Offset Rims: Stronger, Stiffer, More Compliant, Lightweight, Inexpensive

Aug 28, 2015 at 11:12
by ima cAt  
bigquotesIn light of the BOOST 148 rear hub size becoming more and more popular on newer models of non-plus 27.5 mountain bikes, I wanted to shed some light as to why Boost isn't as great as it purports to be, nor as great as it could be in one particular aspect. That is because more than 99% of bicycles in the world have wheels that are under severely imbalanced spoke tension*, and, on this front, Boost only slightly alleviates this problem. Graphics included.

A simple answer that we as an industry have missed are asymmetric rims.

a. asymmetric rim

Asymmetrically profiled rims are different from your typical bicycle rim due to their spoke/nipple holes being offset from the center of the rim. Companies like Asian Cycle Express, Bontrager, Mcfk, Nobl, Nox, Sram, Tandell, WTB all employ this profile into at least one of their rim designs. And, trust them, it is for a good reason.

Here is your tl;dr, as this is slightly heavy in content:
+ Asymmetric rims with offset spoke holes allow for better triangulation of spoke angles and balance of tensions in a wheel build, resulting in a stronger, stiffer (laterally due to proportionately more lateral tension), more compliant (radially due to proportionately less radial tension), and more reliable wheel.
+ This slight redesign in how we produce rims could potentially have delayed the introduction of Boost rear hubs because the spoke angles would be relatively balanced (compare 70:100% to 56:100%), and 29er wheels would not suffer from lack of stiffness.
+ Boost and Asymmetric rims can co-exist. As wheel builders will probably know, neither option fully remedies the issue of imbalanced spoke tensions in wheel builds. But if used together, we can all enjoy wheels that are just as strong and stiff, while being significantly lighter (subjective) through the use of less spokes and lighter rims.

Now, let's get to the entrée of this meal.

To avoid confusion, let me first say that Boost is necessary on a 29er or 27.5+ bicycle if you want to design frames with:
1. tight/short chainstays and big tires
2. strong and stiff pivots for suspension
3. properly aligned chainlines
4. non-noodly wheels

With that out of the way, back to the topic of discussion: Asymmetric Rims with Offset Spoke Holes.

I'll let the graphics do the talking for me. This is the reality of almost all wheels with discs.

Here is your typical front wheel with a 100-63 ratio between the left and right spoke tensions:
1. Your avg front hub laced to Avg Rim 100-63 Benefits of asymmetric rims

Here, the typical rear wheel (DT240 142mm hub) with a 56-100 spoke tension ratio:
2. DT240 laced to Avg Rims 56-100 Benefits of asymmetric rims

This imbalance is the result of massively disparate angles the spokes make from the rim to the spoke holes; 7.2 degrees is 80% wider than 4 degrees. Since the spoke tensions are balanced laterally between the two sides when building the wheel, a milimeter-deflection of the rim in one direction would result in a humongous deviation of spoke tension from the 100% side. Thanks to overbuilt rims, fatigue isn't a big issue anymore, but having every other spoke with a much higher tension also tends to unnecessarily stress nipples, are harder to build up (conventionally), and especially after a while, are more annoying to true. Not to mention, good luck if you bend or warp the rim away from a high tension spoke...

Take note that although the DT 240 hub was designed so you could use the same spoke length for both sides, it comes at the cost of a 2:1 ratio between the two sides' spoke tensions. As you can see, the disc side flange is set wide and larger than the drive side. It is safe to say that having a non-symmetric spoke triangulation would build up weaker wheels which are less stiff than ones built with a perfect triangulation. *Too stiff addressed later/below.*

Great hub, as I still have one to this day, but I cringe every time I look at it.

Moving on! Now, here are the same two hubs laced to a 3mm offset rim with the same ERD.

The offset rim produces a 100-77 spoke tension ratio, compared to 100-63. Note: spoke length decreased from 1.4mm to 0.7mm.
3. Avg front hub laced to offset rim 100-77 Benefits of asymmetric rims

The rear 240 hub laced to an offset rim gives a 70-100 ratio, compared to 56-100.
4. DT240 lace to offset rim 70-100 Benefits of asymmetric rims

As said before, this hub design focuses on identical spoke lengths for both sides, so the offset actually increases the spoke length discrepancy from 0mm to 0.6mm; it isn't a big deal since it is well within the margin of error.

So what does this mean? Has the bike industry missed a great bike-hack this whole time?

I certainly think so. In every case, even with most singlespeed hubs, using an offset rim allows for a wheel build that has its yin and yang better balanced. It means we can have wheels where we only use one spoke for all sides. Or wheels where we have wacky spoke lengths but a perfectly even spoke angle from both sides. Lighter front hubs with the disc side flange attached to the disc tabs.

bigquotesDoes it sound too good to be true? To save 100 grams from even the lightest wheels in the world without any reduction in stiffness, strength or reliability? Better designed wheels can be stronger, more reliable, stiffer or more forgiving than the current crop of great wheels we have today, using less spokes and nipples, lacing lighter rims with less holes drilled to lighter hubs that aren't wider for no tangible reason. The possibilities are many, and it is this very mindset that I hope to see grow, even if bicycles are pretty darn dialed today.

Irony section:
+ Symmetry in spoke angles, but asymmetry in the rim profile? Although I do not have data to prove this, I certainly hope that rims derive most of their strength from material than a symmetric profile.
+ Skeptics will argue that wheels can get too stiff... so you want to introduce a new rear hub size and build wacky triangles in your wheels to make it compliant?
+ I really feel that Specialized and other companies using 135/142mm rear ends on their DH bikes are going in the right direction. The 150/157mm rear hub sizes and 83mm BBs are unnecessary, although that brings up the same type of issues that Boost 148mm brings: obsoletion of thousands of wheels. Except, do you want mountain bikes to forever have two different rear hub standards for no good reason?

Other notes:
+ Directional rim drilling (giggity) so that spokes aren't straight out of the rim then have to bend towards the spoke hole
+ I think many people would be happy in the long run if all mountain bikes (xc, enduro, dh, plus, etc) rear ends became 148mm for the rear hub. How many more unnecessary hub "standards" will we see in the coming years? Roost hubs and frames anyone??
bigquotesA new rear hub size every few years is the archtypical embodiment of "oh shit, we definitely could have done better..." Predicting the future isn't easy unless you're Steve Jobs, but taking a flimsy stance with 'standards' is further fostering dissatisfaction and despondency that's all too common in our articles that simply mention Boost.


  • 1 0
 Well written article, good read tup

I just want the industry to pick a set of standards that work well and do it's best to stick with them, or at least make it easy to convert between them. For example they could have easily made 15mm thru the same width and brake spacing as the existing 20mm standard so that both forks and hubs could be interchangeable between the two using nothing more than a different axle. Or even better, just kept 20mm and scrapped qr9 entirely. 15mm axle on a 180mm fork *cough new lyrik cough* is stupid.

I also miss being able to swap wheelsets between my 20 front 142 rear DH bike with my 20 front 142 rear AM bike. Unfortunately the new DH frame has 150mm spacing.
  • 1 0
 Thanks Jamie! Yeah, the biking industry seemingly changes the standards with little to no reason, with whatever reason being somewhat stupid in the first place (which leads me to believe that it really is to squeeze money out of people by forcing them to change tons of parts or whole bikes).

I've also been tinkering with the idea of running a single speed (short freehub) hub on a 150mm frame so I can use a normal derailleur with both the limit screws pushed in and run 6 cogs on the freehub. Benefits of this is the ability to run the disc line and the chain line further in towards the center, meaning 7.5mm more clearance on each side. I already have a wacky brake adapter from the past that is 4mm wider than standard and all you really need are spacers to fill up the 8 or 15mm of axle space. Just an idea Smile
  • 1 0
 I wonder if the difference between Boost hubs with symmetrically drilled rims and 142 hubs with asymmetric rims might simply be where the spokes are likely to snap under lateral torsion. Likely to break at the hub in a boost system because of the increased spoke angle but in the middle on an asymmetric system? It's interesting to note that at premium price end MCFK and Reynolds (for 2016) have gone asymmetric. I foresee a split coming - market forces from the big players will force Boost on all 29ers and 27.5 bike models at all price points, making it a new standard across the board for oem frame and wheelbuilders to work with. Asymmetric will only continue on niche high end bikes so long as there is a good s/h market in high end 142 frames and the odd couple of companies, like Ibis, hedge their bets between standards as, for instance, with their new 2016 Ripley framesets.

Personally I'd rather stick with a 49mm chainline on a 142/asymmetric system, rather than the 52mm chainline on the Boost, which of course requires a new and (currently) expensive change of crankset and fork. It may be that in the next couple of years as the lycra conscious fashionistas have to move over to boost we'll see a lot of quality s/h 29er frames comin up for sale coz they're seen as old fashioned. That would make me very happy as there is currently a plethora of great 142 x 29ers around from SC, Ibis, Turner, RM etc.
  • 2 0
 Keep in mind that hub flanges as well as spoke holes may be machined and drilled towards each other; almost all high end hubs feature the angled hub flange and quite a few rims (namely the WTB rims) have directional drilling that allows the nipple to pivot.

As you've said, there is a new wave of standards coming fast, but people are getting tired of new "standards" especially when their 135/142mm hubs have given most people no problems. Whether there is a new standard after Boost is just a matter of key industry figures realizing that there are much better ways to fix these wheel and clearance problems.
  • 1 0
 I forgot to mention 3 things that play in my mind about Boost: that 52 mm chainline is more off centre in the crankshaft lateral plane. It's gonna apply more unequal pressure on the bearings on either side and probably wear the bearing prematurely and unequally. We might actually feel the crankshaft rocking?

Weight: SRAM is disingenuous when it states that there is no weight penalty of Boost hubs over standard 142 hubs. That's not true on 3 fronts. Firstly there is an, albeit slight, weight penalty of a few grams on the hubs themselves. BUT what about the weight penalty of both a bigger and heavier frame AND forks?

Aerodynamics: AFAIK no-one, bar non-one, has even touched upon this. The front profile of a Boost fitted bike is simply bigger all round.The extra width of the Boost hubs, increased spoke length and spoke angle, the increased seatstay width and fork width have to have a deleterious effect on aerodynamics. A wind tunnel test would easily demonstrate this. So what? You're not a roadie right? This might be of more legitimate interest to XCers than DHers or Enduro merchants.
  • 2 0
 The bearing wear is negligible (imo). Weight will always go up unless the new standard is narrower! (hah) Aerodynamics isn't a big thing in DH as it is in XC, but I can't really see "Boost" front catching on either... But yes, it is as if these BOOST pushers have never considered any downsides! It should make these "Ferrari engineers" furious that Boost is going on the market.
  • 1 0
 And a parting shot - what happens if you use Boost hubs with asymmetric rims. The mechanics/physics of it is doin my brain in. Do they negate or complement each other?
  • 1 0
 Boost hubs (if they follow the same, archaic design philosophy, or the lack of) will have wider flange spacing than the 135/142mm shell, but that's about it. Most Boost hubs will feature the same ~6mm offset we have with current full-bodied hubs, and will fully benefit from asymmetric rims.

What could happen is that someone with good intentions (but still completely misguided) will offer a Boost hub where the NDS flange is actually put 3mm inboard and the driveside flange 3mm outboard (following the freehub's edge), which would make the flanges/hub have 0mm offset, but completely losing on the benefits of the wider real estate for a balanced wheel build.

Either way, asymmetric rims are more important to wheel builds than Boost is, and they can co-exist.
  • 1 0
 'I can't really see "Boost" front catching on either'. Yes theminsta! I can see another scenario, at least among enthusiatic home bike builders, who don't want to run to the expense of asymmetric wheels. That is a hybrid - boost back end for the additional stiffness where it is really needed but non-boost front end because of re-usabilty of a favourite fork and the lack of additonal expense in a fork change. Interesting times ahead in the next coupla years.
  • 1 0
 Haha that was basically my plan! But I personally feel that a 142mm hub and an asymmetric rim would do me fine, and the 3mm of clearance either side a bigger benefit than having (double) stiffer wheels (wider flanges + even tension).

Everyone may be talking about gearboxes, etc. right now, but there's a bunch of benefits to be had with wheels!
  • 1 0
 Ha! Ha! It was my plan too! Was thinking Ripley with Boost rear end for the twofold benefits of stiffer rear and short chainstays but a standard 100mm front end. But I too am now thinking 142 & 100 hubs and asymmetric rims might well do for me - on a RM Instinct. Pisser about the pressfit bb on that tho.
  • 1 0
 Syntace already offers asymmetrical rims but they've gone one step further with their new EVO6 asymmetrical rear end. which allows for symmetrical wheels and one size spoke on both the front and rear wheels. Other manufacturers will be doing this as well.
  • 2 0
 If you want the stiffest configuration possible for your 29er or plus bike, you don't want to minimize spoke angles at all.
  • 1 0
 Dam im now confused.
Just bought a Santa Cruz Hightower. Rear end will be boost due to the frame dictating that. Front will be non boost 29er forks.
I was thinking of going for the offset ACE rim & hope pro 4 hubs. Am i correct in saying that one side is drive side? Do current spoke calculators allow for this?
Sorry that my comment isnt as knowledgeable as the others.
  • 1 0
 You mean which way to lace the rim's offset? If so, yes. Front wheel should have the offset of the spoke holes towards the drive side and the rear should have the offset of the spoke holes towards the non-drive side. And you can just manipulate hub flange distance-to-center values to get the correct spoke lengths. I'm sure half of the spoke calculators have the offset field for the hub as well.
  • 1 0
 Which is stronger:

1) Non-Boost Hubs in Boost Frame, Offset 6mm Drive Side, Dished back 3mm with Normal Spoke Drilling Rims

2) Non-Boost Hubs in Boost Frame, Offset 6mm Drive Side, Dished back 3mm with Asymmetric Spoke Drilled Rims

Basically, could there be a wheel strength advantage to keeping that old 142 wheel around, getting boost adapters (like MRP or Wolftooth) that offset the non-drive side 6mm with a rotor spacer and require a 3mm dish. I'm wondering if I'd be better of getting Asym rims or normal drilled rims to use with boost adapters on a boost frame.
  • 2 0
 The 6mm rear triangle offset (assuming towards drive side) would negate the need for asymmetric rims.

The difference in flange displacement on mtn hubs is usually between 12~16mm, meaning an offset of 6~8mm is what you need. But keep in mind that there are (what I assume to be) negligible benefits between a 2mm-dished wheel and a true zero-dish wheel. Thus, the 6mm offset is more than enough to have a balanced-enough wheel.

As for the "normal" hub, fitted with Boost end caps that offset the hub 6mm towards the drive side.. you should realize that this setup only gives you a 3mm offset. The rotor spacer included should be 3mm to confirm my statement. With that being said, an asymmetric rim (with the offset holes closer towards the rotor) would give you very well balanced wheel.
  • 1 0
 @theminsta: So basically, Asym rims would still benefit even 142>148 adapted wheels. Good to know.
  • 1 0
 Revisiting this: my GG Smash requires a NDS offset of 3mm with 148 rear hub. If I ran 142 hubs converted to 148, Id need to run 6mm offset to the NDS.

Asym rims still would hold an advantage? Or not? Is that moving the rim more centered over the flanges, makin for more symmetrical spoke tension and no need for asym rims?
  • 1 0
 @PHeller: I'm assuming you mean that the hub center is offset towards the rear derailleur as the other direction makes no sense. If so, using the 142mm hub with it slammed to the disc side will 1) f*ck your chain line up 2) give you a worse than normal wheel build. If you somehow find a 3mm offset brake adapter and slam the hub to the drive side, 1) you get the right chain line 2) better wheel build
  • 1 0
 @theminsta: Nope, the GG products are offset 3mm DS, or at least you dish them 3mm NDS. You pull the rim towards the disc.

If you added a 142 hub with an adapter, the hub would be 6mm offset towards the driver side, the rim coming back 6mm towards disc to be centered.
  • 1 0
 @PHeller: That's rim offset. The hub offset is as I said: to the drive/der side

Moving the hub 6mm to the drive side would result in a 3mm offset. Rear hubs are usually offset 6mm to the NDS so an asym/offset rim would still be very useful.
  • 1 0
 @theminsta: So would a asym rim still be ideal in a situation where the spoke tension and angle is become more even?

I thought then benefit to an asym rim is that they worked better with traditional "uneven" spoke tension?
  • 1 0
 Quick question! Can you use offset rims for non boost hubs and front wheels. I am building a vault j rear hub and a sram predictive st earring Hub. I wanted to use RaceFace offset Arc rims as they are available. I see no preliminary problems we just have spoke lengths that change in wheel build calculations.
  • 1 0
 I'm with you on this one, other solutions to this is older Specialized Demo, Cannondale and now Pyga offset rear triangles or Hope 150mm hub. I wish there are more 25-up offset rims on the market. Smile
  • 1 0
 Offsetting the rear triangle is a great and simple solution, but many people couldn't understand the concept well enough I believe. As soon as people smarten up, we will see more wide and offset rim options, better designed hub bodies, offset rear triangles.... but until then, we will have to suffer the ever-changing "standards".
  • 1 0
 Its worth the read. Gained so much information from here. Thank you for sharing!
  • 1 0
 Carbon rims are already stiff. Asymmetric is not necessary.
  • 1 0
 If asymmetric design on alu rims make them stiffer, than asym carbon rims are stiffer than symmetric ones. Which means you need less material for lateral stiffness (imagine lighter rim that feels the exact same as a heavier/"stiffer" one.

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