9/7/14 6:30 PM

Spoke/drillings and disc brake.

I find myself wondering/browsing about the issues of the disc brake wheel. Thinking the spoke beds on the carbon rims had better be really strong for the forces applying the brakes is going to put VS a rim brake.

SO some Pre-101 thoughts, considering I have only built a just over dozen wheels to date, all road rim brakes. EDIT-put new rims on my 29er disc wheels too, so correction.

So MTB is existing examples of course.

Front asymmetrical VS symmetrical bracing angle/flanges. The front wheel on my SL4 is laced 2x rotor side, radial non rotor side. Thinking 2x both sides be better maybe? It is also 28 spokes front 32 rear, which make good sense I think. Rear is 2x both sides.

So I am thinking a 24 hole/spoke set like the ones I am spying on eBay may not be for a porker like me.

I believe they are 2x across the board, although a bit ambiguous making contact probable if interest stays.

But seriously thinking 28 Spokes might be a better choice 2x all. So I am thinking now after looking at a lot of data a similar weight AL rim may be a better choice. Aero difference insignificant probably from 27mm to 38mm on said spied internet wheels. My original thought on getting the carbon being I do not have to be concerned with brake performance and cork pads with the discs. I say this based on my experience with the ULDV-46 Reynolds and blue pads I had briefly. I decided AL rim give me better brakes, and I did not even try them in the wet. My weight ?

Also found a set with 31mm AL rims same overall weight as the 38MM carbon wheel on eBay. Not sure I can calculate the ramifications on the 35 layers of carbon in the eBay wheel/rims spoke beds.

Other thing the data has got me thinking is asking the seller with the 31mm AL 28/28 wheels to use Sapim Race instead of CX-Rays speced. I think the 80 grams of extra spoke weight maybe a good direction. Less elasticity on the spokes, but maybe how stiff the rim might be should be the determining factor.

Other choice for me is just getting parts and building the set myself. Taht eBay seller has those rims in different drilling spec 16-32.

Also found it interesting the Sapim site suggests not using the Laser spokes on disc wheels, but the CX-Ray apparently they got no issue with. Funny considering the CX-Ray is a flattened Laser. Does cold working the Laser to be an oval/blade align the grain or something.

Also notice one particular builder that uses both the CX-Ray or Laser on the same wheel optionally. The spec for tension used in the build was notable lower in the build with the CX-ray than the Laser. Wondering why that might be.

9/7/14 7:17 PM

I weigh at least as much as you, and I always thought 32 spokes 3 cross in an aluminum rim was as low as I dare go. maybe I am wrong, and I don't know if carbon changes that. A road bike may be more demanding than a mtb, no?

Thats 1 reason I'm building the 7 as a rim brake bike- I can use lighter wheels without concern.

I had a Mavic Speed City? wheel once, that was disc and it would flex to the left under heavy braking. So bad I took it off.

9/7/14 7:24 PM

braking forces are quite small and gentle compared to hitting a pot-hole, methinks a spoke bed that can withstand normal riding on rough roads will handle disc braking forces just fine.

a cx-ray is a flattened laser with a +300% up-charge, hence it being OK for use in *any* application!

9/7/14 7:44 PM

So Walter, you don't think braking in a turn for example makes any diff if the wheel gets more load on the disc side. Not to mention that side stay/leg is probably stiffer. Or in the case of my radial non disc side front wheel et al?

And in my minds eye, braking at the hub instead of the rim has got to put a lot more load on the spoke/nipple to rim interface. My question is, will the fact that the rubber will let loose of road surface at a point happen before said stress might be of issue?

The lack of symmetry of forces makes my brain warm up admittedly.

9/7/14 7:54 PM

i just think the magnitudes are smaller than you see in stuff that wheels already handle well.

9/7/14 8:02 PM

What about brass vs alloy nipples? I always used brass in disc wheels, but maybe I'm too conservative.

I'm not sure you can compare impact forces,which reduce tension, to braking forces,which increase tension..??

9/7/14 9:09 PM

to braking forces,which increase tension..??

Exactly why a low spoke count in a carbon rim worries me. Pulling the spoke/nip through... Seen is on non disc road wheels. Albeit too low spoke on shallow and probably over tensioned.

9/8/14 2:23 AM

You can roughly calculate the additional spoke tension caused by braking. Maximum possible braking force on a bicycle is around 0.6G before you go over the handlebars - the exact figure depends on where your centre of mass is, whether you are ascending (which will increase the max because your centre of mass is further behind the front wheel), or descending (the reverse), etc. The 0.6G is the approximate figure for a rider seated on the saddle on a flat road.

Lets make a few assumptions:

1. The front brake is powerful enough to apply this 0.6G, so that all the braking force is in the front wheel.

2. Weight of bike + rider is 85kg.

3. Wheel built as 32 spoke x3 - I measured the 32 spoke x3 front disk wheel in one of our tandems, and the spokes are at an effective radius of 30mm - i.e. if you extend the line of the spoke and take the length of the perpendicular to the axle centre, that measures 30mm.

4. 700x28 tyre. This has a radius of 340mm.

5. The hub shell is torsionally rigid enough that the braking force is shared equally on both sides.

With these assumptions, we first calculate the braking force of the tyre against the road to achieve a 0.6G deceleration: f=mass*acceleration=0.6x9.8x85=500 Newtons (G is 9.8 metres per second per second)

This force is applied at a radius of 340mm, therefore the spokes at their 30mm radius at the hub are operating at a mechanical disadvantage of 340/30=11.3 times, and this force is being carried by 16 of the spokes (the other 16 spokes exit the hub in the other direction and don't contribute to the braking effect, in fact they will lose tension while braking).

Therefore additional tension on each of those 16 spokes at maximum braking is 500*11.3/16=354N == 36Kgf (one Kgf is by definition 9.8N).

So if the wheel is built with the spokes initially tensioned to 100Kgf, in a panic stop where you are at the point of going over the handlebars, the tension of half the spokes in the front wheel is increased by about 35%.

It should be simple enough to do the calculation for different weights, spoke counts, and hub flange diameters - just substitute the different values in the formulas.

9/8/14 5:38 AM

if there are spokes going light due to an impact or braking, then there are spokes going heavy by that exact amount. the distribution may be different, but it'll be there. the strength of a wheel comes from the spokes that are in tension. in braking the pulling spokes are tighter, the pushing spokes are looser equally around the wheel. hitting a bump the upper half of the wheels is generally in higher tension and the lower half of the wheel is generally in lower tension -- in the bump case, the distribution of that drop in tension is probably a bit focused on the impact point.

i always use brass...primarily better corrosion resistance, as i'm generally easy on equipment.

love nick's sample calculation exercise...brings me back to college physics!

9/8/14 8:39 AM

Something to think of for disks VS rim.

When we rim brake a wheel we essentially create a new support point for the wheel at the rim. In fact the latteral loads would be only on the bottom half of the rim. The brakes support the latteral flex of the upper half.

Disk brakes do not get this support from the brake pads on the rim.

Rims flex laterally quite a bit. Think about brake rub when out of the saddle climbing and sprinting if you doubt it.

So, my point, the braking loads in tight conditions are different for rim brakes than rim brakes when compared to straight line braking.

I agree that any wheel that can take a pothole whack can take disks. I still think the loads are different and can manifest handling problems because of the paradigm of design. The wheel design should start over from scratch and see what engineers develope.

9/8/14 9:03 AM

A radial laced wheel can take a pothole whack but I'd be afraid of radial on a disc brake wheel.

The assumption that the hub shell can distribute the brake force to both sides may be wrong. The Mavic Speed City wheel I had flexed 1/2 inch or more to the left under heavy braking.

9/8/14 9:20 AM

i guess you could radial spoke on the non-disc side but that's not a good idea IMO.

fwiw...motorcycles have been using hub-based braking (drum and disc) on spoked wheels for decades.

9/8/14 12:49 PM

With a much more robust design. Even when weight differences are taken into account.

9/8/14 1:17 PM

If i build or but, no radial for me. Only front non disk wheels. But the stock front wheel is radial NDS and 28 spokes. 2x DS. I did not feel any wonkiness. But then again it was a docile ride. Not like the last two Scott outings with one nose wheelie stop, and Friday's few rear skids where a disc wheel could/would get a more extreme load.

1st few rides on the bike were with 3x 29er wheels.
Also felt fine, but no sudden brake grabs, but I'm sure there would not be any issues with the 29er wheels. 32x 3cross.

Is it harder to skid a knobby on hard pack than a road tire on pavement I guess could be a good question.

If a disc road wheel will skid before the forces actually become extreme enough to matter...
I think this may be the case. Certainly not nearly as non issue front if correct.

Seems like the more crosses the better with the disc wheels.

But I think my usual 2.0/1.8/2.0 drive side and 2.0/1.5/2.0 NDS road [non disk] builds I do may be fine on the back, especially with 32x and 3x crossing. But probably won't bother as less dish differential on a disc rear anyway. Just go all 2.0/1.8/2.0 or maybe even 32x all 2.0/1.5/2.0

Although Sapim sez not to so the later with the Laser. but they are OK with doing it with CX-ray... Again I don't get that unless they just want to sell more 3.00 per each spoke over the .85 cent ones....


Looking at an XTR Center Lock set used. So may just go that route with new rims spokes and swap the tubeless stans rim on those to the CX. They are under 1600 grams as is on a postal scale according to seller. Plenty light for me for a disc wheelset.

9/9/14 4:34 PM

Does anybody know of an underbuilt wheel failing under disc brake loads? Surely someone has gone low spoke/alloy nips. I have not heard of a failure.

9/9/14 5:28 PM

Just ordered XT Center lock hubs front 65% off, rear 55% off. If I don't like the new build for the Roubaix I will do it again and give them to my 29er. ;)

With the parts I have should be 1/2 lb lighter than the stock wheels. And get the original 28 spoke 1/2 radial front off there...

And if I decide to use some A23s, maybe 120-130 grams more lost for the set.

The usual, spend some and loose some, or to loose a lot spend a lot.

I spec-ed out some king R45 ISO hubs with cx-rays and H+ Sons rims and the parts cost scared the shit out of me... 6-7x what these wheels will cost to be
within 100 grams probably...

9/10/14 5:32 AM

especially if all you're looking for is reasonably light weight. I've built a couple sets of clinchers that are under 1300 grams using Kinlin A-200 rims, generic Taiwanese sealed-bearing hubs (rims and hubs from bikehubstore.com), a combination of Sapim Laser and Race spokes with alloy nipples (from Dan's Comp), and Ti skewers (from Ebay). Total cost was ~$250/pair.

These are lighter that any commercial alloy clinchers I've seen recently, use no proprietary parts, are very easy to service and have held up very well for a couple of seasons to date.