The Trixie Chronicles

[H8rDave]

It's time to build a new car. I've built a few cars and trucks, and since it's been a few years since the last one, it's time for another go. This will be one that has been on my Bucket list of 'cars to buy or build'. I've been looking into a lot of the driving events going on, and I really want to build something competitive, safe, and fun to drive. Reading over all the rules for events like One Lap, USCA, GoodGuys, SCCA.... its hard to build a 'one size fits all' car. And, studying all of the 'fast' cars in those events, horsepower is the dominant force. So, for build plan decision #1, it may appear that I am going away from the norm, I want about 475 hp. But, I do have a plan.
The average 'fast' car weighs about 3400 lbs., and has 600 hp (or more). That nets a 5.66 lbs. per hp. My plan is to build a 2300 lb. car, with 475 hp. and net 4.84 lb. per hp. for pure performance. The mythical 3400 lb. car would need 702 hp, and be able to manage it, to equal the performance. Realistically, build plan decision #1 is now trumped, knocked to the #2 spot. And the new build plan decision #1 must be, Everything is about weight. Everything. The concept of weight will be a part of every decision in the build. Body, chassis, drivetrain, safety, comfort, ... everything. The lightest street car I've built weighed in at just over 3k. 3063lbs to be exact. How to shed 763 lbs, or more daunting, how to shed 963 lbs (so my 200 lbs fit into the math) will be a tough job. I'll do my best to show the build step by step. Including the info that I gather for each decision. Hopefully it can help some of the other guys here. Some info I may hold back on in the beginning, but only to keep this more interesting. I have enjoyed many of the build threads here for quite some time, maybe now I can repay the favor.

[Cop Magnet]

What kind of car are you talking about? That's the biggest factor in overall weight--your starting point. I admire your quest to seek the "purity" of weight over horsepower. No question, adding horsepower is easier and cheaper. Good luck!

[H8rDave]

What kind of car are you talking about? That's the biggest factor in overall weight--your starting point. I admire your quest to seek the "purity" of weight over horsepower. No question, adding horsepower is easier and cheaper. Good luck!

Thanks. One of the big concerns for weight over HP is the concept of lateral acceleration. In the middle of the corner, and in braking, a lighter car wins (all else being equal) every time. But there are many factors. On my bucket list there are a few cars that I think may fit the bill. 66 Nova, 65 mustang, C2 vette, A/C Cobra, '32 roadster. Most of my other B.L. cars just start out too big. There are plenty of 'vette's being built, so, I may toss that one. ridetech's 33 and the new Jet hot '32 are close, but neither address the wheelbase problem (too long), but a track style '32 would be bad ass. - No fenders = less weight, and no tire clearance issue. I have to keep in mind that we build Hot Rods to have their own personality, look, and style. So, pure performance isn't the only concern. It has to be fun, fast and cool. Lets throw in "safe" as well. I have a wife and kid who depend on me, and they like things like food and heat. I am factoring in tire size as a big concern at first. The current "315's on all corners" sounds good, but that's a Band-Aid for other ills. Bigger tires means bigger wheels and more rotating and un-sprung weight. And this bring me to a new division in the Weight plan. There are different kinds of weight. While I have to consider that all weight is bad, "un-sprung" weight is worse, and "rotating un-sprung" weight is terrible.

Here's some base info. My mythical 3400 lb bomber has 52.5% nose wt, or 1785 lbs on the front, and 1615 in the rear. In the front, the total un-sprung wt may be close to 180 lbs (given current Pro-Touring builds) of un-sprung wt in the front. That means 1605 lbs. in sprung wt, or a 8.9 - 1 ratio. OK, that's too high. In the rear, the 'bomber's' 9" & coil-over will have 315 lbs of un-spung wt, and 1300 lbs sprung, for a ratio of 4.17 - 1. Too low. the mythical Bomber will push into every corner, and be loose on every exit. My goal is a 7 - 1 ration both ends, and that's where I need to start.

[feeble]

I've been thinking about my next car and weight will be a major driving theme. I had to make a lot of compromises on the suspension and weight distribution on my current build to keep the stock Handler body shape.

I concluded that my next build should just be a scaled up go-kart.

[H8rDave]

Shawn, how much will the Cobra weigh? I've seen some V8 MII's under 2800 lbs.

I've set a target for un-sprung weight at 80 lbs per corner. The math works out good at that number, yielding a 7 - 1 ratio of sprung-to-un-sprung weight, and a total car weight of 2240. At first, 80 lbs seams like an easy goal, and then reality hit. A light weight 18 x 11 wheel is 23 lbs, and a 315/30/18 Falken is 32lbs. so I'm already at 55 lbs, and I've got nothing to bolt the wheel and tire to. So, I have 25 lbs per corner. on the front, this includes the spindle, hub, rotor, caliper and mounts, bearings seals and pads, sway bar links and then half of the weight of the A-arms, coil-overs, and tie rods. That's a tall order. In the rear, IRS is the only way I'll even get close. I'm leaning toward a C4 'vette. I have one, and there are plenty of parts for these all over the country. But, I've got a 25 lb. target for the uprights, hubs and stub shafts, bearings, seals and u-bolts, the complete brake assy, and half of the drive shaft and links. Another benefit of the 'vette IRS is that the hub has the speed sensor already in it, so if I decide to play with ABS or traction control, at least the pick-up is there. The C4, at 62" WMS, may be a little wide. I may need to narrow it don a bit to 58", but that looks pretty easy. I guess I need to dig out the IRS unit and start weighing things.

[bryant]

you can goto cantilever coil overs and put the full weight of the coilovers on the sprung chassis. sure this adds a little more total weight for the additional parts needed but it lessens the unsprung weight. that brings up the question of where is the trade off?

[andrewb70]

In for the discussion.

To keep unsprung weight down there is no reason to go with 18" inch wheels and tires. Go with something smaller and the weight of both wheels and tires will be less. It'll be super hard to get the other unsprung components to be less than 25 pounds. Obviously 14" 1.25" thick rotors are not needed.

Andrew

[jerome]

I'd build around a tire size and tire that does well in competitions you want to do. For example, in autocross, this seems to be the leading tire:

http://www.tirerack.com/tires/tires....=g-Force+Rival

If you wanted to go wide, 17" isn't going to get you there. Max 255s in 17".

[Mike Hervey]

Hi Dave-I'm new here, but I've been lurking for a while. Your concept is the exact same idea I have, but I am only in the planning stages right now.

I'm hoping to build my 1974 Triumph TR6 ino the same type of car you describe. Not sure how an "import" would be received on this forum, but those are my plans. As the stock frames are very flimsy with the stock straight-six, I'm planning on a custom chassis, and power from a small block Ford. In reality, the only thing I'll keep from my existing car is the body and some interior bits. The closest thing I've seen to this is a very nice Sunbeam Tiger that has been posted about on this forum, but I can't find a lot of information on it.

Stock TR6's weigh about 2,300 lbs. A SBF with aluminium heads and a T5 will be about an even swap for the stock motor and transmission. A roll bar/cage will add some weight, but my goal is to be under 2,500 lbs. There are some fiberglass body panels available in the UK.


Good luck, and I look forward to following your build.

[Twentyover]

Andrew x 2

H8TR-
In rotating weight, the radius of gyration (how far the mass is from the center of rotation- all you recent engineering grads forgive my 40year old definition) has a pretty significant impact on how an object accelerates and decelerates. Think about turning a 40b barbell weight vs a 40 lb dumbell weight.

Hervey-
Tracking with you on the English Sports Car deal. Hope you visit britishV8.org Me? I just spent a house payment getting some steel billet unground cam blanks from the UK for my MG

[H8rDave]

Mike, Jerome, and Greg, While I agree with the tire/wheel size comment, there seems to be no good middle ground. If I step down to a 275/35/17, I can a few pounds, but at a large percentage cost in contact patch. the overall diameter difference is minor. If there was a 295/35/17, or 295 40/16, I would consider them, but I haven't found one yet in a 200 tw. Looks like 315/30/18. Falken says the next generation 615 tire will be a little lighter. I weighed a heat cycled and trued (shaved) 315 and it came in right at 30 lbs. Forgeline says their Monoblock wheel will come in at 19 lbs. So, there's a little good news. Then I started weighing some other items. C4 outer upright and hub assy = 24 lbs. I think I can trim 1 lb. so, 23. 12.88" dia. x .812" wave cut rotor, hat, caliper, pads and mount = 11 lbs. , then half the weight of the links, half shaft, coil-over and sway bar link = 12 lbs. If I put the coil-over on a rocker, and add the link, the it drops to 6 lbs. All total is 40 lbs of unsprung weight from the suspension and brakes + 49 lbs. of wheel and tire = 89 lbs. And she's over weight already.

After a little more thinking and planning, I realized that I was looking at a 50/50 front/rear balance, when the goal is really 48/52, so front wt would be 1075, and rear wt would be 1165. And at a 7-1 ratio that gives me 83 lbs per corner in the rear. 89 lbs will net a 6.55-1 ratio. Not bad, but not up to my ideal. I need to cut 6 lbs out of parts that are already fairly light, and the hub/bearing axle shaft are most of the weight, and cant get much lighter.

Next problem. I am thinking about using a Coleman modular spindles and aluminum hubs. - But, the aluminum hubs are only avail in 5 x 5" bolt pattern. I could make my own hubs, but it's a pain, and I want to finish this car before I die (I'm in fine shape, just sayin) So, I wonder if anybody makes custom C4 stub axles that I can get a 5 x 5" pattern in? I also need to narrow the C4 to 60", or 1" per side, this looks pretty straight forward.

Time for more homework. - BTW, thanks for the input guys.

[bryant]

how about carbon fiber wheels? http://www.carbonrev.com/

[H8rDave]

how about carbon fiber wheels? http://www.carbonrev.com/

Those are pure bad ass. But the 15g price tag may keep them out of reach. I did some searching and they are the only company making a carbon fiber wheel, so, it's them or nothing. They say that the 18" x 11" wheel ways in at just over 5 Kg, or 11.23 lbs. While the thought of dropping 8 lbs. of rotating un-sprung weight is giving me Mahogany like wood, the wallet hit is the source of a wet noodle. It is tempting though. A set of monoblock Forgelines will set me back about 6g, so really, all I have to do is trim 9g out of my current budget and I'm set. HAHAHHAHAHA, like that's possible. Or, I could sell something, like a kidney, or some other no essential organ.

[jerome]

I think to be at your target weights, you may be thinking too much along the lines of the current pro touring trends (big wheels, wide rubber, V8, relatively long wheelbase and track width, etc.).

This shouldn't be too hard of a task to have a great performing lightweight car if you start off thinking about cars with dimensions more similar to kit cars (FFR, Superperformance, Ultima, RCR, etc.), early 911, datsun Z car, miata, lotus, or any other lightweight sports cars (japanese, british, etc.).

I would probably settle on a body with reasonable wheelbase and track width for your goals first and then plan out how you would do the chassis.

[maro67]

A suggestion for what type of car to use as a starting point, a model T Ford. More specifically a track nose t bucket. Ialways thought it would be cool to make a tbucket with the same drivetrain used in a foula ford race car.

[Twentyover]

If you're only carrying 600 lbs/corner, you really need a 315 tire or 295 tire? Are you going to get enough heat into it to really get it to grip?

[H8rDave]

Good question Greg. I have spent a lot of time on the tire question. After gathering way too much info on the subject, I may have started to understand something about tires. Now, I am thinking about the contact patch vs. the load much more than the tire size itself. I'll do my best to explain.

Start with a 3000 lb car that has 50/50 bias, so 750 lb. per corner, and bolt on a 275 tire. The tread on the tire is 10.2" across, and lets say the contact patch is 2" wide. (I settled on 2" but it really doesn't matter. As long as two tires are close to the same diameter, with the proper air pressure for the car, the width of the contact patch will stay fairly constant) So, the contact patch on this car is 20.4 sq.in. Divide the two (750lbs. / 20.4 si. =) 36.7psi. Now, if that car is in a 1G corner, and 80% of the total weight is on the outside tires, or 1200 lbs, the load on the contact patch becomes 58.8 psi. Using these for comparison, we can look at what happens when you bolt on a set of 315's to the same car.

A 315 tire is 11.7" across the tread, and with a 2" wide contact patch would net 23.4 sq.in. on the ground. with 750 lbs on the corner, the contact patch now has 32.5 psi. (much closer to the air psi inside) and in the same 1G corner, the load would be 51.2psi. To help put these numbers in perspective, the tire "max grip" numbers are needed. And, they are all different, and really hard to find. But, I did find some, and by working the numbers around, I found a reasonable average for 180 to 300 treadwear tires is around 52 psi over the size of the contact patch. So, once you cross that number, without aero to help, the tire is done. No grip left.

So, for Trixie, at a slim 2000 lb goal, 2200 with driver, we have 550 lbs. per corner. Back with the 275 tire with a 20.4sqin contact patch, and we have 26.96 psi static, and 43.137 psi in a 1G corner. Thinking about the loading of the contact patch, a light car with 275's has a lot more grip avail than a heavier car with 315's. And with that I am one step closer. A 275/40/17 is the 25.6" dia. compared to a 315/30/18 at 25.4". This will help in gearing and transferring previous geometry without having to start from scratch. Also, this is a pretty common size tire. As for rotating and un-sprung weight, a 275 falken weighs in at 26 lbs. compared to the 315's 32 lbs. and the 17 x 9 1/2" wheels can be found down in the 16 lb range, where as the 18 x 11 will come in at 20 lbs, for the lightest offerings. It looks like the 275/17 combo is the ticket for Trixie's magic slippers. Saving 10 lbs per corner of un-sprung weight will be a big step toward the goal.

I am working on a custom hub to put a few readily available parts together, and I'm almost done with IFS and steering geometry. Soon I'll post some picts, and this should get a lot more interesting. -- Merry Christmas