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I'm looking into building a car specifically to target the local speed series (time attack) as well as the future possibility of running in the Pro Am classing of the WTAC. The rules are here Rulebook updated for 2017 - World Time Attack Challenge Sydney if anyone wants to read, but I believe if the car looks cool the organisers are fairly lax with them. I have a direct line to the head scrutineer so I'll run a couple of things past him before I get too far.
Andy Forrest's WRX ... check out that diffuser!
I think the key to making a Miata competitive in this class is to target reasonable power but in a very light and low chassis with as much aero as possible. So here are my design guidelines for the build:
Lowest weight possible - the limit is 750kg for a NA chassis. This means I'll have to drop around 140kg's from my current track car which is already pretty light.
Big power - 450 - 500whp will be the goal. I think my current 5 speed dog box will handle that sort of power (plus it's light)
Tyres - Limited to either 225/50R15s or I think the better option is 245/40/R17 softs or 255/40R17 mediums. The Yokohama Advan AO50 tyres they are are extremely good throughout their life (just under second off a new 245 Hoosier R80)
Aero - Big front splitter, with side winglets, flat floor, big and high rear spoiler.
Chassis - Widened 300mm track, engine moved back and down, body dropped on chassis 50mm, tubular front and rear.
Some of the key limitations are:
Must keep floor, frame rails and firewall. (allowed modifications for seating, cage, transmission and driveline, exhaust and engine)
Must keep chassis rails up to the shock towers front and rear, and must keep shock towers.
All internal sheetmetal must remain as oem unless its required to be removed for safety, cooling, fuel tank placement, etc etc I think this is the where I'll need to get creative so that I can justify some of the removal of the heavy parts of the body.
I have already started hacking up a chassis. The first stage was to separate the outer skin of the car from the main chassis, so this includes the rear panels, sills and windscreen. I've already removed around 80kgs of metal from the insides of the sills, double skins on the wheel arches, and front door bulkhead. The floor and firewall and chassis rails will be kept together and cleaned up to the bare minimum. I'll probably get the chassis sand blasted with the coarsest medium possible to try and shed some more weight. Once cleaned up and as light as possible, I'll pin the body back onto the chassis but 50mm lower. This will bring the bottom of the sill pretty much inline with the chassis rails on the floor. The key here is I'll run a roll cage tube down the inside of the sill justifying the need to cut and shut this part of the chassis. It will also be the major structural member for chassis strength and rigidity.
I also plan to cut the doors in half and solidly mount the bottom to the body to cover the side intrusion bars. These will be diagonally trussed for added chassis stiffness and safety. Only the top half of the door skin will open up for driver access so this can be super light with fixed lexan windows. I can also replace the windscreen with Lexan which will also shed a bit of weight.
The engine can move backwards so that the block sits 51mm behind the main section of the firewall and I'll drop it down as low as possible. I might need to make a dry sump to get it low enough to clear the dropped bonnet. I could also bump it over to the passenger side a little. Not sure if it's worth it though. For the engine build I have to decide if I want to turbo or Rotrex it. I've been wanting to build a HP Rotrex engine so I think I'm favoring that option at the moment. We'll see as I progress the chassis.
Front needs to look something like this:
The biggest question mark at the moment is trying to decide on the suspension setup. I can push the wheels out 150mm each side so I could run longer tubular arms with different hubs if I want. Suspension and brakes are free so I can do whatever I want there. If I run the 17's then I might need drop spindles to get the car geometry right but I suppose I could just rework the entire lot for optimal geometry. With the 50mm body drop and the 245s on 17s I'll basically have no guard left on the front. I can remove metal for bump clearance so that's pretty much the entire tub on each wheel arch. With the extra 300mm body width and the body cut and dropped 50mm plus big aero it will look aggressive.
Like this?
Aero wise I can run a massive front splitter merging into a flat floor then a rear diffuser at the back. If I run tubular suspension it means I can get a much more effective rear diffuser starting to lift under the drivers seat. I'll make a custom wing at the back with my 3D printer. Either a large single element or a dual element. I think drag will be a bit of a concern still so I don't want to go too large, especially if I can get the rear diffuser to be effective.
AB's infinity splitter. I like the smooth transition from the splitter to the bumper.
That pretty much sums up my plan. Any ideas on what else I could look into?
That's a pretty intense build! I'm pretty clueless when it comes to aero, so no comment there.
In regards to power, TSE's kit likely has the most on track time testing relevant to your goals. So that experience could ease the growing pains in hitting your power required for "x" track/event. Which, in turn helps you concentrate on driving the crazy *** machine instead of chasing potential issues. I suppose k24madness has his higher hp rotrex setup with quite a bit of time on it as well, but I think there's more TSE examples now?
For suspension, will you be running a cantilever/push-rod setup with tubular control-arms, drop-spindles, and Xida-ACE? I know Keisler(https://keislerautomation.com/) has options to run MR2 hubs with his spindles, which are a lot stronger than OEM miata and that could even open doors to a 5-lug swap to have better 17" wheel options. 225/50/15 and 245/40/17 are taller than normal, do the Advans run short or something? I like how they have limitations of only using 16 tires for an event.
This looks awesome.
I don't have much technical advice to offer, but I would caution against using a socket cap screw without counterboring.
If it is subjected to dynamic forces, the cap can weaken and shear.
I would not use them on control arms as your last picture shows.
2. Big power - 450 - 500whp will be the goal. I think my current 5 speed dog box will handle that sort of power (plus it's light)
For the engine build I have to decide if I want to turbo or Rotrex it. I've been wanting to build a HP Rotrex engine so I think I'm favoring that option at the moment. We'll see as I progress the chassis.
If you are sticking with a BP motor, this is a no-brainer: TSE EFR kit. There are a ton of applications where I love the Rotrex as an option, but a no-holds-barred WTAC big power build that needs strong midrange is not its place. The EFR car will be much faster.
Originally Posted by Madjak
The biggest question mark at the moment is trying to decide on the suspension setup. I can push the wheels out 150mm each side so I could run longer tubular arms with different hubs if I want. Suspension and brakes are free so I can do whatever I want there. If I run the 17's then I might need drop spindles to get the car geometry right but I suppose I could just rework the entire lot for optimal geometry. With the 50mm body drop and the 245s on 17s I'll basically have no guard left on the front. I can remove metal for bump clearance so that's pretty much the entire tub on each wheel arch. With the extra 300mm body width and the body cut and dropped 50mm plus big aero it will look aggressive.
Because this is a relatively low-power approach that is sensitive to drag, tunnels/diffuser is a big priority (very little drag penalty). Wider track with long tubular arms and tubular rear subframe that is narrowed down as much as possible. Split the tunnels around the differential.
As is always the case, the key will be in making as much front downforce as you possibly can. If you accomplish that well, you'll likely still need a very substantial rear wing and that is OK.
Absolutely look into different spindles right off the bat and design around those. The factory Miata spindle flexes substantially, limits your hub size/options, etc. and is a big limitation for a build of this nature. Depending on how deep you want to go with bespoke parts (things like this are what make a car take years more to complete) you could design your own spindles, or you could source spindles from another car. The Nemo Evo for example, ran spindles from a Ginetta to save on design time/cost.
Hopefully see you at WTAC in 2018. I have found Steve (rules) to be very responsive. Over the past 3 years, at least in Clubsprint class the rules are being enforced, they even had scrutineers measuring ride height in the garages.
It it is a great event and to date I have made some friends, great contacts and had a ball despite being under 2lts and 200 RWHP.
It will be good to see your progress. I have given up on pro am class, and instead doing Clubsprint. I should be there next year. Plan on doing the Vic time attack too. I also have swapped to an NA fir the weight benefits.
Yeah, the NA is definitely a better start than the NB for WTAC.
I just noticed this in your OP -
"Either a large single element or a dual element. I think drag will be a bit of a concern still so I don't want to go too large".
A dual element is by default more efficient, have a look at the CFD numbers on APR's website for example, the GT-1000 makes more down force AND less drag than the GT-250 per given angle.
We've got a basic Na8 going into clubsprint next year.
We think we can be competitive. The record for class was dropped to 1.36 by an Evo this year. The car we have ran 1.44 first try as driven to the track when it was a daily. Weve gone to 17s and 245s. TSE stupid bbk. Splitter, diffuser fastback and big 2d wing. We also have gobe from 270whp to (hopefully) 450 with an upgrade from gt2860 to gtx2867r and 98 to e85 with appropriate internals and supporting mods.
Need to get lots of track hours in between now and next year. That will be a huge part of it.
Perhaps we should use this thread to make updates and the forum can rip our bad ideas to shreds?
See you there hopefully. When you look at the top cars in clubsprint very few are owner drivers. Further the aero needs to be very carefully considered. A twin element would be better than a big single as for the reasons previously mentioned. The wing being designed for my car also proves that you can have more downforce for less drag. However my car is HP limited so I need all the drag and weight reduction I can get!
Despite your excellent fab and engineering skills Madjak, I can't help but think your underestimating what it takes to be competitive in Pro Am. having witnessed the progression over the past 4 years, I would say to have a shot at podium you need 700+ hp, definitely 295's all round, and a heavy investment in real aero, which is significant drag hence the need for power. Expect the class to be quicker again next year.
I feel like this level of competition went beyond amateurs and regular budgets (sub $100k) a while back. Unless you just want to run around for fun and to test your own skills against the clock.
If things go well we will end up with a few MX5's there. Brendans car might not fit into Clubsprint any more, but it would be good if he could make it there in open.
As to the low mount, I think it's way better. Mine is so high, it's just what I had at the time. With low mount the weight is a lot lower, and it's so much better for the down pipe.
11-28-2017, 08:52 AM
WTAC (World Time Attack Challenge)
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