Disc Brake Road Bike, What's Feasible?

[MacB]

As per the title I've been musing over this for a while, looking at things like the Genesis Croix De Fere, Orbea Diem Drop, Kona Sutra and Honky Inc, Marin Toscana...to name a few. There are also a few of the flat bar road/hybrids with discs and the recent change to UCI rules for cross racing, to permit disc brakes, will mean more coming out. But they all lack something componentry wise and, most especially, in weight, all coming in at 25lbs plus.

I'd like a drop bar bike with Avid BB7 road mechanical disc brakes, compact chainset 34/50, 10 speed 12-27, cross top levers and clearance for up to 32mm tyres. Coming in at sub 20lbs!!! By my reckoning:-

2.41lbs = bars, bar tape, extra padding(for my delicate handies), Dura Ace shifters, Tektro Crosstops, handlebars and all cables including outers.

7.98lbs = chain, 50/34 chainset, BB, Pedals, BB7 brakes including rotors, front and rear deraillers, headset, seatpost clamp, stem, spacers, saddle and seatpost.

2.20lbs = 700x32 tyres with rim tape and inner tubes, allowing about 350g per tyre

So that comes to 12.59lbs leaving only 7.4lbs to cover frame, forks and wheelset in order to reach a total weight of 19.99lbs, is it doable? I can find carbon disc forks at 1.5lbs, so that leaves us 5.9lbs for frame and wheelset. Or have I overestimated for some of the componentry weights?

 

[HJ]

The Kinesis Crosslight3 Disc fork is 693g which is only 13g over your target weight.If you saw of the canti mounts you could probably get it below your target...

 

[MacB]

The Kinesis Crosslight3 Disc fork is 693g which is only 13g over your target weight.If you saw of the canti mounts you could probably get it below your target...

It was a Kinesis carbon disc fork that I got the 0.68kg from on a website.

Problem is that I'd like something really nice, Colnago or similar(maybe without the name in 400 places on the frame though), just with disc brakes and a teensy bit more clearance. I noted that the Kona Haole road frame is only 1.3kg in steel, so that would give us a total bike weight, excluding wheelset, of 16.95lbs.

Leaving 3.04lbs or 1.38kg for the wheelset including skewers, I can easily reach about 1.8kg or 3.96lbs for a wheelset. It's strange I can find all sorts of bikes around the 15-16lb mark. The weight differences for the disc brakes and bigger tyres don't come to anything like enough to tip these over the 20lb mark. Maybe I'll need to revise my weight target up to a max of 22lbs.

 

[Jezston]

Very interested in what you glean from your research, MacB! Currently ride a Kona Dew with mech discs and looking for something lighter and better with drops for my next bike - absolutely fell in love with the Croix De Fer, beautiful looking thing ... but not really any lighter than my £400 Dew!

Currently looking custom build myself with cantis or vees on the rear and disc (or even hub?!?) at the front, but options do look VERY limited.

Disc brakes now legal in cross? Now that is exciting news. Look forwards to what the manufacturers make of that!

 

[MacB]

Well. having read a horrendously large, and often semi literate, amount of guff around this subject.....my conclusion is.....need to wait until the new rulings filter through via the manufacturers. Unless you go down the full custom, ie expensive, route the options are very limited. I expect we'll see the first bigger steps in Cyclocross and there's a fair bit of bleedover into touring. If the sales are there then it should hit Audax/Sportive next and finally, if ever, road race. The three main negatives are:-

braking power - road bikes stop well enough already and the limitations are around the size of the contact patch, ie tyre to road surface interface
Weight - don't want anything that adds weight
not needed - basically what exists works fine

On the flip side I've found little information to support a benefit to speed of tight clearances, though I accept I'm no engineer. It does seem to be a lot to do with brake pull and reach. Disc brakes would remove this as a concern and also concerns about brake performance in adverse conditions and rim deterioration. There're also arguments around rotational weight, ie, though the wheel is heavier, the rims are lighter as they don't need a braking surface. American Classic have a 29er (700c mtb/cyclocross) wheelset using the Notubes Alpha 340 rim for disc brakes that weigh in at 1.367kg total. At these sorts of numbers the addition of disc calipers and rotors is well offset. You could even argue that things like carbon rims become more feasible for everyday use as there won't be the damage to the rim via braking. Doing some rough calcs:-

mechanical road discs - 700g for the BB7 calipers, rotors and bolts, allow 400g for frame fittings heavier fork etc and 100g for the extra cable, gives a total of 1.2kg or 2.64lbs in weight. Offset against this would be what's being removed, say 400g for a rim brake set inc pads. That brings the difference down to 800g or 1.76lbs before you start looking at weight savings that could be made on rims. Plus these estimates are based on pretty high end road stuff rather than the more run of the mill offerings. I'm also sure that manufacturers would start looking at ways of reducing disc brake weights as well.

So, without spending silly amounts of money, I can see a disc brake roadbike being feasible with no more than a 1lb weight penalty over a non disc version, and that's just on current technology. Cleaner rims, longer lasting pads, better all weather braking, easier clearance for guards and bigger tyres, more tolerance if slightly out of true, what's not to like? You could even ride your nice bike in cruddier weather with guards and a bigger wheelset and then whack in the lightweight carbon fun wheels for serious stuff.

 

[GrumpyGregry]

lightweight carbon forks and discs.... Hmmm, there is a whole stack of stress factors the engineers need to get just right to prevent catastrophic failure and my face on the tarmac. As a clydesdale I think I'd want to wait until the second- or third-generation of lightweight rigid carbon disc forks came on the market.

My cynicism sometimes make me imagine things but.... didn't last year's Genesis Croix de Fer have carbon forks whereas this year's has steel? I wonder why? Of course it could simply be cost and trying to hit a specific price point but.....

 

[MacB]

lightweight carbon forks and discs.... Hmmm, there is a whole stack of stress factors the engineers need to get just right to prevent catastrophic failure and my face on the tarmac. As a clydesdale I think I'd want to wait until the second- or third-generation of lightweight rigid carbon disc forks came on the market.

My cynicism sometimes make me imagine things but.... didn't last year's Genesis Croix de Fer have carbon forks whereas this year's has steel? I wonder why? Of course it could simply be cost and trying to hit a specific price point but.....

Fair enough add another 300g and you get a steel fork like the Civia or the Surly 1x1, my original premise still remains that you could go full mechanical disc brake for a much smaller weight penalty than the current OTP offerings would suggest.

 

[HJ]

lightweight carbon forks and discs.... Hmmm, there is a whole stack of stress factors the engineers need to get just right to prevent catastrophic failure and my face on the tarmac. As a clydesdale I think I'd want to wait until the second- or third-generation of lightweight rigid carbon disc forks came on the market.

My cynicism sometimes make me imagine things but.... didn't last year's Genesis Croix de Fer have carbon forks whereas this year's has steel? I wonder why? Of course it could simply be cost and trying to hit a specific price point but.....

Trying to hit a specific price point would also explain the change from 105 to Tiagra...

If you are really worried about the strength of lightweight carbon fibre make sure you only ever fly in aeroplanes with steel wings, as you just can't trust these modern ones with carbon fibre wing struts...

 

[HJ]

I have just estimated the final weight of my Genesis Croix de Fer rebuild project and it works out at 9.8Kg (21.6 lbs), and I haven't even tried to source the lightest components, so it must be possible to shave weight off that.

Of course I haven't actually built the bike yet, so only time will tell if my estimate is right

Oh and by the way the carbon fork on the 2009 Croix de Fer is 617g...

 

[MacB]

I have just estimated the final weight of my Genesis Croix de Fer rebuild project and it works out at 9.8Kg (21.6 lbs), and I haven't even tried to source the lightest components, so it must be possible to shave weight off that.

Of course I haven't actually built the bike yet, so only time will tell if my estimate is right

Oh and by the way the carbon fork on the 2009 Croix de Fer is 617g...

do you know the CdF frameweight HJ?

 

[HJ]

Ah, now that is one of the uncertainties, the kitchen scales don't go over 2Kg and the bathroom scales don't weight under 5Kg, but I think it is about 2.2Kg ± 200g

 

[MacB]

Some more 'interesting' stuff having gotten a few replies back from various websites and manufacturers:-

The Notubes Alpha 340 road rim - claimed at a weight of 340g, hence the name, real world weights seem to be in the 350-360g range based on postings from Weightweenies. Notubes have verified to me that this rim can be run with tubeless, clincher tyres with sealant rather than inner tubes and standard inner tubes and clinchers. Erto is 622-17 and they rate the rim for 700x21 to 700x52 tyres, the rim does have a brake track as well. Drillings are generally available in 28h and 32h, there are 24 and 18 around as well but I believe they're only available as a complete wheelset. It comes in at around 1275g for the lightest, 1322g for the medium and 1367g for the heavier wheelset, including skewers. But they do use the American Classic hubs and I've read quite a few negatives around their customer service and quality control. I think I'd rather sacrifice a few grams and go with something like Shimano XTR hubs. I do like the look of 'boutique' style ones from hope, Chris King, Royce, Phil Woods etc, but they all seem to be noisy when freewheeling.

So a wheelset using these in 32h and XTR disc hubs would be 1600g including skewers, add in rim tape, tyres(700x32), inner tubes and cassette and you've got 2900g.
Then 1100g for steel forks, 700g for the BB7 brakes including rotors and fitting bolts and 300g for brake/gear cables gets you to a total of 5000g so far.
Then allow 2160g for handlebars with STIs and crosstop levers, padding, bar tape, stem, spacers, headset, seatpost and clamp, saddle, bottle cages and front and rear deraillers, this takes you to a 7160g total.
Allow a further 1400g for chain, chainset, BB and pedals takes you to 8560g or 17.72lbs and all you've got to add on is the frame. Now a solid CroMo frame comes in at around 5lbs in weight, for a reasonable price you can get a steel or titanium frame weighing in around 1500g.

So that's it, all in weight of about 10kg or 22lbs and you'd have a pricetag of about £1600 plus the frame, so anything from £1750 to £3000+ depending on the frame you want. Spend towards the upper end and I reckon you could shave in at about 9kg but you'd struggle to get much below that without going carbon on frame and forks.

 

[HJ]

I went for Shimano XT hubs which I expect to be around 820g (with skewers) the pair, will weight them when they arrive, then I was going to use Mavic Open Pro rims (OK so I am more road oriented) 870g the pair, spokes (64) 257g (tbc), 220g for the cassette, then looking at 640g (on road) to 900g (off road) for tyres, rim tape 45g, and tubes about 240g, which gives a wheel set of about 3Kg.

 

[MacB]

I went for Shimano XT hubs which I expect to be around 820g (with skewers) the pair, will weight them when they arrive, then I was going to use Mavic Open Pro rims (OK so I am more road oriented) 870g the pair, spokes (64) 257g (tbc), 220g for the cassette, then looking at 640g (on road) to 900g (off road) for tyres, rim tape 45g, and tubes about 240g, which gives a wheel set of about 3Kg.

Interesting, I may be out on spokes, I was using:-

XTR hubs with skewers - 535g
Alpha 340 Rims - should be 680g but I allowed 720g
Spokes and Nipples - I've allowed for 64 at a total of 400g
Rim tape - as per yours 45g
Cassette - I'd allowed 250g
Tyres - I was going with 700x32 at 700g the pair
Tubes - allowing 250g

That gets me the 2.9kg, Open Pro rims would add another 150g but they're only rated up to 28mm tyres, I know you can run bigger but that's from the Mavic site. Whereas they do rate the Open Sport rims up to 700x32, which would be a weight penalty over the OPs of a further 110g. I am tempted by the Open Sports as they are fairly tried and tested whereas the Alpha 340s are only just out. Plus I can get them for £45 the pair as opposed to £165 for a pair of the Alphas, a full £120 off the wheelset price.

 

[GrumpyGregry]

Trying to hit a specific price point would also explain the change from 105 to Tiagra. If you are really worried about the strength of lightweight carbon fibre make sure you only ever fly in aeroplanes with steel wings, as you just can't trust these modern ones with carbon fibre wing struts... ''

agree about the price point stuff btw though that isn't what they told me when I spoke to them about buying a frame and fork.....

but do you really think bike/fork makers spend the same amount of time doing structural stress analysis as aerospace manufacturers? I don't ;-) (Having broken a couple of Trek carbon mtb frames back in the day I know they don't)

1st generation rigid carbon disc road forks will be slapped together from rim brake designs with a bit of rule of thumb reinforcement here and there "hoping" it will do the job. Rather you than me.