New Build for WTAC

[NitroDann]

@greenmachine

Please help me learn about choosing effective gear ratios, where do I start?

Dann

[Magpie]

As mentioned if you download the Racelogic Circuit tools https://www.vboxmotorsport.co.uk/index.php/en/circuit-tools-driver-analysis-software#try-it and open the attached VBO file you will have an idea of how to do a 1:41.7 lap time of SMSP.

[lightyear]

I will be going out to the shop tomorrow most likely. See if they have finished the fastback NA boot mould. The outside will be done for sure. As soon as I get a finished NA boot I will let you know. I am hoping this week.

[madjak]

For WTAC I think the optimal tyre for a MX5 is the A050 255/40R17 Mediums or the A050 245/40R17 softs. I'd skip the 15" rims altogether and set up the car for the larger wheel straight off the bat. The issue with the 15s is the 50 profile the Advans come in which don't work so great for the roll centers and ride height. If you run the 225/50R15 then you end up having to run the car high up off the ground or needing drop spindles to get it right. The 17" rims have the same issue however you get the benefit of the wider tyre and shorter sidewalls.

Plus the rules let you cut metal for tyre clearance which in a NA means you can pretty much remove all the wheel well saving a fair bit of weight.

[Black_Penguin]

For Clubsprint you are limited to the A08R range.

Sent from my Pixel using Tapatalk

[plohl]

^ which is half the problem with clubsprint...
The cars are basically open class cars from a few years ago with street tyres. If you were to look at the rules and build a car to be competitive in the club sprint class, the mx5 wouldn't make the short list. Not saying one shouldn't enter, but it doesn't fit in the rules that well and it is starting on the back foot.

Please help me learn about choosing effective gear ratios, where do I start?

Are you actually going to change gear ratios?
Or just the final drive?
Easiest place to start is a plot of speed vs RPM.

You need tyre OD, gear raito and final drive.
The equation is

v(km/h) = (OD(m)*PI*RPM)/(16.67*Gear Ratio*Final Drive)

You'll need to include any primary reduction, but I doubt you'll have any.

There's probably enough data around to get an idea of top speeds for the main straight for a given lap time. Figure out where this sits in a standard gear set up and work from there. The more data you have on speeds around parts of the track, the more of an idea you can get about what gear you need to be in and where the engine speed is. Corner speeds should be reasonably similar to cars of similar weight.
Given the car is running massive tyres, I guess it's going to need a high ratio diff to help make up for the increase. For example, with a 5 speed box, 255/40R17 + 4.7FD is pretty close to a 205/50R15 + 4.3FD.

What sort of data logging will the car have?

[NitroDann]

Its hard to say what changes will be made
The car hasnt even completed one lap on the control tyre with its new power.

I tend to agree with you about starting on the back foot, but then the history of the class shows the success of civics.

Its unlikely to get a new gearset but its kens money not mine. Currently it only has an ms2 onboard.

Dann

[madjak]

Its hard to compete against a well built civic. I have the fastest time around Barbagallo for any MX5 and I'm still 2.5 seconds off the pace of the fastest u2l civic. They can pretty much get nearly 300whp in a 700kg chassis which is unbeatable on restricted street tyres. Slap on a rotrex or turbo on each for 500whp plus where FWD starts to limit times and then a MX5 starts to have a chance if its light enough.

I think the target WTAC build is an extremely lightweight MX5 with 500whp, big tyres and lots of aero. It has to be driveable at the limit and it might be competitive if everything works together.

For me that build starts around the tyre.

Edit: 300whp not 200

[NitroDann]

Yes the 500whp civics have traction control also.

The limiting factor here MIGHT be the gearbox, its as much power as I can make with this gearbox really, and the ratios are only as good as they are.

For the record Madjak, those are my exact thoughts. 500+ rwhp/tonne and as easy to drive as possible, and let the driver cut the laps. It already has 17x9s with the AD08r 255s. It has huge brakes too.

Dann

[madjak]

Dann, my plan is still for a high reving, high compression engine with a big C38 Rotrex pushing out around 450whp. Ideally a non MX5 engine would be better because it is such a damn heavy lump of steel but I don't really have the funds or patience to build a different engine. The best we can do is get the engine to sit as low and back as possible. I have toyed with the idea of a billet block as I could make one pretty cheaply, but then I may as well make a 3.0L V8 with motorbike heads.

I have thought about running a turbo but I think I'd be quicker with the linear powerband of a Rotrex. Also there are packaging benefits and it's friendlier on the boxes.

From a gearbox point of view I plan on running my sequential dogbox. I don't think I have the standard MX5 limits with the straight cut teeth and it supposedly supports up to 400ftlbs but then if I see case flex it could still cause issues.

In terms of chassis, I figure you can cut away all the front from the shock towers forwards for cooling purposes, cut away the wheel wells to get clearance for tyre and remove most of the boot for cooling. The shock towers will need to be modified a little for tyre clearance too. Maybe run some custom suspension arms and spindles to get the ride height down with the 17" rims. The rear parcel shelf can be modified to fit a fuel tank and my plan is to cut out the sills and replace with a roll bar. Really the only parts of the chassis retained will be some of the firewall, the floor, transmission tunnel and chassis rails up the to shock towers.

All up I think under 800kg is achievable with lexan windows etc.

[NitroDann]

Have a look at the dyno chart again before being so sure about a turbo cars lack of linearity.

Unfortunately most of what you suggest is banned in clubsprint.

Dann

[madjak]

Yup ProAM is my target... I know the arguments for turbo vs rotrex and lets not go there in this thread. Maybe the only way to prove it is to actually show you how fast a 450whp rotrexed MX5 can be. And by fast I don't mean straight line speed!

[greenMachine]

Hmmm by Eipeip, on Flickr

@greenmachine

Please help me learn about choosing effective gear ratios, where do I start?

Dann

Dann, based on a stock 6-speed and readily available diffs.

Thanks to our favourite black and white bird , we have a very useful chart.

Lets assume for the purposes of this discussion that this is an actual datalog from your car at SMP-GP in a baseline configuration - you want to know where to go from here.

It shows that you have a speed range of 200-70kmh. It shows you are not only flat through T1, but accelerating through it before slowing for the slowest corner, the T2/3 complex. If you are not on, or nearly on, the rev limiter before braking for T2, you have thrown away some acceleration and probably top speed. The fact that you do not slow through T1 means you can go faster there, if only the car could go faster (ie accelerate harder out of T12 onto the main straight and/or carry more speed through T12. That is point1 - gear for maximum speed - if you are not on the limiter in top on the fastest part of the circuit (in this case the main straight), you have foregone lap time.

If you can accelerate to top speed sooner, you will average a faster speed between the exit of the last corner, and having to slow for the next (in this example T12 to T2/3, which is a looong way) - and having a higher average speed over a long distance will gain massive lap time. Point 2 - gear to get to top gear/top speed as fast as possible - any time you are not at top speed (when the option is available) is costing you, minimise this.

The same two points, but mainly the second, apply every time that chart trace turns upward. What you would be doing, is steepening those traces, going faster between corners/arriving at the next quicker.

So hypothetically, you are hitting the rev limiter in 6th halfway down the main straight, and holding it there (just under actually) until you brake. You may not be going as fast as you were (on the basis of the chart you probably are, or faster even) but you have averaged a higher speed.

Some practical considerations: Is the engine happy to run for an extended period at full load and full revs? Are the brakes up to the harder use? Becauuse the car has a shorter (higher numerical) diff ratio, is the driver happy with the frequent shifts that entails, does he miss gears?

(TBC)

[greenMachine]

Continuing ...

In practical terms, we are both budget limited and limited in what we can work with. We have a limited range of diff ratios, and our 6-speed gearboxes are limited in their single gearset. So lets forget the gearbox, and treat it as a black box (sorry). That leaves the diff to play with (we can also introduce other variations such as tyre size, but lets not go there for now, except to say you should be testing in as close a configuration to what you plan to use at the event).

As a tuner, you probably have a feel for the real world performance of a high power turbo car, and the effects of aero drag. You may already have some preferences for diff ratios for such a car, or it might be the case of 'this is what is in it now, that's where we start'.

What we really need is some data from the car at this track. So the first thing is to do that, and generate some graphs like Magpie's. Then we look at the graphs, and see what they tell us. How fast, where is the car braking, where is it reaching top speed, what revs is it doing at top speed. Looking at the data, is there evidence that a change of diff ratio might make it quicker? If a shorter diff, can we get the power down and get the benefit of better acceleration, or are we grip limited? If a longer diff, will the higher terminal velocity offset the slower acceleration? The data will have the answers, but that means a diff change (or two), and more runs to get the data.

If a diff change can't be done there and then, then do some more runs, adjusting the wing AoA, splitter height and width (one at a time, and writing down both the start point and the change, as well as the result) and anything else that can be adjusted. Bear in mind that you want to be able to revert the car to 'diff test' configuration for the next test of a different ratio, so that the effect of the new ratio can be isolated.

That's probably enough for now.

One question though, what is the data setup in the car? Will it produce the data that lends itself to this sort of analysis? Dann will already appreciate, but others might not, that this is entirely different to datalogging an ECU for engine data - we need accelerometers, accurate (high frequency) GPS as well as selected ECU data like speed, and revs. Hopefully you will have the car set up already with a data recorder of some sort, and someone who is able to interpret and analyse the data. No doubt you will be doing the same for the engine.

[NitroDann]

What happens when you go extra tall and move the whole track back one gear and top out 5th on the straight?

If 3rd to 5th looks like a better set of rations than 4th to 6th?

Dann

Dann