Valves...

Although I've still got 4-way adjustment (high and low speed bump and rebound), there's still going to be times when there isn't enough adjustment available within the bleed and blow-off valves to match into the car. So, instead of the solid piston and through rod design of the original, the picture above is of a standard shim valve and orifice piston. This will be the ultimate determining factor for high speed damping - which gives the option of using the high speed adjusters to control the 'knee point' of the damping curve.

Virtually every damper uses shim valves somewhere along the line as, although they're horrendously non-linear (although a quick study of the various text books will give the suitable formulae - sadly the IMechE site is down at the minute, so I can't tell you if it's in Roarkes or not) they're a nice consistent non-linear, which needle valves tend not to be. The disadvantage is that, other than by supplying a pre-load (and a lot of cheaper dampers provide adjustment this way), you can't change the characteristics except by cracking the damper open and changing the shim diameters, stacking arrangements and thicknesses. For a top end damper, you often get a choice of 30 different shims to build into your stack, which gives an eye-watering number of options and excessively long times spent playing with the damper dyno trying to get the characteristics you want.

The basic design is still going to be twin tube, with all the flow passing through the valve block. The problem with this is that as the damper moves in and out the volume of the working chamber decreases and increases. Now hydraulic fluid is not renowned for it's compressibility, so we need a reservoir to hold this extra fluid. In 'proper' twin tube dampers, the annular ring around the working chamber does the task, but I want these dampers to work irrespective of orientation - the classic way of telling that a twin tube is a twin tube is to invert it, pump air into the working chamber and feel the lack of damping that occurs. So we'll need a second floating piston somewhere, with a gas pressure chamber on the other side.

Gas pressure can be a useful thing, in that you can use it to support some of the weight of the car and you get a slightly (more if you don't have a big enough chamber for the gas) rising rate as well. WNTL? It's another non-linearity which may need tuning, and I don't have huge amounts of free time...

Back to the drawing board...

OK, so I got a price back from my tame manufacturing partner for the design of the six way adjustable damper which made my eyes water a little and, notwithstanding the fact that it was for an engineering one-off, even with economies of scale wouldn't really be viable in the sense of being able to sell it to any target group other than rich idiots.

So we're off on a voyage of discovery into how to make something cheaper. The first thing to go is the plethora of adjusters. I'd envisaged lots of precision drilled barrels which would give consistence between units. Of course if you take a cylindrical component that can be turned out a huge rate and then have to carefully mount them in a dividing head and drill 16 holes (8 for the indexing mechanism and 8 different sized ones for the orifices) in exact positions, then it's going to cost a lot more to build one. If you're building thousands then you sort out jigs and fixtures, but I can never foresee this being a mass production item, not matter how bling it is...

Now, I still want separate adjustability of high and low speed damping at a sane price, so that I can take a unit off the shelf and valve it for most applications. If we can't have miniature drilled orifices to squeeze hydraulic fluid through, then we'll need a needle valve that you can screw in and out to change the size of the orifice. Not as repeatable and you'll need a damper dynamometer (a few thousand pounds) to do setup.

High speed adjustment will be similar to the original design... OK, so a short CAD session later we have mark II of the adjustment valving:

I've shrunk the component count down from eight individual bits down to three (not including the coil spring and various sealing O-rings) and there's no nasty indexing and drilling required. With the exception of an exhaust hole in the blue component and the hex adjuster on the top, it's all lathe manufacture and thus reasonably rapid (and hopefully cheap)

Away from the tiny precision bits, I'm rationalising the design of the main body. Instead of having a massive boring job, I'll use standard off the shelf tubes interfacing with turned caps. Minimal cost, and bar a few threads, no machining. It'll all be fine...

...tumbleweed

has been blowing through this blog for far too long. I could blame pressures of work (somehow I've ended up with four jobs at the same time) or the fact that some of my creativity has gone commercial. Anyway, the pretty picture above is one of the fruits of all that labour.

One of the four jobs has been developing suspension kits for various people. The problem with doing this is that you often have to make do with whatever a damper manufacturer has sent you, particularly in terms of adjustability. Sometimes you get lucky and you can get the dampers set exactly how you want them and other times you just can't get the rates you want.

So, in finest engineer fashion (i.e. slag off the competition, insist you are the only person on the Earth who can actually do the work properly... have no dress sense or sense of rhythm), I've decided to fill my gap by designing a damper to work just the way I want it. I've even got a manufacturing partner lined up, which is a degree of organisation I'm not renowned for.

So, quick run down of the features:

1) It's a through rod damper. Virtually all dampers aren't, so as the piston moves into the body, the shaft it's attached to changes the working volume, which has to be accommodated. Two common methods are employed. Either you have a second concentric chamber as a reservoir (a twin-tube damper) or you have a second piston with a gas spring to one side which can be compressed to take up the slack (a monoshock). This one has the piston in the middle of the shaft and a seal at each end of the working chamber. As the piston moves, there is no change in working chamber volume. Very useful as it means the design can work on low pressures and therefore react quicker to changes in velocity. It also means that the seals can be off the shelf O-rings and DIN seals.

2) No internal valves. The attractive block bolted to the side carries all the valves. So if you want to change something fundamental, you don't have to strip the whole thing down, you can simply depressurise the unit and swap the valves about. Fluid can only flow from one side of the piston to the other via the valve block. This means you get a large flow of fluid and it's far easier to control a large flow than it is a small one.

3) No shims. Commonly, dampers use thin sheets of metal blocking holes at provide the damping force. These tend to be horrendously non-linear. This makes tuning an interesting affair and lots of rebuilding tends to go on. I've engineered this using simple tappet valves and adjustable orifices.

4) Six way adjustment. OK, this is probably overkill. As well as controlling the gradient of the Force-velocity curve in high and low speed situations, you can adjust the knee point where the two curves switch over. All of that adjustment comes without stripping down valves, which makes it ideal for someone with very little time. Of course having six different knobs to twiddle makes it a nightmare for someone with no idea what they're doing.


I've also tried to make the unit as manufacturable as possible. Similar units from Koni and Ohlins retail at around £500 each, and I'm trying to get the price point to something around 40% of that and if any machine shop with a CNC mill and lathe can churn these out in their thousands, then I'm laughing.

First New Bit...

In finest bottom up design methods, I'm starting at the end and working my way forwards. The nicely rendered bit above is the output shaft for the reversing box. It'll sit coaxially with the input shaft (hence the big hole in the middle) and can be driven two ways. The first (and most common) way will be for dogs to engage in the slots on the front and drive the output shaft directly. These dogs will be on a sliding collar splined to the input shaft. In this fashion there will be a non-geared direct connection between engine and output shaft.

The second way will be for a gear to turn the output shaft. If we take the drive from the input shaft via a pair of gears to a layshaft and then via two extra gears to the gear on the output shaft then we'll reverse the direction of travel of the output shaft relative to the input shaft and hey presto - reverse gear. Now given that bike engines tend to major more on power (by virtue of stratospheric rev limits) than torque, a degree of speed reduction (and hence torque multiplication) might well be a good thing as the driver won't need to necessarily slip the clutch mightily to get the thing moving in reverse.

Now for all this to work with a minimum of nasty mechanical graunching noises, all the gears will have to be in constant mesh. This will mean that the gear on the input shaft must float and only be connected when we want reverse gear. If we use the other end of the sliding collar to do the connect then we have a workable design. Everything else is metallurgy and calculations...

Oops...

I know it's been a bit of a while but somebody pressed my 'get a life' button again and I've changed jobs (while still teaching as well) and a home (re)construction project has been filling what is euphemistically termed spare time. Never mind, I'm back now and I suppose I had better get on with some non-paying real work.

One of the requirements for RGB racers is a working reverse system, something that the majority of bikes (and all bikes if you don't class a Honda Goldwing as a motorcycle) don't seem to have fitted. If you've gone for a longitudinal installation of your engine then it's relatively easy to have an extra gearbox between output cog and differential. Unfortunately, I seem to have plumped for a mid-engine layout which makes for a much simpler differential layout (using chain and sprockets just like the donor bike) but does hamper the ability to go backwards under the influence of the engine. A lot of racers use a second starter motor acting on the drive chain but these seem to have a relatively large failure rate (and failure when tested is an automatic disqualification) not to mention issues with how the torque is delivered. The fact that I'm a mechanical engineer rather than an electron herder seems to be pushing me down a purely mechanical option.

So knocking out a quick specification for a black box, I get the following list of desirable features:

1. Lightweight and compact
   
2. User switchable between forward and reverse
3. Minimal transmission losses in forward mode

And that's about it... I'm still at the brain storming stage but I can envisage a system that is basically the reverse gear and top gear from an old gearbox. With one of these you have direct drive from input to output in forward mode and a small geartrain in reverse.

Adjustment...

Sorry there's not been a post for a little while but life has been a little frantic around here, with a mixture of the start of term (my beloved leader is currently tearing his hair out as we've got about 50% more students than we needed/wanted/expected) and the need to rebuild our outhouse before it collapsed taking the house with it. Even worse than that, I haven't got a SolidWorks model to throw up as a picture. Normal service will be resumed soon.

Anyway, getting back on track, I want to make my suspension design adjustable for two reasons: firstly, I have no doubts that any chassis that gets manufactured is going to have dimensional accuracies measurable by thumb widths and the suspension will need to be tweaked back so that it matches what was originally designed. I have a friend who makes recycling lorries for a living and he works to plus or minus 5mm. Secondly, you will want to fiddle with camber and caster settings to get the right grip balance for corners (why you'd want to tweak them will come later when I do the spring rates).

The classic way of performing suspension adjustments is to screw your balljoints (If you're in a cheap formula) or Rose/Heim joints (as stolen from the Luftwaffe at the end of World War II and gifted to either of those two fine engineering companies - one UK and one US) if you're working with cash. If you happen to follow Formula SAE, you'll now that one thing that makes judges tear their hair out is the REIB problem - 'rod end in bending'. Formula SAE goes for light weight - very light, as any car over 200 kg doesn't get through to design finals - which means that the rod ends would look ridiculously tiny on remote control cars. If the rod end isn't fully screwed home the stresses on it cause horrendous distortion and early failure.

I'm unlikely to be using 6mm joints on any design soon, so we can simply spec up the joints to account for any excess bending - in effect over-engineering the joint. If it does bend, it's probably better that it deforms rather than the suspension arms or, god forbid, the chassis. Rod ends are relatively cheap (and as an aside, don't engineer anything in brass for the foreseeable future - I've just ordered a load for the workshop and it's twice the price of Aluminium) and more importantly, easy to replace.

By screwing the joint in or out we can effectively change the length of a suspension are and thus the camber angle of the wheel. If we just have a simple threaded suspension arm, we have to dismantle the suspension to adjust it. This is not a trivial exercise, and more importantly it at least quadruples the effort required to get a car squared away as you have to go through vast cycles of disassembly-adjustment-reassembly-measure to get the numbers you're looking for. Many cars have adjustment ladders - effectively a turnbuckle with a left hand thread at one end and a right hand thread at the other. These add to the length of the arm, and more importantly add extra weight as you have to over engineer to potential joints in bending.

There is another option - a coaxial turnbuckle (I promise to forget about students tomorrow and CAD one up for your general delectation). All the threads are co-axial so adjustment is made by turning the middle portion. Clever thinking also tells you that you can play silly buggers with the thread pitches to give yourself significantly finer adjustment than a normal fine pitch thread.

As for caster, I considered all the sane and insane options and decided to go with having spacers on the inner mounting arms and thus the ability to shift arms longitudinally to angle the upright. SusProg suggests that I'll need around 20mm of movement in the top arm to get 7.5 degrees of caster. Any more than that and the driver will need to go to the gym a lot to be able to turn the wheel accurately - even with the low weight.

Making sense of the Regulations...

Now, I know that I'm supposed to make sense of regulations, or at least try and find all the useful loopholes in them, but the RACMSA 'Blue Book' makes about as much sense as a bucket of chocolate frogs. I'm trying to work out how small I can make the tubing for the spaceframe chassis, and depending on which interpretation of the rules you like. Section Q covers all the safety features you could or should fit on a car and paragraph 1.3 mandates CDS steel tubing, with a minimum yield strength of 350 MPa and either 45x2.5 mm or 50x2mm diameter and wall thicknesses. Yet paragraph 1.5.2 for Sports racing cars says a minimum of 48.3mm with a wall thickness of 2.6mm and 1.5.4 (b) for non-standard cars says 32x1.5mm. Which is right? I think we should be a Sports racing car rather than a 'non-standard' car, but there are significant weight penalties for using thicker than necessary tubing.

So if we look at the cross-sectional areas of the four options we get values of 333mm², 301mm², 373mm² and 143mm². The maximum compressive force that these members can take before starting to permanently deform (for those who remember A-level physics, the equation is of course Force = Stress x Area), varies between 50 kN for the lowest member and 130kN for the largest area. So which is adequate and which will give you a few short microseconds of terror before a basilar skull fracture puts you out of your misery for good?

The human resistance to acceleration was found out in nicely empirical fashion by Dr John Stapp of the USAF who strapped himself to a rocket sled and pulled 46.2G in a frontal deceleration. Using Newton's second law (Force = Mass x Acceleration), if we get our car down to the minimum weight of 560kg, we get an impact force of 254 kN. If that impact force is suddenly applied (and you can bet it is), the initial impact stress is actually double that (so effectively we can only use half the limiting impact force if we want no margin of safety). I'm vastly oversimplifying the reality, but SolidWorks doesn't come with a copy of DYNA3D or PAMCRASH, which is what the automotive industry use for this sort of thing.

Anyway, if we have two roll hoops, with extra front and rear stays, we'll have 8 struts resisting an impact force.
If we use the smallest diameter tubing the cage will yield under such loading, whereas if we use the largest tubing we have a factor of safety of around 2. Is it really worth saving weight if you get one of those once in a lifetime accidents - and you want to be around to tell the grandchildren all about it? I'd be inclined to use the lightest of the large tubing (which has other advantages, such as being less likely to buckle than the thinner diameter - so that's 50mmx2mm thickness round tubing.

For frontal impacts we can trade deformation for force - i.e. limiting the force and hoping the car stops before the metal tubing makes contact with our delicate feet. F1 expects a peak rate of deceleration of 10g for the first 150mm of deformation, and 20g for the first 60kJ or energy absorption. For our car 20g and assuming that our non-CDS metal tubing (so a yield strength that could be as low as 275 MPa) will yield under such an impact, we need an impact force of 110 kN and thus a cross sectional area of about 400mm². There will probably be 4 frontal force members here (and a nice big chuck of energy absorbing honeycomb to keep the force at those levels for at least the initial phase of the impact). So each tube needs a cross-sectional area of 100mm². So it's a case of running through the sizes to work out what tubing will work in these areas. A quick bit of maths lets me know that I need tubing that weighs around 780 g/m. In circular tubing that means 7/8"x16g, in square that means ¾"x16g and in rectangular we can use 1"x½"x16g.

I finally got out to measure someone's chassis and bodywork at the weekend and the picture at the top is a quick revision of the chassis, less the back end and using thinner tubing. With a 50mm diameter cage on the top, but using only ½" round tubing, the whole lot weighs around 34kg. The chassis was using 1" tubing and had a lot of extra tubes knocking around (it was originally a prototype chassis), so weighed considerably more. Even with a ramping up in tube size, I reckon we can get significant weight savings in place to get the car close to (or even below) minimum weight.