Front wing stays (and ways to stop them breaking)

Introduction

Modern sevenesque cars are usually fitted with motorcycle front wings rather than the old-fashioned 'clamshell' wings. The motorcycle front wings are small, tidy, and don't have any nasty aerodynamic tendencies. However, they do have one significant problem - their stays tend to break due to stress fractures. The main problem is that each wing is only supported on one side and tends to bounce up and down: some stay designs are better than others, but eventually the vibration will concentrate on a stress point such as a weld, a fastener or a change of shape or section. After a period of time (varying according to the stay design) the stress concentration will start to crack the metal, and eventually break it. In my case, the edges of the break showed the classic signs of stress fracture - curved, shell-like shapes going across the broken surface from one edge. It was also the nearside wing stays that always broke, as they were the ones that suffered the most punishment from kerb-side potholes, drain covers and so on. I was lucky that only one of my stays would break, leaving the wing flapping on the remaining one; other people have had their wings fly off completely, only to see them run over by a following car.

Possible solutions

There are a few possible solutions to this problem:

• Make the stays stronger - this may solve the problem, but it will mean more unspring weight on the front suspension and there is a significant chance that the stress point will simply be moved somewhere else.
• Share the flexing along the length of the stay - by using a tapering section for the stays, or by arranging several lengths of steel to act like a leaf spring, the flexing can be spread along the length of the stay so that no one point gets all the stress. This could be a valid solution, although implementing it could be a challenge.
• Stop the wing flexing in the first place, by supporting the wing on both sides - in other words, remove the cause of the stress. A bonus would be that the weight of the stays could be minimised and the unsprung weight reduced. However this is a complex solution that requires stays on the outside of the wheel - tricky to set up, and the extra stays are not to everyone's taste! This sort of support is vulnerable, so minimising the risk of damage to the wheel or axle would have to be considered during design.

I decided to try the third option, having seen it successfully implemented by another builder, Dave Greeves.

Methods for supporting the outer stays

That builder's elegant solution was to replace the centre cap of each front wheel with a new cap that is securely fixed inside the centre hole. This new cap supports a ball bearing in a holder, and a bolt is fixed into its inner race. The outer stays are fastened to the bolt with a nylok nut. One thing rankled with this design - the front stub axle is rigidly fixed to the same suspension upright that supports the inner stays, and if I could fix the outer stay support to the end of the axle then I wouldn't need a bearing.

I had a good look at ways of fitting something to the end of the stub axle, but realised that there were several significant issues that couldn't be ignored:

• There wasn't much thread available at the end of the stub axle, once the hub nut, locating washer and split pin had been fitted. I would only have about 3 or 4 threads available, which wouldn't have offered a secure location.
• Normally the hub bearing is protected by a tight-fitting cover - if a support was fixed to the end of the sub axle then it would have to pass through this cover, meaning that a seal would have to be provided. This seal would have to provide the front bearings with good protection from water, mud and dust, and would run against an unlubricated rotating surface - it would be very difficult to achieve a seal that was 100% effective.
• Any accidental impact on the support could result in damage to the stub axle - and these are now quite hard to obtain.

In the end, I decided to see if it was possible to overcome these issues. After much head-scratching and doodling I came up with a design that uses a replacement hub nut made from hexagonal stainless steel, machined in the lathe to provide an extension to support the stays. This design uses the standard dust cap with a 15mm hole drilled in it, and various seals on a 14mm shaft to keep the muck out. Originally I had intended to use V-seals inside and out, but there was insufficient space inside the cover so I used an oiled felt washer inside and a V-seal outside. Both of these work very well when there is some radial mis-alignment - it is impossible to get the dust cap perfectly aligned every time it is fitted, and the wheel bearings also have a small amount of play (by design), so some latitude is always required. The new hub nut has a circle of holes near its base so that a cotter pin can be fitted.

First the new hub nut was fitted, adjusted and secured with a cotter pin, then the felt seal was added. This was followed by the hub dust cover, then the v-seal. The v-seal would normally stay put on the shaft, but as backup the stay carrier both supports it and provides it with some protection. The stay carrier was held in place with an M8 socket cap-head screw and a spring washer. Note that the screw does not need to be tightly fixed - only sufficient to stop it coming undone.

Plastic was chosen for the 'hub cap', both to avoid corrosion and to reduce any risk of scratching the wheel. I was fortunate in that I was donated some samples of plastic used by professional model makers - it's a very hard and tough polyurethane that machines beautifully. This was designed to be glued onto the stay carrier, so it wouldn't rotate with the wheel - it's not obvious, unless you look closely. It also means that removing a wheel is very easy, as the hub cap comes off with the stays.

Any accidental impact on the support is likely to destroy it, but the wheel and axle should not be damaged as the M8 screw forms a deliberate 'weak point' where the stay carrier meets the hub nut shaft.

Design

The wheel hole and protective hub cover were measured to get the basic layout of the available space. The following picture shows that there is only a little space available.

The following diagram shows how the components are arranged:

This was then broken down into detailed drawings for the two components:

Finally a decorative hub cap was designed to finish everything off:

Manufacture

Hub Nut

The hub nut was a relatively simple piece of lathe work, but the correct machining sequence had to be followed. The material I used was 1" A/F hexagonal stainless steel bar.

1. I mounted the bar in a 3-jaw chuck and removed the corners sufficiently to provide a clean round surface, using a 45-degree tool to put a nice chamfer on the edge of the remaining hexagonal portion; the turned section ended up very slightly less than 1" diameter (this was not a critical dimension).
2. I reduced the diameter to form the 14mm bearing surface, using a straight knife tool. Again, this was not a critical dimension as the v-seal will fit any diameter between 13mm and 15mm - but the finished diameter must fit easily through the hole in the dust cap, so 'as close to 14mm (0.551") as possible' is a good target.
3. The 7.1mm tapping hole for the M8 thread was drilled right through the piece, so that it would be visible when the nut was cut off.
4. An M8 tap was used in the tailstock chuck to start the threads - they were to be finished in the bench vice, but it is always a good idea to start threads straight and true in the lathe. This should NEVER be done under power - unless you have some impressive hole tapping machinery!
5. The piece was cut off the bar stock - I chickened out of using a parting tool, preferring to hacksaw it off at the bench.
6. I re-mounted the piece with the hex outwards and faced the piece off to the correct length, again with the 45-degree tool. This was also used to give a matching chamfer on the other edge of the hexagonal portion.
7. The hole was checked to ensure that it was central - I was lucky that 3-jaw chuck was accurate enough, as the bearing surface did not have to be EXACTLY concentric with the inner thread, but it could have been necessary to use a 4-jaw chuck and dial gauge to get it right.
8. A 14.5mm tapping hole was drilled for the 5/8" UNF thread - I had to be very careful about the drilling depth, as it had to be deep enough to fit completely over the end of the stub axle without drilling the end off the workpiece (there's about 25 - 30 thousandths of an inch to spare!).
9. A 5/8" UNF tap was used in the tailstock chuck to manually form the first few threads - again, not a job to be attempted under power!
10. The piece was removed from the lathe, taken to the bench and the threads were finished. Note that the 5/8" UNF thread has to go down as far as possible into its hole, as the stub axle is fully threaded and there's no depth in the hole to spare.

This was followed by drilling the 12 evenly-spaced cotter pin holes, using a jig bolted to the baseplate of my pillar drill. The jig was set up so that the holes were exactly the correct distance from the mounting face of the workpiece (0.5") and exactly central to its diameter. Six holes were lined up by ensuring that each vertical face was at 90 degrees to the baseplate (using a square) and the others at 60 degrees (using a spare nut between the baseplate and a face of the workpiece). Each hole's location was 'spotted' with a centre drill to locate the drill bit - there was a risk of the drill bit wandering on a smooth curved surface. This technique is shown in the following pictures:

Stay carrier

There was nothing complicated about machining this component to the diagram, apart from the following:

• The head of the socket cap-head fastening screw was measured and the measurements of the outer recess were adjusted to suit. I chose not to use any washers under the screw's cap.
• I set the stay carrier in the lathe's 4-jaw chuck, slightly off-centre. I then used a small round-nosed tool to cut a groove around approximately one-third of the way around the circumference to act a location groove for the stays - this can be seen in the picture below. The groove made setting everything up for welding much easier as the stays could be locked into it.

Stays and wing fastenings

The stays were made from 5mm diameter stainless steel round bar. Each pair was made from a single piece of rod, bent into a 'V' with a radius to match the the stay carrier, and then TIG-welded to the stay carrier. The stays were bent to suit the wheel and mudguard positions with the aim of clearing all the moving parts, but without them sticking out too far and risking accidental damage.

Each stay was fastened to the wing using tabs fixed to the existing inner stays, and the connection to the tabs was made using loops formed on the ends of the stays. The loops were made while the steel was red hot, using a simple bending tool held in the vice. This tool had two 5mm pegs held in a block of steel with their centres just over 10mm apart - with this device each loop could be formed in about 5 minutes.

The finished wing stay system

The following pictures show the wing stays in place. I like them, even though I know that they're not to everyone's taste! It is fun watching people trying to work out how it works though...

Having tested these on the road, it is clear that the wings no longer flap around at all so (hopefully) the chances of the original stays cracking should be remote - time will tell.