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Disc brake upgrade

Matthew Cochrane2020-03-21 14:45:23

I'm thinking of upgrading my one-year-old Dawes Duet to disk brakes.  The supplier says the upgrade can be done, but I need a new set of 26" wheels, and if I want to get them from Ebay or somewhere, it'll reduce the overall cost.  He mentioned "100 front" and "135 rear".  The list of wheels on Ebay is endless and varies from 1-£300, and I haven't a clue what to look for.  Anyone have any advice?

Martyn Aldis2020-03-21 15:33:21

Our Dawes Discovery Twin came with a 135mm spaced rear hub, the normal Mountain Bike spacing of the time, (this is the width Over Lock Nuts so you may see 135 OLN quoted), 36 spokes and fairly basic rims and hubs. This was OK for a few months but when we toured with luggage we had rear spoke failures and then free hub failure. As I had got a bit more knowledge by this stage I realised the frame was actually made with 145mm spacing as it was the same as higher models in the range, such as the Double Edge. A local shop supplied us with an Shimano XT mtb disk capable hub which I spaced to 145mm with a longer axle and the shop built it up with 36 DT Swiss Alpine III spokes and a Mavic DH rim. I put on a very powerful Hope disk brake and this worked well for a time but then the rim started to split at the spoke holes. Another DH went the same way so I gave up on that brand and the disk and rebuilt the wheel myself with Velocity Deep V rims. This has worked very well over seventeen years, wearing out three rims, but keeping the same spokes.

What does this mean for your situation? My take is:

Basic MTB wheels aren't really adequate for a tandem with a two adult crew unless the riders are very light, weak and don't ride steep hills or carry luggage. A disk brake increases spoke loading.

The difference between left and right side spoke tensions is very much greater on 135mm spacing than 145mm spacing and this difference is the weak spot of rear wheels with 7+ cogs. If your frame is really 145mm spaced or a compromise you could go to 145mm. If you are stuck with 135mm you could use differential spoking using thick spokes on the right and a butted spokes on the left. If you Google differential spoking, you should find information on this - Chris Juden did very clear write ups when he was at the CTC.

As the spokes carry the braking force with any brake at the hub you need to source strong rims with a good thickness where the spoke holes go through the metal. We have had no problems with Velodity Deep V or old style Velocity Cliff Hangeras and the rather low braking track thickness would not matter if you are mainly using your disk brakes. These rims are expensive new but sometime come up unused on ebay. We have Ryde Andra rims on our Thorn tandem and these are super strong also heavy and not expensive in the normal finish.

So to sum up, if you are going to use the bike a fair amount you will probably need to get something a good bit more specific than a basic MTB wheel. Double weight, the torque from tandem climbing in low gears and the reverse torque from disk braking all push the limits on solo wheels but careful choice of MTB components can get you a good result if best wheel building practices are applied.

I like having three brakes and on our Cannodale we have front V brake as well as the two disk brakes that came as standard. I'm working on putting a single rear disk on the old Dawes again mainly to save rim wear as much as anything.

Hope this helps,

Martyn

James wilson2020-03-21 15:58:24

I believe they are referring to wheel spacing-the area between the frame where the wheel fits. The frame rests on the "axle" and the quick-release or nuts tighten down the frame the axle. If you have more or less gears that can change the spacing. 

Chris Bishop2020-03-21 17:45:23

Why do you want to upgrade? We had an entry level Dawes with just two V brakes for several years, went up and down a few hills on it. Eventually we upgraded to a generally better tandem. The Dawes needed a serious renovation or replacing and we decided to replace. I think the wheels you have are 48 spoke front and rear which is what we have for touring with luggage look for similar. You will have sent a lot on an entry level tandem, money might be better saved for when you upgrade?

Paul Emsley2020-03-21 18:14:37

Does your frame have the necessary fixing lugs for the disk calipers?

Martyn Aldis2020-03-21 18:14:38

Yes, if 48 spokes these wheels are likely to be a better bet than moderate priced off-the-shelf solo MTB disk wheels. If it is tandem hub it could be 145mm spaced and if  you are lucky it could even have a thread on the left of the hub which would let you fit an adaptor for a disk brake as brake number three.

I take Chris's point about over upgrading. We have had a lot of happy use and I have had a lot of happy tinkering with our Dawes but really the quaility of the components as supplied new was not up to regular tandem use and had there been a choice we would have done better with a model further up the range with better components but the same frame. In fact the frame is now the only bit left of the original 2001 bike.

Chris Peachey2020-03-21 18:25:40

I would doubt the advantage of the upgrade you are suggesting.

As already pointed out the rear tandem hub on your machine is almost certainly 145mm wide so you would need a disc hub of that width. Sourcing this via E-Bay is risky. Why would someone be selling a decent disc/tandem wheel?
Suitable tandem wheels from a reputable source will not come cheap.
I would recommend checking your existing brakes. Better brake blocks can make a big difference. Also check the cables and outer casing are not causing a problem.
On long descents a third brake is an advantage. Disc brakes can fade seriously if applied for long periods, ie 10+ mins downhill.
If you still want discs consider replacing the tandem rather than an expensive upgrade that would add little to it’s value.
 
Chris
Technical Officer.
 
PS…£300 would be the minimum for decent tandem wheels!
Bob Cordran2020-03-21 19:31:34

I would echo the two Chrises' comments.  I have a Duet and looked into upgrading to discs a year or so ago.  From memory the forks do have the right lugs (but don't take my word for it as they may have changed this since I bought mine).  However, on investigation, it soon became clear that it would be way more expensive to upgrade properly than could be justified on a tandem of that price.  As Chris P says, the wheels alone will be north of £300, and then you need to have the brakes themselves, levers/hoods, hydraulics...    I can't remember exactly how much it would have ended up costing, but it was more than I considered would make sense given the overall spec of the bike and what I'd paid for it (new) in the first place.

Matthew Cochrane2020-03-22 10:04:36

Many thanks for all the helpful comments.  I've enjoyed reading them.  The supplier confirmed the lugs do exist, but what your're all saying is the main expense will be upgrading the rest of the machine to match, and for the money we can do better.  We will put the upgrade on hold, see how the summer's riding goes and consider trading up.  After all, why ask for advice if you don't intend to take it?

Cheers

Matt

 

Chris Lee2020-03-22 14:33:08

We have a ~20 year old Dawes Discovery  on which replaced the rear wheel and had the front wheel rebuilt with a disk compatible hub so we could upgrade to disk brakes. 

Other than replacing parts we've worn out (chains, cassettes and brake /gear cables and tyres) the components are original except for the rear derailleur.

We've used the Dawes for over 16000km of very heavily loaded touring and had only minor problems with it.

David Ebling2020-03-22 15:27:19

My take is that if you aren't happy with the brakes, there's no point downgrading the wheels to something not up to tandem usage. If you're willing to pay for decent quality wheels then it may be worth it. Get the biggest rotor you can. I rate Shimano hydraulic discs. Cable discs require quite a bit of faffing for pad wear. As you have flat bars I think, you wouldn't have to buy expensive STI shifters/brake levers to go to hydraulic discs like you would with drop bar bikes.

A more cost effective upgrade would be some Avid Speed Dial 7 brake levers, the best V-brake cantilevers you can find, and some swiss stop pads. Decent quality brake cables won't hurt either.

Alternatively Magura Hydraulic rim brakes will improve the braking a lot. They can run quite close to the rim though so wherl trueness is critical and if your wheels flex lots they could rub. I don't have experience with them on tandems so might be worth asking around before going down that route.