Mid Engine SVX concept. - Page 13

Dessertrunner · #**181** 02-07-2013, 01:42 PM

Don't want to have a top engine mount, so have beefed up the gearbox one with a new heavier design.
Its tacked ready for welding when the other parts are in place.
Tony

Sean486 · #**182** 02-07-2013, 03:15 PM

Aren't you done yet? Come on!

Seriously, awesome work man. Looks great.

michael · #**183** 03-09-2013, 05:02 PM

Updates???

Dessertrunner · #**184** 03-10-2013, 08:54 PM

Sorry guys have been working just not posting,
Here is a 3d pdf of the final brake, disc, upright & wheel assembly.

Hope it all makes sense.
Tony

Dessertrunner · #**185** 03-10-2013, 10:23 PM

Okay brake capliers are wilwood 120-4322 &23 they are the type used on a GT III and NASCAR here are the details.

"The cast aluminum GT III Caliper is ideal for NASCAR Nextel Cup, Busch GN, Craftsman Truck, ARCA Supercars and SCCA road racing. The double bridge bolt design is extremely rigid and provides drivers with a firm, responsive pedal and superior stopping power. The differential piston bores have been positioned to maximize clamping force, provide even pad wear and enhance caliper stiffness. Weighing only 8.4 pounds, the GT III incorporates stainless steel heat shielding throughout the bridge area and piston bore facing, along with heavy duty pad abutment plates. The GT III utilizes the thick GT pad (11.3 cubic inches, 1.10” thick), and is a direct bolt-on interchange with the 5.25” mount GT Series calipers. Wilwood’s exclusive short track pistons are available for the GT III Caliper. Recirculator tubes are also available. 1.88"/1.75" piston diameter. 1.38" rotor width. "

Front and back of the car will have the same calipers, same size disc and wheels.

Discs are the same as late STI 13in or 326mm diameter.

At this point I am aiming to run ABS as you can see fromt from the plan.

Tyres at this time are 275/35 - 20in, there has been a fair bit of debate on this issue. Bazza and I came to blows on this point as he beleived that 275/35 on 16 or 17in rims would give me faster lap times. I don't agree, happy for anyone to express a opinion.

Tony

neverLift · #**186** 03-10-2013, 10:31 PM

Unless the 20" wheel is designed to be lighter AND have less rotational inertia, 16/17's will be faster. And if the 20" wheel is designed to be better than a 16/17" wheel, the 16/17" with equivalent design would be faster.

You can't fight physics.

Upright assembly looks cool though!

Dessertrunner · #**187** 03-10-2013, 10:35 PM

Quote:

Originally Posted by neverLift

Unless the 20" wheel is designed to be lighter AND have less rotational inertia, 16/17's will be faster. And if the 20" wheel is designed to be better than a 16/17" wheel, the 16/17" with equivalent design would be faster.

You can't fight physics.

Upright assembly looks cool though!

Do you mean the physic of the weight or the physic of the grip?

neverLift · #**188** 03-10-2013, 10:43 PM

Of the weight and more specifically, how far away it is located from the center of rotation.

The tires themselves should be more or less equal in terms of mechanical grip. But the 20" tires take more energy and therefore time to accelerate and decelerate. Moment of inertia I=mr^2, where m is the mass and r is the distance of the mass from the axis of rotation. The mass I'm referring to is that of the tire and the outer barrel of the wheel

Dessertrunner · #**189** 03-10-2013, 10:56 PM

In Australia our V8 supercars went from 17in to 18in and they say they expect lap times to be quicker. 24 hr Le Mans cars are 19in and the rules stop them going to 20in, as I understand it NASCAR tyre outter diameter is simlar to 20in with 275/35.

So I can't get my head around what is the right thing. THere has to be a gain from a larger over all diameter other wise they would need rule to stop people putting smaller tyres on not larger.

Its confusing

Tony

'E' · #**190** 03-11-2013, 02:48 AM

Does it have anything to do with engine speed, gearing and overall speed? I have 19's on my Svx and that adds 10kph at 100kph at the same revs that would yield 100kph on normal rims

neverLift · #**191** 03-11-2013, 07:59 AM

Quote:

Originally Posted by Dessertrunner

In Australia our V8 supercars went from 17in to 18in and they say they expect lap times to be quicker. 24 hr Le Mans cars are 19in and the rules stop them going to 20in, as I understand it NASCAR tyre outter diameter is simlar to 20in with 275/35.

So I can't get my head around what is the right thing. THere has to be a gain from a larger over all diameter other wise they would need rule to stop people putting smaller tyres on not larger.

Its confusing

Tony

Most of the time, brake set-up size is the limiting factor. The teams want as large of discs as possible. So the wheels need to be a certain diameter to clear them. Of course there is a tipping point where the brakes are more than big enough, and you're just adding weight. Most of the time though, teams always want more brake.

As for rules dictating size, there are other, non-performance reasons too.

Quote:

Originally Posted by V8 Supercars

"The tread is still the same width, it's just missing half an inch out of the sidewall. It won't look a great deal different from what we have now.

"Having a bigger tyre gives a lot more flexibility as to the variety of braking packages you can fit inside the wheel as well.

"Cars all over the world are being made with 18-inch wheels now so we're going with the flow and moving along with that.

"Most race cars around the world, particularly in GT racing, use 18-inch too so it's more of a common size.

Source

smc · #**192** 03-11-2013, 10:02 PM

Agreed with neverlift. The rule on wheel diameter is primarily there to limit the size of the brakes. Kinda like in gravel rally. 15" diameter wheels. You have squeeze some punny brakes in there. But a good rule of thumb when it comes to brake size. When your at speed, if you can slam on the brakes and lock them up, then you dont need bigger brakes. R-compound tires and in many cases, non power brakes would require larger brakes to aid in it.
A great example of this is the stock SVX calipers. Most people this our cars have crummy brakes. And up until recently, I would agree with them. Until I put on a set of quality pads. Even with my r-spec tires, i could lock up all 4 wheels. 80 to 0 mph was damn good.
... Sorry, getting off topic here..

In terms of wheels, you only need to go as large as your brake system requires for clearance. Yes, you get less rotational inertia from the lighter tire/wheel package, but you mainly gain a more compliant tire carcass. A tire needs to have sidewall. If your tire looks like you painted it onto the wheel, chances are you will be All over the track. The trick is finding out what is Best for YOUR car and the track conditions. Bumpy track, more sidewall. The tire can conform better and take more abuse. A silky smooth track, less sidewall for crisper steering input.
Its really a game of trial and error, and in Many cases racing regulations. ALWAYS check to make certain you are in compliance with what ever racing sanctioning body you are racing with.

This also tends to bring up the thought of tire width. Most people tend to think a wider tire is better. This is rarely the case. A crazy wide tire on a car is there to cover up bad handling characteristics. Be it bad camber fluctuations in suspension travel, bad roll center, or akerman angle. A wide tire has far more surface area in contact with the tarmac at any given time. Most think, "Ahhh, that means more tire to grip with!" yes and no. Yes, there is more tire. However the pressure per cm of that contact patch pressing down on the road is far less than that of a skinnier tire. Thats why in tarmac rally, our tires are not all that wide. In reality, that fat rear tire is not giving a heck of a lot more grip that the front.

I talked to a few fellow racers on this, who quickly bring up formula 1 as an example. Huge rear tires! However with out the downforce to squish all that rubber to the ground, they handle like absolute crap. Now take a look at other formula classes. Similar cornering speeds, less areo, thinner tire. Most of our cars will Never have that sort of downforce aiding us in racing

In the end, it all comes down to choice and track conditions. Wide, skinny, lots of sidewall, little sidewall.

Get a set you happy with and experiment over time. Talk to others with similar setups and so on.

Cheers
SMC

bazza · #**193** 03-12-2013, 12:29 AM

Quote:

Originally Posted by smc

Agreed with neverlift. The rule on wheel diameter is primarily there to limit the size of the brakes. Kinda like in gravel rally. 15" diameter wheels. You have squeeze some punny brakes in there. But a good rule of thumb when it comes to brake size. When your at speed, if you can slam on the brakes and lock them up, then you dont need bigger brakes. R-compound tires and in many cases, non power brakes would require larger brakes to aid in it.
A great example of this is the stock SVX calipers. Most people this our cars have crummy brakes. And up until recently, I would agree with them. Until I put on a set of quality pads. Even with my r-spec tires, i could lock up all 4 wheels. 80 to 0 mph was damn good.
... Sorry, getting off topic here..

The bigger brake you can stick in there gives far superior control and yes better pads are under-rated.

Quote:

In terms of wheels, you only need to go as large as your brake system requires for clearance. Yes, you get less rotational inertia from the lighter tire/wheel package, but you mainly gain a more compliant tire carcass. A tire needs to have sidewall. If your tire looks like you painted it onto the wheel, chances are you will be All over the track. The trick is finding out what is Best for YOUR car and the track conditions. Bumpy track, more sidewall. The tire can conform better and take more abuse. A silky smooth track, less sidewall for crisper steering input.
Its really a game of trial and error, and in Many cases racing regulations. ALWAYS check to make certain you are in compliance with what ever racing sanctioning body you are racing with.

Agreed. However I explained to Tony that at Winton (where I'm the quickest Subaru two years running now so I must be doing something right and not pissing in the wind lol) that I spent 55% of a lap turning and 18% braking. So big brakes are a waste of time at this track. Wide tyres however.. worth 3 seconds (proven).

Quote:

This also tends to bring up the thought of tire width. Most people tend to think a wider tire is better. This is rarely the case. A crazy wide tire on a car is there to cover up bad handling characteristics. Be it bad camber fluctuations in suspension travel, bad roll center, or akerman angle. A wide tire has far more surface area in contact with the tarmac at any given time. Most think, "Ahhh, that means more tire to grip with!" yes and no. Yes, there is more tire.

I can advise you that going from 195's to 205's to 215's to 225's to 235's and now to 275's that in the case of a Subaru AWD, wider is ALWAYS better ON tarmac. There is no doubt a point exists where you need to consider steering geometry - maybe above 300's. Also if anyone has ever read Carrol Smith's books - he also states this amongst is other findings. There are a few "tyre grip vs force" graphs which are quite useful and interesting.

Quote:

However the pressure per cm of that contact patch pressing down on the road is far less than that of a skinnier tire. Thats why in tarmac rally, our tires are not all that wide. In reality, that fat rear tire is not giving a heck of a lot more grip that the front.

I found that the pressure density doesn't really matter on tarmac circuit work. It's more about the available grip of the tyre. Skinny "215" tyres run out of available grip easily and mine only handled around 1.35G's. Similar setup with 275's went up to 1.80G's - of course it's unknown how much is "wide" tyre alone, however you'd assume a fair bit would be.

At the end of the day, the rubber is the link between racecar and track and you only have so much grip for X amount of rubber.

Dessertrunner · #**194** 03-12-2013, 01:01 AM

Now I am confused,
Would it be different between a front engine car and a mid engine, can't argue with Bazza's lap time point, also difficult to understand why most of the race cars use a higher side wall profile.

Anyway from my point the mid engine project is a work in process so I am sure I will try different rim/tyre sizes over the next couple of years. The main issue is to design the body and suspenion to fit the biggest tyre I think I would use then go down from there. Easier to go that way then trying to go larger later.

Mesuared the Brakes etc on a rim last night, looks like can fit a 18in rim over the calipers, 17in might be to tight. Given that the V8's went to 18in for more choice that sounds like a plan down the track.

Tony

Dessertrunner · #**195** 03-12-2013, 05:58 PM

Getting a bit frustrated with the project, need to get the lugs/pivot point for the laterial arms away from the discs a bit better which then means the plate I am using is not thick enough. Currently its 75mm and I need to be 100mm. Current thought are to do up a bearing hub simlar to what is on the newer STI's. It looks like it will work just more cad time getting it all sorted out. On the postive side the bearing pocket is quicker to cut on the CNC lathe then the mill, also the new dsign for the main upright is a lot simpler which means it will require less machine time on the mill. Will post the new design when I get it sorted. I might be asking a bit much as I am trying to keep the Abs sensor as well.

Tony