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Ask Pinkbike: Tire Sag, Negative Volume Spacers & Bearing Lifespan?

Aug 5, 2021 at 9:56
by Seb Stott  

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.




Tire sag

Question: PDXooo asks: I've noticed that when people ride with low tire pressures there's a noticeable amount of "tire sag". Won't such sagging cause a wheel to be\feel smaller than its actual size? In other words, isn't using lower tire pressures effectively reducing the overall diameter of a wheel? Honestly, I feel as if MTB riders are encouraged to use 29 wheels to counteract the loss in effective wheel diameter due to running the encouraged lower tire pressures!

I've noticed that when I sit on my bike, my tire profile at the contact patch is essentially flat - does this fact not make the profile of a tire, weather round due to a small inner rim width or flat\square due to a wide inner rim width......irrelevant?



bigquotesIt's great to see you're thinking about the tire in suspension terms like sag. Sag is the amount the tire (or suspension) deforms vertically under rider weight. Stiffness is the ratio of the deformation to the force applied, so sag is a good measure of the stiffness relative to the weight of the rider, and the stiffness defines how easily the tire/suspension will be able to absorb bumps,

A mountain bike tire sags by somewhere in the region of 15mm when set at an appropriate pressure. The outer radius of a 29x2.5" tire is about 375mm, so this sag reduces the effective radius by around 3.9%. For one thing, this reduces your effective gearing by 3.9% compared to an infinitely stiff tire, so this needs to be taken account if you wanted to calculate your gear inches very precisely. But a 27.5" wheel is about 5% smaller than a 29" wheel, so any reasonable variation in tire sag is not going to overcome this difference.

Furthermore, more tire sag makes the ride smoother over rough terrain (despite the smaller effective wheel radius) by reducing the tire stiffness and allowing it to deform more easily over bumps, so it definitely wouldn't be a good idea to reduce tire sag in a 27.5" wheel to make it feel more like a 29er. I actually did a test years go comparing 3" tires on a 27.5" wheel to 2.3" tires on a 29" wheel, such that the overall wheel diameters were almost the same. The 3" tires were dramatically smoother and offered more grip and less harshness on rough, rooty or loose trails. They actually rolled faster too on bumpy ground, though they were slower on tarmac. On a vaguely related note, this article found that four different sizes of the same road bike tire had near-identical roiling resistance when setup with the same amount of tire sag, showing that it's an important metric for rolling resistance too.

As for the second part of the question, you're correct that at the contact patch, when using any reasonably low pressure, almost the whole width of the tread pattern is pressed flat against the ground. This is worth remembering when you see some tire reviews talking about an unpredictable loss of grip as the tire transitions from the center tread to the shoulder tread when leaning into a corner - unless you're running very high pressures, the shoulder tread is in contact with the ground the whole time.

However, that doesn't make tire profile and tread wrap (how far round the sidewalls the tread goes) irrelevant. More tread wrap (a rounder tire) means the shoulder tread can grip better and more predictably at very high lean angles (this could help explain why some pro riders are still using 25mm rims); on the other hand, a squarer tire profile will create a wider contact patch, where more of the shoulder tread is in contact with the ground at lower lean angles, which may mean more grip when the bike is closer to upright. This explains why tires designed for low grip-conditions are squarer than dry-weather tires, which are designed to excel at higher lean angles. Motorcycle tires have a very round cross section to cope with lean angles as high as 45-degrees. 

Schwalbe 27.5 images
This image shows the elliptical shape of a tire's contact patch. Bigger tires have a wider, larger contact patch.
Note that the tread of the same 2.35 tire flattens out but remains about the same width while the casing grows from 56mm to 62mm as the rim grows from 22.5 to 40mm..
With the same tire on a wider rim, the tread pattern gets more square. This may not be ideal for high lean angles unless, like Maxxis' Wide Trail tires, the tire is designed for wider rims with more tread wrap.



Whistler SRAM tuning camp
If you remove the air sleeve from a RockShox Monarch shock, you'll find space to add red volume bands. Adding bands above the black ridge (as shown) makes it firmer at the end of the travel, but adding them below the ridge (nearer the damper shaft) makes it firmer at the start of the travel, which might be helpful in this case.

Negative volume spacers for a light rider?

Question: Becka asks: I'm not using anywhere close to all the travel on the rear of my 2015 Canyon Spectral even with 35% sag and no volume spacers. If I run more than 35% the climbing becomes terrible. The rear feels too harsh and I'm getting bounced around on rough descents. I weigh 56kg and I have a RockShox Monarch shock with no volume spacers. I've heard that a damper re-valve could help but I will be buying a new bike soon so I don't want to pay out for a special super-light shock tune.


bigquotesThat's a pretty common problem because most bikes have the same damping tune for all frame sizes. Since you're at the lighter end of the spectrum, the shock/bike isn't perfectly engineered for you. The ideal solution might be to get a lighter compression valving tune, but that's not cheap and if you're planning to sell the bike soon it might not be worth it.

I have a cheaper suggestion. In your situation it's definitely appropriate to remove all the volume spacers from the positive chamber (as you have already done), but you could also try adding spacers to the negative chamber. This will make the suspension a bit stiffer in the start of the travel (which isn't ideal), but it will make the spring softer later in the travel without increasing the sag. This will make it more forgiving on bigger hits while also helping the bike sit higher in its travel when climbing.

When you remove the air can sleeve from the Monarch shock, you'll see it has two spaces for volume bands: one above the step in the middle and one below. Adding bands to the top of the chamber (positive side) will make it firmer towards the end of the travel, but adding them to the bottom (negative) part will make it firmer in the beginning of the travel. Normally that wouldn't be something I'd recommend, but in your situation it should help open up more travel without increasing sag. It's not a perfect solution, but if you have some bands lying around it's free to try, easy to do and easily reversible. 
Effect of negative chamber volume length in the air spring curve. Bigger negative chambers EVOL Debonair Vivid are more sensitive in the first 1 3 of the travel and have more mid stroke support. Thus they need more force to fully to bottom-out.
This graph by Andrextr shows the calculated spring curves for different negative volumes at the same sag. Decreasing the negative volume makes the spring firmer before sag, but softer after sag. This will allow you to use more travel and should make the suspension less harsh in some situations.




Big clean up job at the Norco tent with a dash of new bearings.

How to make frame bearings last longer?

Question: Adam asks via Instagram: My Whyte S-150 likes to eat pivot bearings and it's getting hungrier. Even with fancy bearings there's slack in the main pivot after a month or so. Your thoughts please?


bigquotesMy first piece of advice would be to buy decent bearings, as you have done - cheap ones are a false economy and result in the headache of changing them out more often.

A month is really poor innings for a set of decent bearings though. I'd first check you've done the pivot bolts up to the correct torque (you may need to ask the manufacturer what this is). Also, make sure you haven't lost any of the fiddly little washers which sit next to the bearings and provide the correct spacing (this is easily done!)

Assuming you've done that, you could try packing the bearings with grease. The bearing manufacturer doesn't know these bearings will be used in a frame, where they'll only ever rotate back and forth by a few degrees, while potentially being exposed to high loads and water/dirt ingress. They're designed to be able to cope with high RPM and millions of revolutions in their lifetime, while keeping running friction to a minimum. This is appropriate for a hub bearing, for example, but not so much with pivot bearings.

According to an engineering friend who knows a lot more about bearings than I do, even good quality bearings don't have enough grease for the pivot bearing application. So he suggests carefully picking one of the rubber seals off with a sharp pick or Stanley blade, packing the bearing full with waterproof grease, then replace the seal. This may damage the seal, so if you do this with new bearings before installing them, put the damaged side facing inwards. Another tip is to use some anti-fretting compound on the outside of the bearing to make removing them easier later on, and also to hold the spacers in place while putting the frame back together.

Other than that just be careful when washing your bike. Don't jet wash the pivots or use too much detergent near them and if, possible, dry your bike off afterwards by giving it a shake or even a towel down before putting it away. I hope that helps.



92 Comments

  • 77 0
 On my previous bike, I ran four tests on pivot bearings, each six months long. I ride in the US PNW, and ride a lot in the wet. I tested washing my bike with water (only) or brushing it down after the dirt has dried. I ran each test twice, once through the summer, and again through the winter. I stripped the bike down every six months to inspect and clean.

Long story short, the quickest way to have bearings start going is to brush the dirt off when dry. What I saw was the dirt would pile in around the bearings at a much quicker rate than spraying it with water (garden hose.) Washing it with water resulted in less dirt around the pivots and they lasted longer.

I also tried another test where I had excess grease on the face of the bearings on one side of the bike, and the other side of the bike did not. While the side with the grease collected dirt, the bearings did fine. On the other side, dirt worked their way into the bearings.
  • 25 3
 I just dont wash my bike...lol
  • 19 0
 This is beautiful empirical data, thank you! I always had the suspicion that garden hose washes were the way to go, but could never prove it against the brush the dirt off after it has dried crowd. I will now recite this as gospel to anyone within earshot at the trail head.
  • 17 0
 Oh yea... overpack those pivot bearings with tenacious grease, slather some on the bearing faces and don't disturb the sludge barrier that it creates. Works wonders for preventing contamination.
  • 1 0
 This kind of confirms my suspicions, I always figured water was better than dirt. Joking aside, I washed my bike like this all winter and lost a crappy across headset, the seals just weren't up to the task, the cane creek one is fairing much better. Also a bottom bracket bearing. Not a bad attrition rate since I was riding in wheel clogging mud
  • 8 1
 Park Tool grease is also commonly used but washes out WAY too easily. Get yourself some waterproof grease (I have Bel-Ray Waterproof Grease) and your bearings will stay packed for far longer.
  • 4 5
 My only problem with frequent hose washes is drivetrain deterioration. Unsoapy water's surface tension may keep it from getting past bearing seals, but it settles into your cassette and chain quite nicely and takes a lot of effort to get totally dry before you hang the bike up for the day. Seems like drivetrain wear is more expensive than bearing wear.
  • 1 0
 @thustlewhumber: i wipe mine with alcohol or muc off occasionally
  • 1 0
 @thustlewhumber: I did try that method for a little while too, but got tired of all the mud falling off in my garage or having to work on a dirty bike.
  • 2 0
 @rickybobby18: yes, for chains, lube immediately after washing or at least wipe dry. I haven't had any issues with modern cassettes or chainrings.
  • 2 0
 I've never done the back to back testing, but since going water-free, my bike has been silent and crunch-free. I ride in very dry conditions, however, so that may play a factor.
  • 5 0
 @rickybobby18: bel ray has too much brand recognition. Its not special.... Its simply an aluminum complex grease. Aluminum complex grease is shit for wheel bearings as it strings at high speeds, but its OK for pivot bearings and headsets, however it lacks EP ingredients, For motorcycle and bike linkages I like Schaeffers 274 ep aluminum complex grease.
  • 1 4
 @rickybobby18: just remove the chain before washing, problem solved. Master links make it much easier these days than the old pin type
  • 6 0
 @Sambikes11: nothing that's supposed to spin should ever come from Acros, it's a general rule of thumb.
  • 2 0
 @englertracing: Any reason why you use that 274 grease instead of a high tack one?
  • 2 0
 @rickybobby18: air compressor. Cheap and really useful for setting the bead inn tubeless tyre too
  • 3 1
 This is a bit of a tangent, but might be useful to someone. I use a garden hose and sometimes a bucket of soapy water and brush to clean my bike, then put it away still wet - but with a dessicant dehumidifier running in the garage. I can't prove it, but I feel my drivetrain is staying in better condition since I started doing that.
  • 2 0
 @Sambikes11: Those Acros headsets are shockingly poor quality. I don't know how they keep getting spec'd on bikes.
  • 2 0
 @Eatsdirt: base oil viscosity.
274 was recommended by Schaeffers when I called them about it 5 or 8 years ago
  • 34 0
 Helpful Q&A, with good tech details written in an accessible style. Thanks
  • 10 0
 Great, glad you think so. Thanks!
  • 8 0
 Regarding premature bearing wear, most often the bearing spacer or tube that sits internally between the inner races of the bearings to prevent overly excessive preload on said bearings has to be "perfectly" sized, I had an instance on a top end USA brand of frame where this was NOT the case, guess what ? Overly preloaded bearings and premature bearing failure. The fix was a 0.10mm micro shim aquired from a local engineers murchant which I installed between spacer tube and bearing, perfect preload. Unfortunately theres a good chance a lot of mechanics may not pick up on this. Hope this helps someone in the future.
  • 2 0
 Good point. On my YT Capra, the frame spacing for the main axle was undersized, so the stops on the axle were 2 mm wider apart than the stops in the frame. Bearing was toast after 200 km. Some quality control!
  • 5 0
 Some of the double-sealed bearings (Trek uses them) have cages that hold the bearing balls in place. Good luck to you if you remove both seals and drop one of those little balls. It's a major pain to get everything back in place correctly
  • 5 1
 Every time I wash my bike, I towel dry then finish by blowing out the pivots/bearings with an air compressor. Always surprised by the amount of water that blows out - I don't know if it actually increases the lifespan but it sure makes me feel like it does.
  • 19 0
 Be careful because you may also push water into the bearings, just as you would with pressure washing.
  • 4 1
 What are the values in the travel by force plot legend? I thought we were talking about volume, but units are mm? Also, why are shock tuners, eg, Vorsprung, Shockcraft, still using Windows 95 era plotting software? I presume it’s because the tuning software and its native plotting tools are ancient. These plots could benefit from different line types and/or color scaling that corresponds to the continuous variable they encode
  • 8 0
 dude those are just excel graphs...
  • 3 0
 I'm going to guess here and say the chamber volume is converted into linear units by fixing the diametric chamber as a constant. At least that seems to be how rock shox prefers to define it.

www.sram.com/globalassets/document-hierarchy/service-manuals/rockshox/rockshox-suspension-theory-guide-rev-c.pdf
  • 2 0
 @SoDiezl350: This document is gold. Thanks.
  • 5 0
 The shape of a spring curve depends on the compression ratios of the positive and negative chambers, which are a function of the effective chamber lengths relative to the travel. A volume on its own doesn't tell you this unless you know the effective piston area, so effective length is more universal.

For example, let's say you wanted to compare a Fox 34 and a Fox 36 air spring, both at 160mm travel. If they both have the same length of positive and negative chambers they will have the same spring curve (albeit at different pressures), but if they had the same volumes, the 36 would be more progressive.

Also, what's wrong with an Excel plot - it does the job, right?
  • 2 1
 @seb-stott: These plots are just half the fun of the equation since the air spring behaves completely differently at different accelerations. The plots basically only work for a slow motion, otherwise you are going to see quite a hysteresis between pushing and extending phase. These graphs are good for making the use of spacers understandable for endusers, and for the manufacturers respective marketing department. For everything else they are just basically useless.
  • 5 0
 Agree that full negative chamber bands for a light rider is the correct setup. Eating a few cheeseburgers or just riding faster will also help Smile .
  • 2 0
 I finished packing my pivot bearings with grease about 30 minutes ago. I do have a little dental pick that works perfectly for pulling the seals (better than a Stanley blade which I used to use), but they do still get a little bent. Side note, a dental pick set is super cheap on Amazon and is so useful for little stuff like this on a bike. Anyway...

If you have Knipex pliers, or some other kind of smooth, parallel-jaw vise/plier, put the seal in between the jaws and give it a *gentle* squeeze and they'll straighten right out.
  • 3 0
 Sharpen a old spoke also works.
  • 2 0
 "I feel as if MTB riders are encouraged to use 29 wheels to counteract the loss in effective wheel diameter due to running the encouraged lower tire pressures!"

Isn't recommended pressure a function of width (or more specifically, cross-sectional area) and not diameter?
  • 2 0
 I wash bike with water only and no high pressure attachments and after that I towel dry. Then I use my shopvac and vacuum all the pivots and any other tight areas that water can hide. Way better than using an air compressor because you're not blowing any dirt into the bearings. Also the shopvac isn't strong enough to suck any Grease out, just the water. I do a full teardown of my bike every 6 months and re-grease all the bearings. So far this processes has been working great for the past 40 years I've been doing it. A bike mechanic showed me this trick back in the 70s when I was racing BMX.
  • 2 0
 "This image shows the elliptical shape of a tire's contact patch. Bigger tires have a wider, larger contact patch."

No, lower pressure tires have a bigger patch. Wider (taller really) tires and stiffer sidewalls allow less pressure while keeping the rim safe good rims hold onto the bead also allowing less pressure, and that's what actually makes the contact patch bigger. If those tires pictured had the same pressure, the contact area would be very close if not identical.
  • 2 0
 I'm an older, less flexible, slower to react, kind of rubbish, rider who still wants to go fast. Would a longer, lower, slacker, 29er, wide-barred, long-travel bike be the best option for me? Thinking that the stability provided by this platform would provide more confidence to hold speed, especially on the steep and technical.
  • 1 0
 So what about an extra negative spacer for heavier riders. IE move one from pos to neg, higher aircan pressure = more support at sag, less ramp up and more active mid range. My 2p worth. I've done it on a Topaz and it's better for me
  • 1 0
 I'm not gonna argue what is better for you, but in general I have experienced the opposite for larger riders.
  • 4 0
 I looked into this recently, and as far as I can work out, rider weight has ne effect on the number of +ve/-ive spacers needed.

Rider weight sets pressure requirements
Rider style sets volume spacers requirements
  • 1 0
 @gabriel-mission9: it depends on leverage ratios. A frame with a very flat leverage curve might necessitate many spacers for a big rider even with an air spring because the final rate with frame and spring progression still isn't enough for the weight.
  • 2 0
 @justinfoil: Yes. Frame kinematics do come into it. I meant for a set bike though. As far as I can work out, if you have a certain frame, and two riders of very similar skill levels and riding styles, then they will need similar volume token setups, whatever they weigh. Even though they will need totally different base pressures.

I think the common assumption that bigger riders need more tokens is false.
  • 1 1
 @gabriel-mission9: maybe, but it's impossible to say because you can never find two people with say, 50 lbs, of weight different with the same "riding style". Even if they're both plowers, or floaters, or line-snipers, a 30ish% weight difference means even the same "style" is going to put very different forces into the shock. Plus, big riders often want more progression from the spring even if they're, say, a line sniper, because when they do go off line the bottom out is that much harder because they weight more. So avoiding the bottom becomes a slightly higher priority sometimes, hence big guys often going with less volume.

I'm 100 kg and I've bent 8mm titanium shock mounting bolts without a hugely noticable bottom out, hate to think what the shock insides would have gone through if it wasn't stuffed full of volume spacers.
  • 4 2
 Might seem obvious but don't t forget to use MAX bearings in linkages... if you're using regular hub/wheel bearings then that might go some way to explaining why they aren't lasting so well.
  • 1 1
 Exactly young fella, could'nt have said it better myself !
  • 4 0
 If the frame was somehow manufactured off spec it would munch bearings too no?
  • 2 0
 So remember Turner bikes with bushings and a grease fitting. Now Ibis has followed with similar at the main pivot. I had a few Turners and never an issue over years in crappy conditions.
  • 1 0
 Had a Whyte G-170 that ate pivot bearings as well...Whyte make a big deal about bikes designed for UK weather but in reality it wasn't the case. A fun bike to ride but this bearing issue and the really terrible internal cable routing would discourage me from buying another one.
  • 1 0
 Regarding premature bearing wear, check your spring rate. I upped my rear shock spring to 500lbs from 400lbs and my bearing life went up at least fourfold, I was totally under sprung and probably bottoming out a lot passing a lot of impact force through the bearings. Swapping bearings should be quick and easy if you have the correct tools too, I use the cheapest Chinese ones I can get and they last very well now that I have the correct spring rate.
  • 1 0
 wish I saw this last night when I had the main pivot apart for cleaning. The bearings were just a little crunchy and i could have repacked them while I was at it. Just my luck!
  • 2 0
 A lot of bikes are made very cheepely (like white) and this is a problem
With them. Not straight frames and weak ones will flex and eat bearings.
  • 1 1
 I always chuckle when i here pro-bike reviews that list tyre pressures as a-lot are in the 23/28 psi range and pro riders saying that too low pressure is bad, i agree with them, its just that a-lot of public and media still think lower is better and then wonder why their 15 psi rear burps off of the rim.
  • 1 1
 Also worth noting on premature bearing failer is the installation methods used by the bearing insertion operative, use the proper tools AND methods and if you use sockets and a hammer then said tools should be used on you ! ;-)
  • 3 0
 Hey, can I have stickers though??
  • 1 0
 Is that negative chamber volume reducer trick something you can try in the super deluxe? It all seemed like one chamber when I opened mine up.
  • 2 1
 The Super Deluxe still has a positive and negative chamber, but without the dual air sleeve layout of the Monarch DebonAirs and HVs. You could try putting a fat glob of Dynamic Seal Grease on the Counter Measure spring to achieve a similar effect.
  • 2 0
 I'd never heard of anti-fretting compound before. How is it different to anti seize?
  • 1 0
 Also curious about this. Perhaps @andrewbikeguide and @seb-stott can provide clarification?
  • 1 0
 Is the copper stuff I put on my wheel nuts on my car an anti fretting compound?
  • 4 0
 Yes and there is a silver version. On the advice of a BB expert I now use this on the crank spindle where it touches the BB bearings instead of grease, in addition to all my bearings when I seat them. The stuff never washes off, it keeps moisture out better than grease and there is no need for grease in this application as the spindle is meant to move the bearing race not rotate within the race. The full pack for pivot bearings is an old trick and worth noting as there is zero rotation and therefore almost no meaningful resistance to movement if the bearing is full of grease.
  • 1 0
 @andrewbikeguide: Nice thanks!
  • 1 0
 i've been using that stuff by penrite for years on my BB threads and its been awesome, i also wipe the face of the bb bearings(outside) with it before installing cranks, i think ive only ever had 1 BB bearing go bad in my years(i do swap them every year though and keep the old ones)
  • 1 0
 Holy crap, this nails three topics I’ve been wondering about since whenever..!
Great stuff here, it’s a keeper.
  • 2 0
 if you use air pressure to set tyre sag, you're doign it wrong
  • 1 0
 hey where is the ebike podcast the mullit bike podcast ect they seem to say thill do one then never do it
  • 1 0
 I sometimes dry my bike with a leaf blower after washing it. Works great!
  • 1 1
 No problems with pivot bearings if you ride a hardtail
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