Pavement Sprint Car Build

[Graham08]

I've hinted at the existence of this project for quite a while, but never done a full blown build thread because there wasn't much building going on. Now that I'm working on it again, I'm going to start from the beginning with a proper build thread.

This build has unfortunately taken way too long. I started designing the car back in 2006, but building a couple customer cars, a move, building a new shop, and life in general have gotten in the way. Recently, I've started buying parts and reworking the design, so I figure it's time to start this thread to document where I'm at, and where I'm going.

Starting from the beginning, I had spent a season driving my previous sprint car against supermodifieds in the Midwest Supermodified Association back in 2004. I had zero previous driving experience, so the learning curve was extremely steep. I finally got up to reasonable speed, and proceeded to wad the car up like tinfoil at Mansfield Speedway.

Because the old car was a converted dirt car, it was heavy and not particularly stiff. It also had some rear suspension geometry gremlins that I needed to take care of. With the damage done in the crash, I figured it would be easier to start over, so the old car was scrapped.

When I started designing this car, my primary focus was on weight, stiffness, and a wide rear spring base. To that end, the plan has always been to run manual rack and pinion steering, which led to cantilever coil overs on the front end to package the rack. This also allowed the frame to be really short in the front, with the axle, rockers, and coils all mounted out in front of the frame.

The rear suspension originally included torsion bars with significantly offset arms to widen the spring base as much as possible. This is one thing that is changing as I work on the car because I don't want the headache of custom torsion bars, plus you're limited on spring rate by the diameter of standard bars (going to non-standard diameters gets expensive real quick).

With all that said, here are some pictures from the original construction process.

Unlike most "conventional" sprint cars, which are built as two sides in jigs, that are joined together with crossmembers and a motor plate, this car was built from the bottom up. This is a shot of the main rails and crossmembers from front to rear.



The frame table is my dad's. It consists of a large I-beam and square tube crossmembers. We made jigs to better locate the tubes in the front K-member and rear torsion tubes.

Here is a better shot of the front K-member. This car is built with the max allowable offset (4"). In this shot it looks a little goofy because the K-member is offset to the right so the cantilevers are centered on the track width. The centerline of the frame ends up offset 4" to the left of the center of the track width.



Here is the original configuration of the rear torsion tubes. I had originally planned to run 26" by 1-1/8" bars to get the possible rates higher than you can get with 30" long bars.



Here is another shot of the frame while the build was in progress. This is different from conventional construction because it has a main hoop in the cage, rather than some crossmembers that tie the two sides of the frame together. I feel like this gives the cage a little more structural integrity...but the normal way of doing things is also plenty strong.



This is the front of the frame while the build was in process. The front axle will eventually be located ahead of where the top rails bend downward.



I'll continue to add to this thread over the next few days to get caught up to the present time.

[TheBandit]

Originally Posted by Graham08

I don't want the headache of custom torsion bars, plus you're limited on spring rate by the diameter of standard bars (going to non-standard diameters gets expensive real quick).

How are torsion bars packaged for this application? Can you not move the ends along the torsion bar to change the effective length or turn down the diameter of a standard size to fine tune the rate?

What is the new design plan?

Can't wait to see more!

[Graham08]

Clint,

The way sprint car bars work, there are splined ends with shoulders that ride in bushings. Pretty much all dirt cars run 30" long bars with 1-1/8" splines. 29" long bars are also commonly available, and a couple manufacturers offer 26" bars, but they're sort of rare. By "non-standard diameter" I meant the spline size. Stock car sway bars are made in 1-1/4" and a number of other larger splines, but that would all be custom stuff for my application, and none of it is cheap.

The center of the bar (the "effective length") has the diameter adjusted to change the rate. So in solid bars, you're able to get somewhere around a 0.750" or 0.775" diameter as a minimum, and 1.125" as a maximum, in 0.025" increments. Some manufacturers will custom make bars with the center larger than the ends, but the max diameter is limited by the ID of the tube it's going in.

Bars are normally made out of heat treated 4340, so it's not something you're going to modify yourself.

You can have bars custom made shorter...a number of supermodifieds run 18" bars...but then you're at the mercy of the manufacturer on delivery time. Plus, depending on geometry and how the bars get used, the fatigue life can be short.

My problem was that I was going to be limited to a max wheel rate of around 300 lb/in or so, plus the bars, tubes, arms, etc. add up to more weight than the equivalent coil over. So, I'm going back to coil overs in the rear, with the geometry adjusted to get the mounts as far outboard as possible. At least with coils, if I need a different rate, I can get it overnight from several different manufacturers. I think Hyperco's range is like 85-800 lb/in for the 10" long coils I'm going to be running, so if I can't find a rate that works it's my fault.

[TheBandit]

Ah, so the problem was the spline diameter/size, not the rates? For the effective diameter section maybe I'm crazy, but couldn't you spin it on a lathe and grind the diameter down to adjust the rate without significantly affecting the heat treat? Regardless I see your motivation for going to coils.

[entropy]

Watching this one!

[Graham08]

Originally Posted by TheBandit

Ah, so the problem was the spline diameter/size, not the rates? For the effective diameter section maybe I'm crazy, but couldn't you spin it on a lathe and grind the diameter down to adjust the rate without significantly affecting the heat treat? Regardless I see your motivation for going to coils.

Yeah, the max possible rate is the issue with standard hardware. It becomes more of a problem with wings and banking added to the equation.

You're not crazy...you absolutely could grind the effective diameter to get rates in between what's commercially available. It's just not really necessary...a 0.025" diameter change is usually less than 25 lb/in difference in wheel rate, depending on geometry, so it would be a really small effect for the amount of effort involved.

[Graham08]

Here are a couple more pictures of the frame build in process. This is the front 3/4 view with most of the tube bracing added to the car.



And a side view at the same point...

[TheBandit]

One last question here... can I drive it?

[Graham08]

Maybe...it seems like everyone that sees it asks that question! There's a line forming...gotta get it done first.

[Graham08]

Here are a couple more photos of the front shocks and rockers.



The rockers are 1:1 ratio, with a very slight rising rate built in. The reason for that was to keep them from going over center into a falling rate which tends to make the car evil. The shocks are Ohlins STJ's that I bought used. I've got a couple sets of them, but in hindsight I probably would have been better off with Penskes. The Ohlins stuff never really caught on and I'm not sure if they even make them anymore.



The rockers are built out of a combination of 1" x 2" x 0.065" 4130 tubing and 0.065" 4130 sheet. Looking at these pictures brings back painful memories of making these things with a jigsaw, cutoff wheel, and drill press. I'm much better equipped now than I was a few years ago. They have needle bearings at the center pivot, and are symmetric so the same piece can be used left or right.

Someday I would like to make another set out of streamline tube just because. Of course then they would have to be lefts and rights so the teardrop faces the correct direction.

[TS3g]

Looking good Graham! The overall design looks great....that baby ought to be stiff like you intended. Looks like it might be a little tight in the elbow area, but I'm sure you designed the car with that in mind so it shouldn't be a problem. On the cage, are you going to add a brace/gusset from the top halo to the rear halo (a Scootek rounded gusset would be sweet looking there )?

I don't know think you went wrong with the Ohlins, they are big time in the dirt racing world (maybe not open wheel so much, but definitely on dirt late models). I know in some of the entry level classes, guys take Ohlins internals and adapt them into Bilstein bodies to meet rule restrictions.

[Graham08]

Originally Posted by TS3g

Looking good Graham! The overall design looks great....that baby ought to be stiff like you intended. Looks like it might be a little tight in the elbow area, but I'm sure you designed the car with that in mind so it shouldn't be a problem. On the cage, are you going to add a brace/gusset from the top halo to the rear halo (a Scootek rounded gusset would be sweet looking there )?

I don't know think you went wrong with the Ohlins, they are big time in the dirt racing world (maybe not open wheel so much, but definitely on dirt late models). I know in some of the entry level classes, guys take Ohlins internals and adapt them into Bilstein bodies to meet rule restrictions.

Thanks!

Actually, the elbow area is something I have to address. I ended up having to cut a tube out to get a Butlerbuilt full containment seat in there, and there's more work that needs to be done. It became really obvious once the steering wheel was in there.

There are a couple gussets missing in those pictures...they'll show up as the thread gets updated. I agree on the bent tube gussets...it will have to wait for the next car, though. I have become a lot more aware of those sorts of things since I started this build.

I know the LMJ (remote reservoir) Ohlins are big in the dirt late model world. I didn't realize guys were swapping guts in to Bilsteins, though it should work because they're both 46 mm pistons. When the car gets a little closer to running I need to call Ohlins and get the scoop on what's actually available now.

[Graham08]

OK, now for some pictures of the original rear suspension. This stuff is going to be changing soon to eliminate the torsion bars and redo some stuff lighter and better.

Here's the LR, showing the birdcage (bearing carrier), torsion arm, shock, and wishbone. The birdcages were made from large OD tubing that has a fairly thick wall so there is a decent amount of material over the spiral retaining ring that holds the bearing in. The next set will have the OD turned down over most of the width to save some weight.



The torsion arms were made using a commercially available steel end that comes with the splines already broached. This slides into 1-3/8" x 0.120" wall tube, that was bent to get the arms out into the wheels a bit for a wider spring base.

Here's the right side.



The right side is pretty much the same as the left, with the addition of a "Jacob's ladder" (Roberts linkage) for lateral location. Here's a better picture of that:



The Jacob's ladder is one of those good ideas that turned out to be not so good. The way this is built, it won't work because the links have rod ends in both ends...which makes it underconstrained. It flops side to side when you push on the chassis. This is going to be replaced by a Watt's linkage that should package a lot cleaner and be more adjustable.

The long wishbones that provide longitudinal location are staying put. The reason for them is they make the rear axle swing in the same arc as the torque tube, which should limit the amount the torque tube and driveshaft have to plunge vs. suspension travel, which I'm hoping is better for rear grip.

[chloeedad]

Great build...

[Graham08]

OK, moving on a bit, here are some pics on the ground.

First, the front end.



The axle is 2-1/4" x 0.120" 4130, which is pretty standard practice. I would like to build the next one out o 2-1/2" x 0.095" because it saves a pound or two. This rack & pinion was made by Latest Rage. I can't say I would recommend one to anyone. It turned out that the fit of a good U-joint on the input splines was loose, like it's really a 0.620"-36 (metric) spline instead of 5/8"-36, and there is a fair amount of slop in a brand new piece. I ended up replacing it with a quality piece from Woodward recently...pictures of that later. It turns out you really do get what you pay for.



I made the steering arms from 4130 tube and bar. I had just gotten a milling machine, so I had to use it on these. In retrospect, they're heavy, and I have material to make another set out of aluminum. Steering arms and caliper brackets are next on the list of things to do.

The spindles are Winters steel 10 degree "Ford" units, with Sander knock-off hubs. These will become spares eventually, I would like to upgrade to aluminum spindles.



I went a bit crazy machining clevises for this car. I would do some (the frame side) the same way again, where the axle brackets I would like to do in 4130 sheet on the next axle.

Here is the frame side of the radius rods. I think the clevis mounts are a pretty good way to get them in double shear.

[Graham08]

A few shots of the rear suspension with the car on the ground...

Right rear:



Left rear:



Right side wishbone bracket:



The small loop is to push things out to where the wishbone is clear of the body. The next set of wishbones might have sphericals in the front pivot instead of a rod end, just to do away with the shank in bending. But, there isn't that much bending load here since the brakes are inboard, and the torque tube reacts the drive torque.

An overall side view:



The "starter"...when a truck is pushing on it :

[TS3g]

Looking pretty killer man......fantastic fab skills!

Just curious, how much clearance is there between the LR tire and the torsion arm? Looks pretty tight, could be some rubbing with sidewall deflection.

[Graham08]

Thanks!

Those pictures were taken with the LR pulled all the way in. I don't know if you could actually run it that way or not. You might get away with it because left turns would pull the sidewall away from the arm, but with a splined axle it's easy enough to put another 1/4" or 1/2" of spacer in to push the wheel out.

It's all moot anyway since the torsion arms and bars are going away...

[chloeedad]

How close is the tail section to the bolts for the quick change cover? Fab work is beautiful.

[Graham08]

Thanks! Too close. You can just get the high nuts off to change the gear. This is getting fixed in the conversion to coil-overs.