Ask Pinkbike: Long-Travel vs Short-Travel Trail Bike, Measuring Fork Stiffness, Heavy Tires or Inserts?

Author : kacaumantenmanten
Publish Date : 2021-04-09 12:57:31


Ask Pinkbike: Long-Travel vs Short-Travel Trail Bike, Measuring Fork Stiffness, Heavy Tires or Inserts?

Here at Pinkbike, we get inundated with all kinds of questions, ranging from the basic "Can I have stickers?" to more in-depth, soul-searching types of queries like if you should pop the question or what to name your first child. Ask Pinkbike is an occasional column where we'll be hand-picking and answering questions that have been keeping readers up at night, although we'll likely steer clear of those last two and keep it more tech-oriented
Can I set up a 160mm bike to ride like a 140mm bike?

Question: Jhomas.toseph asks on Instagram: If you've got two bikes with the same leverage curve, but one's short travel and the other is longer (say 141mm vs 161mm travel) would it be possible to replicate the feel of the short travel bike simply by increasing the spring rate on the long travel bike? I'm trying to choose between the Privateer 161 and 141, which look to be almost identical apart from the travel numbers. I don't ride flat out fast trails very often, so would generally prefer a firmer more supportive ride. However, I'm wondering how much of the short travel feel I could achieve on the 161 by running higher shock/fork pressures, whilst still having the option of a plusher ride with deeper sag for bikepark/DH laps. Any help would be very much appreciated! Cheers!
I think the basic answer is yes. If you were to run 26% sag on a 161 and 30% sag on a 141, both will have 42mm of sag. And barring any significant differences in the leverage curve or the shock, that would mean they'd have the same spring rate. Obviously air shocks are non-linear so there will be a difference as the 141 gets towards the end of its travel, but in the normal "pedaling region" (say the first 100mm of travel or so) the wheel rate and therefore the feeling of support vs harshness will be pretty similar. You could add volume spacers to the 161 to get it to feel almost identical. You may never use all the 161's travel when set up like this, but that's not necessarily a bad thing - you could just think of the last 10-20mm of travel as "for emergencies only".

Normally, the problem with running less sag than the bike is designed for is it will give you a higher sagged bottom bracket height and steeper dynamic geometry. But that may be exactly what you're after for flatter, less demanding trails. Plus, the Privateer 141 and 161 have near-identical bottom bracket heights and head angles, so you'd be getting the same sagged geometry anyway if you ran the same number of millimetres of sag.

One difference is the effective seat tube angle, where the 161 measures 80- degrees and the 141 measures just under 79-degrees. If you ran both bikes at the same percentage sag, the extra travel of the 161 would cause it to sit at about the same angle once sagged, but if you set the same absolute sag, it'll be a little steeper. That's not necessarily a bad thing though, and you can always slide the saddle back on the rails if you find it too steep.

Personally, I run about 28% sag on the 161 and I find it to be a very comfortable climber. The steep seat angle makes it feel very purposeful on steep climbs and the high anti-squat suspension means its efficient under power too. It's no cross-country racer, but it pedals well and the 141 is barely lighter.
Good question. Manufacturers and reviewers often don't specify exactly what they mean by "stiffer", but you can measure fork stiffness in three ways: Fore-aft stiffness (how much does the axle move back and forth for a given force); torsional or steering stiffness (how much does it twist along the steering axis if a torque is applied between the axle and the stem); and lateral (how much does the axle move sideways for a given lateral sheer force). Manufacturers sometimes choose not to specify which they're referring to in order to make their fork seem better. For example, inverted forks are usually stiffer fore-aft but less stiff laterally and torsionally; their manufacturers occasionally forget to mention these last two metrics when talking about stiffness.

https://acg.instructure.com/eportfolios/37492/Home/4k
https://acg.instructure.com/eportfolios/37494/Home/___Super_Me_HD_
https://acg.instructure.com/eportfolios/37495/Home/
https://acg.instructure.com/eportfolios/37496/Home/HD___Super_Me_UHD__1080p
https://www.guest-articles.com/business/ask-pinkbike-long-travel-vs-short-travel-trail-bike-measuring-fork-stiffness-heavy-tires-or-inser-09-04-2021

Where in the fork chassis most of the flex occurs is hard to answer, but I think the basic answer is "everywhere". Forks are carefully designed using Finite Element Analysis to maximize their stiffness-to-weight ratio, so it wouldn't make sense to have one component in the chain connecting the axle to the frame that was much stiffer - or much less stiff - than all the others. If you look at slow-motion footage of forks flexing, there appears to be some visible flex in all the components (crown, stanchions and lowers).
Heavy inserts or heavy tires?

Question: A nice chap I got talking to on the trails a few weeks ago asks: I get punctures occasionally. Should I bother getting a set of inserts like CushCore, or just heavier-duty tires like DoubleDown casing ones instead of EXO (which has a similar weight penalty)?
It depends on what's causing you to puncture. If it's sharp rocks like flint or slate cutting a hole in the casing, an insert isn't going to help at all. If it's pinch flats, then an insert should reduce the risk of those dramatically, but I can tell you from bitter experience that they're not 100% effective. And remember, if you do puncture and you can't fix the tire with a plug, you'll have to carry the sealant-soaked insert home with you. An insert should help protect the rim from damage to a greater extent than a thicker-casing tire, but again, I can tell you this doesn't always work - I once destroyed a SRAM Roam 60 carbon wheel in the middle of an Alp with CushCore installed after hitting a hidden rock at speed.

The difference in weight between a Maxxis EXO and DoubleDown tire is about 200-250g, while CushCore weighs about 265g per end in 29". However, the thicker DD casing doesn't just add weight, it will also increase rolling resistance. This is because the stiffer sidewall saps more energy when conforming to the ground at the contact patch as the tire rolls along. This difference in rolling resistance is probably more noticeable than the difference in weight between EXO and DD tires in terms of climbing speed. CushCore, on the other hand, is claimed to maintain or even slightly reduce rolling resistance, so the EXO tire with CushCore will likely be a faster setup.

I'd also say that, in the case of CushCore especially, there's a noticeable benefit in terms of ride feel and traction. The tire is more damped, so it skips off the ground less over rough terrain and offers more traction and comfort than a standard EXO casing. A stiffer tire casing will provide more damping too, but with CushCore there's less of a downside in terms of the "wooden" feel over small bumps that you get with stiff tire casings. I've also enjoyed Rimpact's inserts, which offer some of this damping benefit, though to a much lesser extent, at around 100g per wheel. As far as I can tell, Nukeproof ARD, Huck Norris, RockStop and Flat Tire Defender have no real impact on ride feel.

Obviously thicker tires are cheaper than thin tires plus inserts, but some of the more robust inserts (again, CushCore is one of the most long-lived options I've tried) should outlast several sets of tires, and if it saves you from destroying one set of tires in its lifetime, it's practically paid for itself.

 



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