Flywheel – Chamber of Understanding

RX8 Project – Part 9, Flywheels Part 3

So just to finish of the flywheel section here are the the finished custom parts :

Flywheel spacer on the crank, you can see the black dust seal in the centre covering the new pilot bearing underneath.

A wider shot showing the spacer in position among the currently disassembled state of the engine.

And finally the flywheel itself.

In this photo the ring gear and location dowels for the clutch basket have been fitted.

The ring gear was actually a lot easier to fit than it was to remove because you can just put the ring gear in the oven (at maximum, in my case 250°C+ off the end of the scale!) and put the flywheel in the freezer for an hour or so as well – this may not actually be necessary but you want the most possible room between the parts when you fit them together. If the ring gear snags on the way down it because there isn’t quite enough space it can be a real pain to get it off again. Before installing the ring make sure it is the correct way round – all the teeth should have a bevel on one side to help the starter engage cleanly this goes towards the position of the starter motor! Take the hot ring out the oven, check it and drop it into place as quickly as possible but make sure it’s right and fully seated to the shoulder of the flywheel. Once touching the flywheel the ring will cool rapidly and lock in place.

The dowels in question turned out to be the wrong size, I specified them as 1/4″ diameter (6.35mm) and this is what is still shown on the drawing but it turns out the ones I measured had more rust than I thought and the holes in the clutch basket are actually designed to locate on ~~6mm dowels – something I really should have checked!~~ From what I have since found out this is likely one of the many Ford engines which have special dowels ~~which are (from what I can find out) 8mm on the flywheel side but only 6mm on the clutch side.~~ The correct dowels are actually 6.30mm on the smaller diameter so my original measurement wasn’t actually too far off, I just shouldn’t make daft assumptions! Larger end is 7.97mm diameter by 6.5mm long on the ones I have, overall length is 18mm. Tolerances and fits are not my strong point but I’ll probably start with a 7.9mm drill and hope to press fit them.

For simplicity I recommend buying something like this available via eBay as Cosworth clutch dowels by x-power engines:

I’m planning to modify the appropriate holes on the flywheel to use the correct dowels I just haven’t quite got round to it yet!

I should probably also take a moment here to mention flywheel bolts. The Duratec crank has a slightly unusual thread which is M10x1.0mm (M10 Extra fine). This is as it happens the same thread commonly used on brake hydraulic components like bleed screws. Needless to say the stock bolts are far too short as the engine originally just had a thin flex plate so longer bolts were needed. Now various companies will sell flywheel bolts for almost any engine but not for something like this and they rarely specify the actual sizes of the bolts in a kit so I can’t just buy one for something else that will fit very easily. My solution was find the best standard bolt I could and so I am using some 12.9 high tensile socket cap bolts which I managed to find from a bolt supplier on eBay with the right thread. For anyone who doesn’t know 12.9 rated bolts are the highest rating before getting into one off special items (usually using exotic materials) and they really are very strong. As a comparison ARP gives their flywheel bolts as having a tensile strength of 180,000 PSI. The 12.9 bolts are rated to have a minimum tensile strength of 176,900 PSI – a number close enough it makes me think they are likely the same material! The strength figures for these bolts mean at the size I will be using each bolt can be safely loaded to in excess of 7000kg of tensile load indefinitely with no deformation. Their failure point being somewhere north of 9500kg each! Some time in the future I will do a full write up of nuts bolts and other fixtures it’s worth knowing about.

So that’s my shiny custom flywheel, next time you see it it should be bolted to a rebuild engine with a whole host of custom or cobbled bits on it!

RX8 Project – Part 8, Flywheels Part 2

Apologies for the long delay since my last post (more than a month!), life has been getting in the way of having time to do anything on blog of late. The good news is that the RX8 project has made some progress and this blog is still no-where near the current status so there’s still plenty to come!

In flywheels part one I mentioned how I ended up in a situation where I didn’t really think the cast flywheel was save to modify and how a chance encounter led me to a solution. The problem it presented is I’m primarily an electrical/electronic engineer, while I dabble fairly extensively in mechanical things designing a flywheel isn’t exactly something that comes up every day and the precision was critical so I spent a lot of time making sure I got it right!

Critical aspects as I saw them were the bolt pattern to match the crank, bolt points for a suitable clutch and and very accurate outer diameter to allow fitment of the RX8 starter ring gear.

Looking at these criteria one at a time the bolt pattern is an interesting one. At first glance all the 8 bolts appear to be evenly spaced around the crank on a PCD (Pitch Circle Diameter – this means the centre of each of the holes is placed on a circle). After checking my early flywheel model drawings against the real flywheel I noticed that all the bolts lined up except one which was just slightly wrong; ok, approximately 2mm, enough to be considered very wrong!

This suggested the pattern wasn’t exactly what I thought so I started checking exactly what the error was in different directions to figure out what was going on. After extensive measurement I managed to work out what was wrong, the bolts were indeed on a PCD they just weren’t evenly spaced. For even spacing the bolts would be at 45° intervals but one hole was shifted 4° round the PCD so it was 41° and 49° to the two nearest holes. Combined with a 76mm PCD this made the bolt pattern line up perfectly. This is actually quite useful because it means when the crank/flywheel are balanced they cannot be reassembled in the wrong alignment.

The crank also features a location register to make sure the centre of the flywheel is perfectly centred on the crank. The register is a raised lip accurately machined to a specific outer diameter so there is no lateral slop between the parts, in this case I measured this to be 44.40mm in diameter. when I trial fitted this it needed some emery on the crank to fit but this seemed due to surface rust where the engine had been stored in a damp room for a long time. Your mileage may vary!

Next up we had the clutch, I initially planned on using the RX8 clutch as I thought it would be stronger and have more options later but on further research it turned out RX8 clutches are very expensive indeed and anything other than a stock one gets very expensive very quickly and largely need to be imported so I started looking at other options. This took me back to the idea of using a Mondeo 240mm clutch, they’re cheap, readily available and the stock ones will handle a fair amount of power. Admittedly a stock kit is highly unlikely to last long with the amount of power this project could get to but there are readily available uprated covers and plates that could be used. Plus £50 on a project that may never really work isn’t too bad, £300 for a new RX8 stock clutch is more than the car cost! I also already head the factory Mondeo flywheel to take all the appropriate dimensions from which kept the process fairly simple.

The last issue was the ring gear, this is critical because the RX8 has its starter motor on the gearbox side and when because of this the options are either re-use the RX8 starter or butcher the RX8 bellhousing to allow an engine side starter to fit. For simplicity I figured I’d go with the RX8 starter since I was getting the flywheel made anyway. Starter ring gears are whats called an interference fit on the flywheel. In essence the ring gear is intentionally slightly smaller than the flywheel it is designed to fit onto and when the two parts are either pressed or heat fitted (heating up the ring so it expands and can be slipped into place) together. It is a tiny change in size when fitted and just the friction between the two parts that prevents the ring gear slipping when the engine is started hence why this is rather critical. To simplify this I modelled a nominal 290mm for the diameter of the lip this mounts on but supplied the ring gear to the machine shop and asked them to machine to an interference fit. This led to the following design:

RX8 Flywheel V6 – Machining Drawing

After a lot of double checking with these base measurements I needed to get the correct offset from the crank to make sure the clutch plate is in the correct position to be fully engaged with the gearbox splines. This led to me modelling everything to make sure it would all fit where it needed to:

Here you can see how everything stacks up. Between the bell housing and engine there is a 10mm spacer (grey) this represents the adaptor plate thickness. Clearly the bell housing has been simplified but the overall length is correct and the position of the splines (a little hard to see in the picture) and pilot bearing diameter (the reduced diameter) on the gearbox input shaft are correct.

Unfortunately having got all of this looking right and sent it over to the machinist and work starting on it I realised a couple minor mistakes, one was that I’d not offset the flywheel to match the spacing of the bell housing caused by the adaptor plate (shown above but this picture is from a later version) but related to that I hadn’t checked the offset to make sure the starter ring gear was actually in the right position to engage with the starter!

Turned out it was a little off and actually needed more offset but unfortunately the raw material for the flywheel had been delivered and machining had already begun and sadly it wasn’t big enough to allow for this extra thickness so I needed a new plan. The best I could come up with was to add a small spacer to correct this. Luckily this also allowed an opportunity to include a new pilot bearing location. This is a bearing that locates into the end of the crank to support the engine side of the gearbox input shaft and due to the gearbox adaptor plate thickness and the fact of it being a mismatched engine and gearbox the standard bearing was now too far away to support the shaft.

RX8 V6 Crank Spacer V1

This spacer corrects the problems above and still includes the correct bolt pattern, location diameters to keep everything centred. The 35mm internal diameter is the exact size of the bearing I used. This allowed a suitable bearing and a dust seal to be pressed into place and likely stay there, that said there’s a lip in the spacer to hold the bearing up and once the gearbox shaft is in place it physically can’t fall out. It’s probably worth pointing out here that this bearing only actually moves in use when the clutch is pressed, when driving along in a gear the clutch locks the crank and input shaft together and so the bearing is rotating overall but the inside and outside are rotating at the same speed so the vast majority of the time it shouldn’t experience any wear.

The final product to be coming in part 3!

RX8 Project – Part 7, Introduction to flywheels

Having decided what engine I was going to use and deciding to keep the existing gearbox so I could retain the factory carbon prop shaft the next logical step was to work out how exactly to achieve that…

First off the engine I had bought was from an automatic and so had a flex plate rather than a flywheel. This is a comparatively thin piece of steel which gives the starter ring gear, which would be on the outside of the flywheel on a manual, a fixing point and also mates the torque converter to the crank. In automatics the torque converter provides the rotating mass to smooth the engine pulses. So the first step was to get a flywheel that would work. My first idea was to take the factory RX8 flywheel which has quite a deep offset (i.e. it is quite dished) which would help correct for gearbox adapters which would space the gearbox off the engine. So I looked into simply machining off the back of the RX8 flywheel flat and drilling the bolt pattern from the V6 crank into it. While technically this would work there are a few problems.

(Taken from here : http://www.locostbuilders.co.uk/viewthread.php?tid=185939&page=2)

The image above shows exactly what I’m talking about, this is a factory RX8 flywheel modded onto a V6 crank. I believe the engine used in this case is the Mazda KLDE. While this all looks good there are a couple of issues. First is that the RX8 flywheel isn’t balanced as it stands, it forms a balanced arrangement with the rest of the engine and so needs modifying. I don’t have a picture of this but the weighted lip on the rear ranges from thin on one side to very thick the other. Aftermarket flywheels get round this by using balanced flywheel and a separate counterweight. Problem two is that the factory flywheel is cast iron, this has an irregular structure and can have flaws and other weaknesses from new but parts are generally made with a factor of safety to account for this but modification in this way will remove some of this additional strength and change areas of stress. I actually 3D modelled this change to see if it would work:

Original:

Modified:

Having modelled this I performed a stress analysis of this based on the force created by the flywheel spinning at 8000 RPM. This is more than redline as it stands but it seemed prudent to plan ahead! It turns out the stock flywheel is only about 20% stronger than required based on the nominal ‘standard’ properties of cast iron. The new version would be below strength at this speed, dropping to 7500 RPM gave something around 102% strength. Not a number I felt confident in at all! Just to make a point here as people argue the safety of modded flywheels a lot (mostly from “I’ve done it and it’s fine”). I’m not saying it will fail modded like this, in fact the numbers suggest it is (just) strong enough here but there is no margin for error even on the ‘ideal’ material and cast iron does vary significantly. A given flywheel might be fine like this for years, but get a weaker one or one with a flaw or even give it a hard jolt when it’s at full revs and it may just shatter. If it does, be somewhere else!

So after deciding the mod wasn’t really the best idea I realised that all I needed was a flywheel that would bolt up and work. A fairly easy task at face value since it turned out the the standard Ford clutch splines (1″ dia, 23 spline) match the gearbox the solution suddenly seemed simple and I just needed a stock Ford flywheel and clutch for that engine. It turns out there were a few variations of the Ford flywheel depending if you went for an ST200, ST220 or just a vanilla V6 Mondeo but the difference between them seems to be some are ‘lighter’ versions to make the more special cars rev a little more freely. This is achieved by leaving out sections of the outer lip on the flywheel. In my case I had no idea if this project would ever work so I bought the cheapest! This is when another problem emerged:

Note the starter ring gear on each. The top is the S-Type flex plate, the bottom is the Mondeo one. It seems the Ford and Jag use a different starter motor as well. Add to this that the Mondeo flywheel puts the clutch far too far forward to mate to the gearbox and because the starter is on the engine side on the Jag but on the top of the gearbox on the Mondeo (because it’s transverse) – something we can’t do on the RX8 as the gearbox is wrong and there isn’t the room in the tunnel the whole idea falls apart! Modifying the cast one seems to be the only sensible option.

Around this time I happened to have a chat with a colleague at work who is a professional mechanical engineer and 3D designer I know through the job I had at the time and explained the problem and he directed me towards a machinist who did a lot of work for him and was well into cars. By chance a few days later this machinist came into the office and as soon as I explained the problem he just said “ah, we’ll just make you a custom one if you do a design”…. So I found myself with the challenge of designing a custom flywheel which as per the machinists recommendation would be made out of EN24 steel. As a comparison changing the models above to EN24 changes the safety factor to something around 300% from memory meaning we can lighten it significantly later if required and not risk weakening it dangerously.

More to come…