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27,5 fork for fullsuspension E-tandem

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27,5 fork for fullsuspension E-tandem

Old 01-11-21, 10:45 AM
  #1  
longpete
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27,5 fork for fullsuspension E-tandem

I'm looking for a new fork on my new fullsuspension E-tandem. System weight 100+53+32= 185 kg (two batteries and 2200gr fork).
Calipers 18 at home , 20.3 or 22 in the mountains.


For the moment I use a 27.5+/29 boost Suntour 35 Aion 12cm. This fork builds a bit too high (54,2 cm).

Bracket higth is with sag around 32cm. My feet can't touch te ground. My inseam 97.5 cm.

Fork is stiff enough but with maximum allowed pressure of 120 PSI the sag is 5cm instead of about 3,6 cm.

I use ofroad 27.5 wheels and 2.6 tubeless tyres, onroad 29 wheels and 1.4 tyres.


Ideal is a minimum of 35 stanchions, better 36 or 38. Travel 12 or less(rear has 10cm), the fork builds then between 52 and 53cm high.

Easily adjustable travel is a +, so that the travel can be changed in the future at a reasonable price.


What came to my mind :


Pike : a little to lightweight I fear. 12 cm easy to find, Charger 2 damper ok. Model 2020 negative chamber a bit too bigt. 2021 better?

Yari: hard to find in 12cm, damper upgrade necessary taking into account the extra weight of tandem and riders.

Lyrikl en Zeb : travel starts at 15 cm, too much : fork builds to high.

Marzocchi bomber z1: 27.5 starts at 15 cm of travel but i wonder if it would be possible to mount the internals of a fox 34 which are available in 12cm.

This would be an ideal compromise I think.

Fox 36 ebike(internals fox 34) except rhythm : too expensive.

FOX 38 : way over my budget en not yet found in 12cm.


Formula selva 35 : travel 120 possible but complicated fork.


Open for other alterantives at a reasonable price point.


I'm looking for advice of tandem or real heavy (over 125kg) riders really riding one of these forks. So i can find out If

I exceed maximum allowed air pressure to get the ideal sag of 3 to 3.5cm with a travel o 120 (Pike 163, Yari a bit less, fox and Marzo : no idea)

An alternative could also be a fork with more travel and more sag than the usual 25 to 30%. I noticed this helped riding over rocks and other things higher than 15 cm.


One other question : front 2.6 tyre with 1.3 bar and tandem : precise steering gets difficult. Santana advises longer fork rake for stabilsation, while a a 37mm rake make the weelbase shorter and the turning easier.
On this E-tandem with long chainstays I don't feel the need for more stabilisation but I'd like tohave to werk less in tight turnings.


A dropper post could be an alternative to be able to touch the ground, but i fear the presuure of the stoker stem on the front seatpostmight make it difficult for the dropper to drop.

Last edited by longpete; 01-11-21 at 12:17 PM. Reason: writing mistake
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Old 01-13-21, 05:32 PM
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DangerousDanR
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I picked up an coil spring Ohlins fork (new year old) for my MTB on eBay for under $500. If I was currently looking for a new MTB fork, I would be watching eBay.

I am running 27.5X2.8 or 3.0, so I went with a fork that is intended to fit a 29er bike. It replaced a Marzocchi air spring fork which failed due to temperatures below -30F. The Ohlins is a huge improvement over the Marzocchi.

And, while I am under 125 kg, I stepped on a scale a few days ago and had a cringe moment. I was over my 100 kg target weight by a good 15 kg. When I last rode the bike I am guessing that I was about 110, but I may be fooling myself. But when I first put the Ohlins on there, I was in that 125 kg range, and it worked great. Spring preload is easy to adjust.
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Old 03-22-21, 10:59 AM
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Update :My conclusions :
*forget about fox+marzocchi for tandems : maximum allowed pressure way too low for 150 kg or more : over 40% sag, a lot of compression damping reduces sag to 36% .
I tried Ebike+ myself; only positive thing not bottoming out without adding tokens. Ebike+ allows higher pressures than normal forks.(fox not work with weights over 120kg).
*Rock Shox 35mm diameter up to
*120 mm allows pressures up to 194PSI good for 180 kg
*130-140 mm allows pressures up to 163 psi good for 160kg
*150-160 up to 148 PSI good for 160 kg(also valid for th 38 mm ZEB.
Pike 120 easy too find, but a bit too light I fear. Zeb builds too high. Lyrik might be a more stiff option but not available in 120.
A Yari and a gold 35 are heavier, but are thet also stiffer?

*Suntour Aion 35 boost works better than the fox but also too much sag ;: the manual I have says 120 PSI maximum pressure, but there are also manuals where 150 PSI is written:
150 PSI would be the solution for the right sag with the Aion.

Is there really no one else using a suspension fork on their tandem on this forum?
If there is : what fork , what drivers weight, which pressure?

Last edited by longpete; 03-23-21 at 07:21 AM.
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Old 03-30-21, 09:14 AM
  #4  
twocicle
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It is a common misconception that the max psi values provided by the mfr will apply to all travel settings. Even if you phone the customer support line, you'll likely get someone simply referring to the published max psi specifications which really only apply to the fork's longest travel setting, and you can't get them to discuss otherwise because they have legal issues they need to avoid and the guy on the phone doesn't have the authority to say otherwise. The single thing the mfr is telling you, is not to exceed the maximum peak (fully compressed) air rating for the fork achieved when the fork is set to its maximum travel length.

Explanation:

When a fork is set up with less travel the air camber is typically reduced, and so when fully compressed the maximum psi it can achieve is also reduced. For example, if you take a fork with a maximum travel length of 160mm and reduce it to 120mm, in most forks you are taking away 40mm of air camber. That is 40mm less air you have available to compress, thus the fully compressed peak psi value will be much lower than with the 160mm travel setup.

As a simple example, think of a floor pump for your tires. If you block the tire valve so no air will exit the hose (ie: attach to a tire but with the tire valve closed), when you use the full pump stroke and record the maximum air pressure you generated, you will see this is much higher than when you reduce the pump stroke to something less (ie: try 75%). Although you may now think there is some 25% more headroom (extra air) you can add to the base/starting pressure, that is not entirely accurate because air springs/shocks are not linear. However, the important point is... yes, there will be spare psi headroom available when you reduce the stroke from the longest length the mfr used to specify the max psi allowed.

Unfortunately manufacturers do not post max psi values for each travel length, nor do they post max psi values for when volume reducers / tokens are added. For this last point, here is a good webpage describing the effect... https://bikeco.com/volume-spacing-ca...r-explanation/ . I have not located a similar page describing the fork travel setting effect.

Bottom line, when your fork travel is set to less than the maximum travel, in most cases* the published maximum psi (calculated @ max travel length) can be exceeded without problem for shorter lengths.

* Forks where this may not apply are those which use fixed length separate inner air chambers instead of the fork stanchions (lowers). For example the new Fox 38 model. However, the vast majority of forks utilize the stanchions as their air chambers and so will be effected by various travel length setups.

---

Caveat/CYA Statement: The above opinion is purely logical deduction and personal experience. Assess your needs and equipment accordingly.

FWIW: This is the sort of mtb tandem riding we do (onboard cams never show trail steepness). Many addnl videos posted as well...


Last edited by twocicle; 04-09-21 at 01:34 PM.
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Old 04-23-21, 07:15 AM
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longpete
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Considering Rock Shox gold 35 :
*travel can be changed with spacers between 100 and 160.
*2300 grams :a lot of material means mostly stiffer(inner diameter of 32 forks, so more progressive and stiffer)
*price ok
*22 rototors allowed
*lockout on the bar
*damping motion control not as good as charger
*stays higher than Yari.
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