16 speed Brompton: part 7 – clearances

 

1) Tyre tread to fork cross piece.

The tyre is a Schwalbe Marathon pumped up to near normal pressure. The cross piece is the one at the bottom of the triangle next to the rear hinge. The bike is upside down in this picture.

The rear triangle has been opened out to accommodate a narrowed Nexus 8 Red Band hub. The OLD for the Nexus in this configuration is around 122mm. The rear dropouts have been widened by 1mm which is necessary to fit the Nexus dropout washers. The 1mm has been removed from the “back end” of the dropout which sets the hub a little further away from the cross piece than if the metal had been removed from the side of the dropout nearer to the front of the bike.

If you open the fork out to accomodate a hub with and OLD of 135mm then this gap will decrease further.

Clearance between lower cross brace and tyre tread

Clearance between lower cross brace and tyre tread

 

2) Sprocket to end of fork tube.

This is a 16 tooth sprocket mounted with the dished side facing the hub. This is to push the chain line outwards a few millimetres. The white object on the left inside the ellipse is the sprocket and the end of the fork tube is to the right of it. Clearance is minimal but sufficient. The chain has been moved off the sprocket to take this picture.

Rear sprocket to fork clearance

Rear sprocket to fork tube clearance

 

3) Spokes on the non-drive side to rear fork tube.

There is just enough clearance on my bike but I had to put a couple of small dents on the inside of the tube on Damae’s bike. If there is any significant play in the wheel bearings then the spokes will start to make contact with the rear fork (as the front of the wheel will be pulled to the left by the chain). The bike is upside down in this picture.

Spoke to rear fork clearance on the non-drive side of the Nexus 8 hub.

Spoke to rear fork clearance on the non-drive side of the Nexus 8 hub.

The fork spreading tool

Here are a couple of pictures of the tool I used to spread the rear forks on our Bromptons.

Brompton spreading tool

The assembled for spreading tool. The dropouts are clamped between the pairs of large washers.

Brompton fork spreader

The section of steel tube is just over 7 cm long. The ends of the threaded bar slide into the tube. Once the spreader has been bolted to the dropouts, the innermost nuts are then screwed in against the steel tube thus pushing the threaded bar sections outwards.

Tool to spread forks.

Detail view of the arrangement of nuts and washers on the threaded bar. The steel tube section slides on from the right.

Materials required

1 x piece of thick walled mild steel tube, outside diameter 12mm
1 x piece of M8 mild steel threaded bar (minimum length 40 cm)
4 x large M8 washers
2 x small M8 washers
6 x M8 nuts

Tools

Hacksaw or other metal cutting tool
File or sand paper

Instructions

1) Cut two 20 cm lengths of threaded bar. (I halved a 50 cm long piece of threaded bar)
2) Take the steel tube and cut of a piece 7.5 cm long.
3) Dress the cut end of the tube and threaded bar, if necessary
4) Screw on the screws and place the washers in the order shown in the third picture.
5) Slide the ends of the threaded bar into the section of tube as show in the first and second pictures

You are now ready to go!

How far do I need to spread my forks?

This is a bit difficult to say. You need to spread the forks further (under tension) than the 122 mm required to fit a modified Nexus 8 hub. The steel that the forks are made of will bend elastically before it starts to deform permanently.

With the tool in place I needed to wind it out to 138 mm on my Brompton which, when the tool was removed gave me a 122 mm opening. Damae’s Brompton needed the tool to be wound out to 140 mm before the Nexus hub would fit.

If in doubt start with a lower value, say 130 mm. Remove the tool and measure the distance between the dropouts. Then adjust as required.

You might need to bend the drop outs after spreading the forks so that they are parallel. Measure the gap when the dropouts are parallel again and adjust as needed.

Disclaimer – what happens if something goes wrong?

Use of this tool could damage you and your Brompton/bike. You are responsible for your bike and the modifications you make to it. I accept no responsibility for any damages or losses incurred by using the tool described in this post.


Note

I do not recommend opening out the forks to accept a hub with OLD of 135 mm. You will get clearance problems between the tread of the tyre and the cross piece on the fork near to the rear hinge. To fix this problem you will need to modify/replace the dropouts or find another way of increasing clearance between the rear tyre and the cross-piece.

Brompt-o-lieb

Here are some pictures of the piggy-back frame I made to allow me to carry Ortlieb panniers on my Brompton. This was a proof-of-concept frame I knocked up in a couple of hours with no access to tube bending tools. I never got round to tidying up the welds properly nor have I painted the frame. So be warned, it is not a pretty sight. However, it has functioned perfectly whilst fully laden all summer (1100km including the Rallarvegen). The total load on the luggage system was 16-18kg depending on how full my water bag was.

1) The view from the rear of the Brompton touring bag. The frame is attached at the top with a pair of jubilee clips. Most of the weight is transferred directly to the mount at the centre of the Brompton bag frame. This mount, in turn, sits directly on the Brompton luggage block. This seemed to be the most sensible way to do it at the time. I am not sure I would do it differently now, as it still seems to make sense.

Brompt-o-lieb piggy back frame

The piggy back frame from the back. The top bar looks more curved than it is. This is due to distortion from the wide angle lens.

2) Close up of the lower mounting point. There is an L-section piece of metal welded onto the middle of the bar. The bottom of the L protrudes forwards and sits in a little trough at the top of the plastic mount. I don’t know if newer Brompton bag frames have the same feature.

Detail of the lower mounting of the Brompt-o-lieb piggy back frame

Detail of the lower mounting of the Brompt-o-lieb piggy back frame

3) View of the top of the frame and how it is fastened to the Brompton frame. The jubilee clips work a lot better than the fastenings I made myself. The clips pull the frame down so that the L-shape tongue is clamped to the plastic Brompton bag mount. One of the problems of this design is that later Brompton touring bags cover the Brompton bag frame almost completely. You’d have to unpick part of the stitching at the top of the bag to be able to get a jubilee clip or other fastener round the top bar of the Brompton bag frame.

This is one of the reasons why I plan to do away with the Brompton bag completely in the Mk 2 version. The other reasons is to be able to use Ortlieb front panniers on both sides of the frame and save the weight of the Brompton bag frame.

Brompto-o-lieb piggy back frame

View of the top of the piggy back frame

Bromptons on the Rallarvegen

Last weekend we decided to give our 16 speed Bromptons another test. The test was to cycle the Rallarvegen in Norway from Haugastøl to Flåm. We have wanted to do the Rallarvegen since we moved to Norway almost three years ago and the Bromptons would make getting back home on the train a little easier.

We took it gently, mainly because Damae is now 26 weeks pregnant. On the Friday we cycled from Haugastøl to Finse and camped at the DNT Finsehytta ‘campsite’. Saturday we cycled from Finse down to Flåm and took the Flåmsbanen back up to Myrdal. The weather was brilliant and the Bromptons created a lot of interest.

Stan, Damae and their 16 speed Bromptons at the highest point on the Rallarvegen

The trip gallery is here.

16 speed Bromptons: Part 5 – ready for action

Our 16 speed Bromptons are now just that. We built our own wheels earlier in the year and I then went through the process of getting everything to work together. I had already spread my rear forks a couple of years ago and did the same on Damae’s Brompton. A few tweaks were needed on her bike to get sufficient clearance between the spokes where they come out of the Nexus 8 hub and the fork and the rear derailleurs had to be shaved with a ‘Drehmel’ to provide enough clearance for the Nexus 8 gearchanger.

We’ve managed to put around a hundred kilometres on the bikes and they will be used for our summer tour. This will be a sedate affair because of Damae’s condition, taking in part of the NSCR in Germany and Danish cycle route 5 between Fredericia and Grenå. A travelogue will eventually get written as will a full report of the conversion.

From our point of view the conversion has been well worth the effort and money. The gearchange (with the single twistgrip) is fantastic compared to the 2×3 SRAM Brompton setup we previously had. The high range on the 50t chainwheel is good for general cycling, with a bigger range than the old SRAM 6 speed, and the low range on the 34t cog allows climbing up steep hills fully laden. Only time will tell if the setup is reliable.

For the time being, here are three pictures of my bike set up for touring. We’re using my “Brompt-o-lieb” adaptor frame to mount two front panniers on the back of the Brompton bag. We’ve used these a couple of times for short trips and the bikes are very stable even when loaded with weights of around twenty kilograms. At the moment the front bags and contents weigh around sixteen kilograms which is a lot less than we normally take when touring with ‘proper’ touring bikes.

My main worries for the trip are:
a) the Brompton derailleur breaking
b) our home made wheels failing
c) being unable to get the Schwalbe Marathon tyres off the Sun CR18 rims. Getting the tyres on is trivial with the Simson Tyre Mate (recommended purchase) but I managed to break the Var Tyre tool the first time I used it to remove a tyre.

Finger’s crossed, wish us luck (although “Hals- und Beinbruch” might be more appropriate). Here are the pictures:

16speed Brompton conversion

Rear view of my Brompton. Both bikes have two bar bag mounts and we will be using both of them. I fitted a rear rack to my bike, for two reasons:
1) it acts as a damper reducing high frequency buzzing from the rear wheel
2) The bike, when folded rolls better with a rack than without it

Front of Stan's Brompton showing the Brompt-o-lieb frame

The outer edge of the Brompt-o-lieb is just visible in the picture. Loaded with 16kg the bike is much more stable than with 10kg in a Brompton bag alone. The bags can be removed as a single unit with a little extra care and carried around without removing the Ortlieb bags.

The cockpit of Stan's 16 speed Brompton

View of the ‘cockpit’. There is plenty of knee room when cycling. Starting off takes a bit more care with positioning the pedals for the first down stroke. The handlebars are adjustable with a long allen key the lower sections can rotate upwards. In this position the bike still folds. I modified the handlebar stem using a rigid seat post rather than a sprung one as is more usual.

16 speed Brompton: Part 2

My SRAM gears have been having problems recently when back pedalling. The chain would go slack and then suddenly tighten up, or pull the chain off the chainwheel. It just didn’t feel right so I dismantled my Brompton and on checking the SRAM hub’s bearings I discovered I’d overtightened them. I loosened the bearings off and all was well again. However once the rear wheel was off I decided that I’d had enough of the Brompton 6 speed and today, right now was the time to start my Nexus 8 conversion.

I’d printed off this page from the AtoB site-link long since dead which gives reasonably clear instructions for modifying the Nexus hub and spreading the forks. I’d bought the bits to make the spreading jig already so it was just a case of fitting the bits together and bolting to the Brompton rear fork dropouts.

Brompton rear fork spreading jig.

My home made Brompton fork splayer bolted to the Brompton dropouts

Brompton rear fork splayer

Another view of the fork spreading tool

Then with just a little trepidation and with Damae standing by for moral support I started turning the inner pair of nuts. Slowly but surely the rear of the Brompton forks widened and at approximately 136 mm (under tension) I decided to check the width against the modified Nexus hub. I removed to the tool and the forks sprung back to around 120 mm width at the dropouts. The hub didn’t quite fit but I realised that I’d not filed down the outer lock nut on the left hand side yet as the instructions suggested. I took the hub into the garage and removed the lock nut and filed it down and re-fitted the locknut. A little to our surprise the Nexus then fitted in perfectly and I hadn’t managed to break the forks. It was very easy to do. (NOTE: I ended up using the tool again to open out the forks a little more. When I fitted the dropout washers it was quite hard to get the hub in and out. This time I wound the tool out such that the gap (under tension) was 138mm. The Nexus 8 fitted more easily after I did that.)

Nexus 8 in a Brompton rear fork

Nexus 8 in a Brompton

Nexus 8 hub gear in a Brompton

Nexus 8 in a Brompton. Note the absence of the black dust cover. This was replaced later after being modified a little.

Widening the forks had been my biggest worry. If the forks had broken then I’d not only have lost the use of my Brompton, but I’d would have needed to shell out for a new rear fork and then fit them. Now the forks had been widened I turned my attention to modifying the dust cover. This is made of two components and large plastic cover and a smaller metal cup that fits on the outside of the plastic cover. The central sleeve of the black plastic cover needed to be filed down by about a millimetre but the metal cap needed some more drastic surgery.

I fitted the cap to part of the jig and clamped the assembly in the vice. Taking an angle grinder with a thin cutting disc I carefully cut off the shoulder of the metal cap and dressed the cut edges. When I fitted the cap and dust cover back on the hub still just fitted in the forks. Although people do run Nexus hubs without this dust cover I didn’t want to do that as I’m aiming for maximum reliability.

Modifying the dust cover end cap

Modifying the dust cover end cap

I had to try to work out the offset of the rim and hub so I could work out what size spokes I’d need to order. With Damae’s help we measured the clearance between her Brompton rear wheel rim and the non-drive side of the rear forks. This turned out to be 13mm. We then placed the rim round the hub and bolted the hub back into the Brompton forks. With some careful measuring and re-measuring I came up with some figures that I would be able to put into an online spoke calculator. I’ll put the correct figures in the blog when I know what they are ie after building the wheel 🙂

I found that SJS Cycles do some short spokes that should be suitable. This means that it will take at least a few days for the spokes to get here. In the mean time I want to be able to use my Brompton. So I had to find a way to get my SRAM hub back in my Brompton forks. To do this I had to put some spacer washers on each side of the hub. This seemed to work fine and the wheel went back into the forks with only one real problem. As the forks have widened the two speed changer is now too far out to be able to pull the chain onto the larger inner cog. So my Brompton is now a three speed Brompton. Another minor issue is that the SRAM gear change cable and the chain tensioner now foul the front wheel when the bike is folded. This should not be a problem with the Nexus hub in place as the gear changing mechanism is on the inside of the forks and I will be using a different chain tensioner.

SRAM 6 speed in a widened Brompton fork.

The SRAM 6 speed in the widened rear fork. You can just see the spacer washers on the left hand side of the axle.

As luck would have it SJS Cycles had a 50/34 double chainring with steel rings at a very reasonable price. I was getting a bit fed up with the cost of the Brompton chainrings each time I needed to replace them. When I fitted this double chainring a few days ago I noticed that it felt considerably stiffer than the standard Brompton chainring. On accelerating it felt like the power was going straight to the rear wheels. I realised that there must be a lot of flex in the standard chainrings and cranks. I’m sure that any theoretical efficiency losses of the Nexus 8 over the SRAM three speed will be cancelled out by using a stiffer crank and chainrings.

Cheapie 50/34 crank on a Brompton

50/34 spider and chainrings on a Brompton

Modified dust cover on a Nexus 8 speed hub

Here is the narrowed Nexus 8 speed with modified dust cover ready to go into my Brompton