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Cadensdad14 New Reader
6/12/16 6:36 p.m.

I had a thread in the general discussion but I think it's time to start one here for some in depth advice about specifics.

Here's the car. 01 Audi TT 180 HP Quattro.

The engine (yes it's in my laundry room, yes that's my boy, I'm teaching him what's called priorities). It's a 2.7L twin turbo. I have a spare block and a stroker crank as well.

I also have a Volkswagen 5-speed DVZ transmission out of a 2.8L Passat.

Finally, this weekend I procured this subframe and suspension assembly from the local junkyard. It provides mounting points for this engine and transmission as well as providing most everything I need to go multilink suspension.

The idea is to mount the subframe in the rear. I'm planning on running it in a NASA German Touring Series group. The group will change based on how of the engine and weight. By using an Audi engine, VW trans, and Audi chassis I have satisfied the biggest part of the series rules.

In the next few days I'll be dropping the gas tank and cutting out the floor pan. The whole interior is already stripped.

I'm trying to figure out the strut tower mounts. I'm thinking that I create rings to provide mounting, then make a cross bar between the towers across the engine and tie the whole thing in with a roll cage. Then provide mounting points for the subframe in somewhat of the same manner. That way to drop the engine, trans, and suspension its only 14 bolts and it's out. My only fear is that I start to fall into the category of a tube frame chassis and have to pay the penalty.

G_Body_Man SuperDork
6/12/16 6:43 p.m.

Holy Scheisse. . That's awesome!

David S. Wallens
David S. Wallens Editorial Director
6/12/16 7:03 p.m.

Awesome. Keep the updates coming.

HunterBenz Reader
6/12/16 10:50 p.m.

This sounds interesting, looking forward to following along!

Cadensdad14 New Reader
9/25/16 7:05 p.m.

Been a long time since I updated anything on this. Life got in the way for a while. Had to move and other things.

Ive got the cabin all stripped down. I've dropped the rear subframe and suspension and removed the gas tank. Tonight I cut out the boot floor.

I'm a bit proud of my grassroots gantry crane. Worked even better than I thought.

The view from the back. Lots of space for the engine and transaxle. Going to start working on fitting the subframe when I have time.

simontibbett Reader
9/25/16 9:51 p.m.

This is going to be cool.

seyhan3535 New Reader
9/28/16 11:16 p.m.

Waiting on this with eager eyes. Also how serviceable will that 2.7 biturbo be once it goes in?

Flight Service
Flight Service MegaDork
9/29/16 5:46 a.m.

ambitious, I like it.

RossD UltimaDork
9/29/16 7:26 a.m.

This is exactly like one of my daydream builds... but you're doing it!

Cadensdad14 New Reader
9/30/16 9:47 p.m.

I'm hoping to keep the engine relatively serviceable. The hope is to keep everything on the subframe and provide mounting points for the strut towers. Then use disconnects where everything bridges to the engine from the chassis. That way it would hopefully be 10 bolts to drop the entire rear subframe, trans, engine, and suspension. I'm also hoping to use an .080 aluminum panel attached to the main hoop of the cage for a lower firewall and a Lexan upper portion. That way they could be removed to access the front of the engine in the car. Finally, the hatch is heavy. Id like to make a replacement of fiberglass with 4 pins on it to provide quick access to the top of the engine.

Cadensdad14 New Reader
10/7/16 6:54 p.m.

Ok. So tonight I began working the subframe into position. I find that the more opinions I get on a topic the better my final result is. So here we go.

Obviously it needs to be centered left to right. The forward rearward position is pretty set by the wheel position in the wheel well and the angle of the axle to the trans. The one I'm having trouble with is setting the height of the subframe.

I have a b5 a4 and a b5.5 Passat that both use the same subframe. What would you use as a reference for the height? I don't like the idea of using any parts of the wheel because they move and loads vary. At this point the best reference I have is either low point if car or side skirt to subframe.

So now I come to you all, oh great gurus of the grassroots. What say you?

GameboyRMH GRM+ Memberand MegaDork
10/9/16 10:48 a.m.

Side skirt to subframe. The side skirt should be level with the unibody's floorpan and therefore makes a good reference point.

2GRX7 New Reader
10/9/16 2:20 p.m.

Perhaps consider mounting the tire/wheel combo you're looking to use first and foremost. Then make sure that the lower control arms are parallel to the ground. From there, cut multiple pieces of metal equal in length to the lowest part of the subframe (that's now positioned at it's optional point) and spot-weld them to the bottom of that subframe-they'll serve as feet.

You're now creating a jig that'll allow you to spot-weld other pieces of tubing that will help you position the upper arms/mounting plates. You'll be able to move it in or out of the body at will without concern of messing up your geometry.

BTW, are your looking at correcting that front sub-frame geometry so that it will "work" correctly at the rear of the car?

Cadensdad14 New Reader
10/9/16 8:49 p.m.

Ok. Maybe I need to do more research, but my understanding of the suspension is that it keeps the tire in contact with the road.

On the front you have camber, toe, and caster. On the rear you have camber and toe. Caster is tilt that comes into play when the tie rods turn the wheel. Caster plays a big part in camber gains when turning. For example, if you have 90 degrees of caster you would be turning your camber.

On a multilink suspension, the control arms mainly limit the geometry change to camber, making it superior to a McPherson strut system. On a McPherson system a bump changes camber. The tie rods control the changes to the toe to create turn and through the caster have an effect on camber.

If the tie rods are in a fixed position then the control arms focus on camber only.

So then by using adjustable control arms I can modify my geometry to fit my needs on either end. Correct anything that was wrong.

2GRX7 New Reader
10/9/16 11:51 p.m.

I'm thinking about the anti-dive aspects of the front suspension being incorporated into the rear. Even though it is a multi-link suspension, it's inner pick-up points (when looked at in profile) will not line up on a perpendicular plane with the ground.

There will be a certain amount of incline/decline to the upper links that I would think you would want to examine a bit further or you may find yourself chasing, for EXAMPLE, side jacking forces upon acceleration with never-ending spring changes.

The adjustment of the links inner pick-up points will also change the pitch point, or the pitch center of the car-do you want to feel the car's yaw motion to occur under your butt, or directly behind your seat?

I'm no means an engineer, but I've been here before and got some help. It would kinda' suck if I didn't at least say something but I'm definitely looking forward to the results though! I hope this helps.

GameboyRMH GRM+ Memberand MegaDork
10/11/16 1:14 p.m.

Good point about the anti-dive. You may have to tilt the whole subframe to correct that. You can't really go wrong with zero anti-dive/squat, it just means you might have to man up and run hard springs

2GRX7 New Reader
10/12/16 9:35 a.m.

Yeah, with that multilink "A-arm" setup, it kinda means one would HAVE TO tilt the sub-frame down at the rear, although will you still deal with binding at the outer pickup points (at the hub)?

On a somewhat positive note, it looks like the upper links are a part of the strut top hats, and while you may have the same issue with the outer joints possibly binding, you're not going to need to "adjust" the positioning as much as the lower links. And to Gameboy's point, you'll be able to run a more compliant set-up in the rear (more stable/better grip out of corners) and have close to a zero squat. .

Cadensdad14 New Reader
10/21/16 6:51 a.m.

Quick update. I bought the car without tires and wheels, but yesterday I found three 17" alloys with tires to keep this build moving along. Now I can set the lower control arms and begin fabrication. I think I'm going to reuse the subframe up front and make the rear one from scratch.

On even better news I think I've found a pair of the 6 pot brembo calipers like the Porsche Cayenne uses for $100 total.

I love finding car parts cheap.

ssswitch Dork
10/21/16 10:51 a.m.

A mid-engined twin-turbocharged Audi TT with a frunk might actually be a very practical daily driver. I find this build entirely reasonable.

Cadensdad14 New Reader
10/22/16 7:36 p.m.

I mean isn't that the entire purpose. Why would anyone want a car that didn't work as a daily driver?

Cadensdad14 New Reader
11/3/16 9:20 p.m.

So after considering the advice you have all provided I have made some decisions. I have discarded the idea of using the front subframe on the rear. With the amount of engineering already involved it like why not make it a touch more complicated. To remove the anti dive i would have to rotate the subframe and that would raise the center of gravity of the motor. Also, by going full subframe i think i can pick up 4-6 inches of headroom by lowering the seat. Right now in the stock seat my i have about an inch.

I need input on 2 ideas.

im going to start with the chassis floor structure, then work into the roll cage, motor, trans, and suspension, and wrap up with fuel cell, battery and electricals. So im looking for feedback on these 4 designs based on what ive seen on other spaceframe chassis. All are 1.75 x .120 DOM tubing.

Plan 1: This is the simplest. It would be light and depend primarily on sheet metal as a large gusset. Plan 2:Stronger, but heavier Plan 3:I would reverse the diagonals to meet at the side rails to protect the occupants. Plan 4: The strongest. Would not be opposed to reducing wall thickness on some of the non critical components. In the event on a side impat it would provide the most driver protection of any of the options.

Also, i have been consodering setting the rails on a 6-10 degree slope from front to rear and sheeting the bottom. it shouldnt raise the C.O.G. too much and would provide a true ground effect.

Looking forward to your input.

GameboyRMH GRM+ Memberand MegaDork
11/4/16 8:11 a.m.

I would go with design 2. 3 is overkill and 4 is crazy overkill.

2GRX7 New Reader
11/4/16 9:49 a.m.

I'd agree with Gameboy- #2, but maybe consider square tubing, or 2x3 for the base tubes. You can weld to the rockers easier, and your DOM will be a lot less frustrating to weld to a flat base. Heck, you may even consider using .095 wall DOM. Less weight up top, and still legal if car is going to be under 3000lbs (I think that's still in the rules??)

Oh, and on the suspension, figured I needed to help since I opened ma' big fat mouth! Here's a link to suspension software;


....AND, check out this thread;


This guy's work is just simply beautiful. Lots of pics and explanations.

Man, this build has similarities to the build I want to do! Impatiently watching!

Cadensdad14 New Reader
11/4/16 7:37 p.m.

So I've pushed on with a rendering. I know everyone has said that the second image above would be sufficient as far as rigidity for the chassis goes, but i want to reinforce he side tubes to add impact protection in he event of a side impact. There will be a set of x-braced door bars bent out in a form of preload with gussets. I am maintaining the 1.75" tube size, but im reducing the wall thickness of the interior tubes to .095. As far as reducing the cage wall thickness, the TT has a curb weight of around 3,500. Even with the structural changes and losing the interior i dont know if i will get below 3,000. In any case, the primary intent of this structure is to protect myself and potentially others from injury or serious death, so .5 lbs per linear foot is acceptable to me in the required cage bars.

For using rectangular tubes for the sides, if the thought was to use the increased beam strength I would consider that strongly, but i dont think ease of use should be a big consideration.

Mike GRM+ Memberand Dork
11/4/16 11:35 p.m.

Wow, awesome build. Tell anyone who asks that it's a rare badge-engineered Audi version of the Cayman.

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