Credit: John Reynolds, Brandon Turman, and Dylan Stucki
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21 comments
  • Fox

    3/21/2020 3:05 PM

    The Runt works quite well and does what Diaz says it does. It is a great value and a sweet upgrade that goes into your fork in about 5 minutes.

    I agree with the race vid analysis. BT pulls ahead of himself with just a few extra pedal strokes and doesn't get the 2 seconds back during the rest of the run. This highlights perhaps the most important thing in enduro and pedally DH racing, that putting down power is the easiest place to make time. Another thing about a race pace run in what looks like AZ- you better not go off the trail, cuz everything has giant thorns!

    Cool vid, thanks!

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  • boaz

    3/20/2020 10:38 PM

    This is an excellent feature, so much good info. Thanks Vital and DSD
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  • thejake

    3/20/2020 4:24 PM

    Good video. I think having dialed suspension and a bike that fits properly are the 2 biggest things in a bike riding well. For some reason a lot of people aren’t into the custom tuned suspension thing, never really understood that when people drop 4-10k on a bike but then won’t spend an extra $300-400 to get the suspension perfect. They will buy a product that they can put into their fork or shock, like the Runt, but won’t go for a full tune. The fit thing is a different conversation, drives me nuts when I see people running super wide bars with narrow shoulders. It’s ergonomics not rocket science.
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  • bizutch

    3/20/2020 3:45 PM

    Can I send you the bone stock Suntour Aion on my 2018 Kona Process 153SE and see if you can make it magically ride like my rear Cane Creek CS Coil w/ Valt progressive wound spring? To say I ride off the back now is an understatement. The back is oh so nice...but the front...hah!
    Can't afford it...but just to see if you have magic powers.

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  • mixmastamikal

    3/21/2020 4:47 PM

    Your better off just getting a different fork honestly. No sense in dropping money on that thing. But yes a good tuner can definitely make it better.
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  • bizutch

    3/21/2020 6:20 PM

    Oh it was utter sarcasm. I was joking. I enjoy the bike but cant upgrade at a reasonable price without buying both a new fork AND wheel because this fork is 100mm spaced.
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  • ebruner

    3/20/2020 11:40 AM

    I noticed that in the shock slomo and pov footage, the stock shock seems to be running the dpx2 in the medium or mid compression setting vs the tuned dpx2 seems to be running in open. Any comment or validation to that?

    Related but unrelated... Can you speak to the differences in performance between running a light compression tune with a few more clicks of LSC adjuster, vs a firmer compression tune with the adjuster more open. I realize that this is shock design specific info/commenting, so we can keep it specifically to the DPX2. I guess I'm wondering if I'm better off with a lighter tune and keeping the adjuster mostly closed so that we're forcing the shim stack to do the work, vs utilizing the built in bypass circuit that fox and so many others utilize in order to provide a range of adjust ability.

    Me personally, I'd love to run my shocks more open on compression settings, however I'm one of those people that needs the compression to push off of/into. I typically will deal with a bit of harshness in order to have the bike there when I need it to be able to launch. I can almost always tell when I've got the shock so open that it feels like a port orifice damper (mayonnaise) and this has always been a struggle for me with oem (un-tuned) suspension products.

    Lastly... going to get specific to myself here... I have a SC Megatower with the RS SD Ult. I am wanting a new shock for this bike so I can lighten the compression damping up, but still have the bike provide the support I need in the midstroke to pop off of features. What would you do for a shock and tuning. I am not concerned about dollars here... I've considered everything from custom tuning the SD to an 11/6 to a marz bomber cr and putting the fox van rc light/light tune in it.

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  • DSD

    3/20/2020 12:39 PM

    The two shocks were tested in the open mode. The levers are just clocked slightly different between the two shocks.

    As for the compression question, we're essentially opening up both compression circuits. So it will be lighter that the stock tune with the adjuster fully open. Obviously the compression tune depends on the rider weight and riding style. In most cases, you'll continue to run the compression fully open and use the air pressure and volume spacers to get the support you need. This helps maintain the suppleness. It's not likely that you would need to use the LSC compression adjustment. So for example, if you go from your normal trails to the bike park where the speeds and forces are typically a lot higher, you would add air pressure to get more support vs. adding compression which would ultimately effect the initial stroke sensitivity. Hopefully this addresses your question about having more support when you need it.

    As for your rear shock, we can definitely tune that in a similar fashion as the DPX2 in the video. You can gain a lot of sensitivity out of the rear end with the coil spring out back, and the linear rate is much more supportive in the mid-stroke. The Bomber CR is a good shock for the money, but it's limited in tuning potential. We've seeing some issues with oil bypass in the Bomber CR that we didn't used to see with the Van R/RC. We're still searching for a really good, tune-able, coil shock option that also has a climbing switch. Since you already have the Super Deluxe, let's tune it...

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  • mixmastamikal

    3/21/2020 4:53 PM

    I rode the owner of suspension syndicates bike with the runt on it and I must say wow. Well done. I would reccomend it to anyone that is looking to get more out of their fork. However I would definitely not recommend trying one if you dont have the money to buy one. You will want one. Lol. Any chance you will be making one for the 40 or boxxer?
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  • EBlackwell

    3/20/2020 9:19 AM

    I am normally one to put a fair bit of stock in timed runs, but watching this footage there is no doubt in my mind that the tuned set up would be faster over 4+ minutes. That much additional composure in the bike is going to be so much less fatiguing on the body. The heart rate spikes and additional death gripping that happens when your body gets bucked around, plus the increased chatter on the non-tuned set up would likely add up to a lot of fatigue over longer runs and big days. While timed testing on a longer track has its challenges (line choice, increased levels of rider fatigue over multiple runs, pacing, etc), I would bet the tuned set up would start really shining in that environment.
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  • DSD

    3/20/2020 9:42 AM

    We totally agree, longer runs and more runs would have ultimately shown the difference. We may have tried to pack one too many things into this whole feature, which ultimately did not leave us enough time to do a proper timed test. If we had been able to do 3 or 4 runs on each setup and take the average, we're confident we'd see a bigger difference. Also with more time on a great setup, the confidence you find, will allow you to push much harder.
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  • w4s

    3/20/2020 11:05 AM

    The reduction in arm pump is huge, too, i feel like i have much better brake control at high speed.
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  • Explodo

    3/20/2020 8:14 AM

    I'm really glad you did this video and test. I put the runt on my birthday/christmas wish list last year, but of course didn't get it. I have a question though; Do you think a fully tuned air suspension is going to ride better or just be lighter than a good coil setup? It sure sounded like a lot of what Diaz was touting as advances in your suspension capabilities were just trying to get air shocks to ride more like coils.

    At this point, I'm just thinking that maybe I need to move on to a full coil setup instead. I had a full-coil Tracer 20 years ago and loved it. The MTB industry went through years of neglecting coils, but now they're getting back to them and there are many good options for coil setups again.

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  • DSD

    3/20/2020 9:38 AM

    This is a question that we get a lot these days considering the mass offerings of coil spring conversions available on the market.

    What we would say is that for a rear shock, the coil is a really great option. Rear air shocks require tight seals to be able to handle the air pressure, especially at full compression. This in turn creates at lot of extra friction on the entire system. With the coil spring, it alleviates a ton of friction, and gives you un-matched suppleness and mid-stroke support. The limitation of the coil spring has always been being able to find the exact right spring rate for every rider. Most riders find themselves between rates, and depending on riding style and preference they end up choosing between a spring that is either too stiff or too soft. In most cases, riders can get away with the stiffer rate because it's better for the overall suppleness than dialing in the preload. Sprindex adjustable coil springs may be changing the game on this front as far as dialing in your rate. We're looking forward to testing some of their springs here in the near future.

    So, when it comes to forks, we're dealing with a totally different animal. Because of the design, especially self-equalizing air springs (Fox NA2 EVOl and RockShock Solo or DobonAir), modern forks do not require the tight seals that we see in rear suspension designs. So we ultimately have much less friction in the fork, freeing up the potential with tuning the air spring.

    With coil conversions and coil fork offerings, the same issue we see with people being between rates on the rear shock, is even tougher with fork offerings. Most companies only offer 5, maybe 6 spring rates, which makes it even more difficult for riders to find the correct rate. In turn, the companies that are making these conversions and forks are improvising bottom-out mechanisms to essentially help keep people from hitting full bottom out when their spring is too soft.

    The other issue with coil forks is that, instead of being on the outside of the rear shock body, the spring is confined to the inside of the fork stanchion. These fork springs are very long in comparison to a rear shock spring, so as the fork goes through it's travel, the spring needs room to move around, which adds more friction to the system. Visualize pushing rope. To be perfectly clear, coil forks can be a great option for some people, but the likely-hood of nailing the spring rate for the rider weight is slim.

    So, with the RUNT, you can actually take the linear rate of a coil spring, and dial it in to the exact rate you need for your weight, riding style, preference, and the terrain you're riding. Depending on the accuracy of your shock pump, this means you can adjust the L chamber in 2.5 psi increments, which we have found makes a noticeable difference.

    We have had a number of customers that are always trying the latest and greatest go from being on a RUNT, to trying numerous coil conversions, and ultimately ended up back on the RUNT for all the reasons above.

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  • Vorsprung

    3/22/2020 12:20 AM

    Very cool video guys. The Runt is a solid product for sure. However, I would respectfully disagree with a lot of the assertions made in this particular post:

    What we would say is that for a rear shock, the coil is a really great option. Rear air shocks require tight seals to be able to handle the air pressure, especially at full compression. This in turn creates at lot of extra friction on the entire system. With the coil spring, it alleviates a ton of friction, and gives you un-matched suppleness and mid-stroke support.
    Obviously, air springs have more friction than coils. However, the frictional values of rear shocks are generally lower overall than forks - they only have one wiper/shaft seal instead of two, and the squeeze on the air seals is actually comparable to what it is in forks, though at lower pressures and surface areas. However having measured this, once the additional leverage of the linkage is taken into account, the frictional value at the wheel is far lower on the rear than that front. On top of that, the additional weight on the rear wheel and the lower sensitivity to impact of people's feet as compared to their hands makes fork friction a way bigger concern than shock friction. This isn't just speculative, this is measured on the dyno (well not the hand/foot pain thing, people don't seem to like being crammed into the dyno for some reason!).

    The limitation of the coil spring has always been being able to find the exact right spring rate for every rider. Most riders find themselves between rates, and depending on riding style and preference they end up choosing between a spring that is either too stiff or too soft. In most cases, riders can get away with the stiffer rate because it's better for the overall suppleness than dialing in the preload. Sprindex adjustable coil springs may be changing the game on this front as far as dialing in your rate. We're looking forward to testing some of their springs here in the near future.

    I would not agree that the spring rate needs to be as precise as you're making out here, nor that the limitation of coils has been finding the right spring - if that were the case, there'd be the same problems with rear shocks as there are with forks, because the typical 50lbs/in increments are approximately equivalent to the 5lbs/in increments that fork coils are usually offered in (eg with the Smashpot). In my opinion the limitations of coil springs in forks come from the fact that you have a dead linear motion ratio between the wheel and the spring/damper the whole time, whereas with frames there's the variable leverage ratio that can be used to control progression. That means many people end up overspringing the fork JUST to prevent bottoming out, when the ideal spring rate would be lower 99% of the time and they're compromising that for the other 1%.

    So, when it comes to forks, we're dealing with a totally different animal. Because of the design, especially self-equalizing air springs (Fox NA2 EVOl and RockShock Solo or DobonAir), modern forks do not require the tight seals that we see in rear suspension designs. So we ultimately have much less friction in the fork, freeing up the potential with tuning the air spring.
    Agian, not actually the case - rear shocks are also self-equalising, that has no effect on the necessary seal squeeze or friction. Forks do have smaller surface areas and lower pressures, both of which do contribute nonlinearly to frictional values, but again they lack the leverage and weight that the rear end has, so they still come out behind. I mean, easy way to tell even if you can't measure it - how often do you hear people complaining about sore hands compared to sore feet?

    With coil conversions and coil fork offerings, the same issue we see with people being between rates on the rear shock, is even tougher with fork offerings. Most companies only offer 5, maybe 6 spring rates, which makes it even more difficult for riders to find the correct rate. In turn, the companies that are making these conversions and forks are improvising bottom-out mechanisms to essentially help keep people from hitting full bottom out when their spring is too soft.
    The idea of anti-bottoming systems is not to compensate for a spring that's too soft, but to open up the range of useful spring rates so that preventing bottom out is not a tuning consideration. If the spring rate that's optimised for 99% of your riding time happens to be soft enough that you can bottom the thing out hard from time to time, it makes more sense to simply prevent the hard bottom outs than to compromise the spring rate the rest of the time. We also offer 11 spring rates with the Smashpot, though I realise a lot of OE spring options are much more limited.

    The other issue with coil forks is that, instead of being on the outside of the rear shock body, the spring is confined to the inside of the fork stanchion. These fork springs are very long in comparison to a rear shock spring, so as the fork goes through it's travel, the spring needs room to move around, which adds more friction to the system.
    It doesn't actually add friction or stiction - it creates (very) small nonlinearities in the spring rate, that are measurably tiny. There is no additional stiction because the spring itself can still flex in spite of whatever guides it's contacting, until the spring surface slips (which usually happens within such a tiny displacement, like microns, that it's effectively non-existent anyway)

    Visualize pushing rope. To be perfectly clear, coil forks can be a great option for some people, but the likely-hood of nailing the spring rate for the rider weight is slim.

    So, with the RUNT, you can actually take the linear rate of a coil spring, and dial it in to the exact rate you need for your weight, riding style, preference, and the terrain you're riding. Depending on the accuracy of your shock pump, this means you can adjust the L chamber in 2.5 psi increments, which we have found makes a noticeable difference.

    Dual chamber systems like this are not actually truly linear though - they're digressive by nature in the middle of the stroke (which can be a good thing compared to a standard air spring, I would agree) and also falling rate by nature at the start of the travel as all air springs are. The stiff initial rate still doesn't compete with a coil in terms of that initial compliance, for any given mid-stroke spring rate.

    It's worth noting as well that we don't really have a dog in the coil vs air fight, since we make products to suit both. Both have their strengths and weaknesses.

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