Datsun Roadster: 2020 Challenge Build

I knew I had some catching up to do with my reading!

Hardtail, eh?

It's a cart. Should be wickedly fast on the autocross.

Not the smoothest ride, but fast.

In reply to wheelsmithy :

It's not exactly a hardtail, but probably may as well be. 

The entire rear subframe carries the motor, axle, brake, chain tensioner, etc. The subframe is suspended from the chassis by two rubber pucks (you can sort of see them by the corners of the rearmost part of the chassis), and then the subframe is connected to the chassis by one very large spherical bearing at the front.

The fore/aft distance from the spherical to the pucks is probably longer than the distance from the pucks to the wheels, so the wheels will actually deflect LESS than double the pucks in any bump travel. So, while it does move, it definitely does not move much! (Think of how much you can squish a hockey puck, then double that, and that is effectively my max suspension travel in the rear).

I'll try to grab some clearer pics for anyone interested.

Well, had me some fun this morning!

Confirmed planned engine weighs between 130-140. There's going to be fudge factor either way for accessories and mounting stuff but not much. Let's say the motor swap saves 70 lbs to be conservative.

Option 1:

This is about as far forward as the motor goes without modifying the two diagonal frame members. Should still be plenty of room for chain, and the motor would mount to the "standard" sliding mounts, and chain tension done the "standard" way. This is probably the fastest route to a running car. Downsides are that the motor is still unsprung weight, and is further from the center of mass of the car than it needs to be.

Option 2:

This is about as far forward as the motor goes with still keeping good clearance for headers and stuff. I'd love to see the motor here as it's close to center of mass of the car, and the chain length is really normal motorcycle length. Id have to modify those diagonal members, and if I put the motor here the easiest thing to do would be to mount the motor to the chassis, and no longer to the swingarm. That means much less unsprung weight as well (good), but also that I'd be reengineering a chain tensioning method (boo).

Mmmmm, I love Sunday morning coffee in the shop.

Robbie said:

In reply to wheelsmithy :

from the spherical to the pucks

That just sounds dirty to me. 

I vote forward cause that's a lot of unsprung weight. 

The center mass is more forward than you are thinking. On a car this small, the 225 lb driver will make a huge difference. 

Stampie said:

Robbie said:

In reply to wheelsmithy :

from the spherical to the pucks

That just sounds dirty to me. 

I vote forward cause that's a lot of unsprung weight. 

plus, there's the concurs to think of.  Moving the engine, and tweaking the suspension seem like good "look at what I did" execution and engineering points.

It sounds like you were saying you only have an 1/8” inch of suspension travel. Is there a measurable distinction between sprung and unsprung weight?

I would vote for the shortest chain arrangement.  Unless you include something to take up the extra length, keeping it under some sort of tension, of the froward mounted engine.   

Option 1.

Get this thing up and running and moving as quickly as possible.  The gains from extra development time will by far offset any theoretical gains from unsprung weight.

Robbie said:

Picking this whole deal up is why I was down in Indy. I got a late start and Indy guy was very nice and invited me to crash on his couch. In the morning I got to enjoy a delightful breakfast with his whole family before making the journey back to Chicago. 

Brian - thank you. And your tr4 is awesome.

Note: I did not purchase this stuff from Indy guy. 

Happy to have you visit and chat about this and my TR4 in the garage.  Thanks for teaching us the "two words" game.  It's become a family meal time favorite!

Definitely option 1. 

Theoretical gains from weight distribution VS perhaps 2 months of actual testing and/or auto crossing is no contest. 

You also have potential power losses from the longer chain. 

Won't you cook the back of your head and neck on the headers if you mount the engine that far forward?

Wicked! How long until the clean body shows up?

For engine placement, I agree that there's not much difference in spung vs unspung weight when your suspension barely exists. Can you scale the car with driver with engine in each position and go from there?

Robbie said:

Indy-Guy said:

Robbie, you've got me thinking about this build again.  I'll take an update when you're ready to post something.

Well, you posted this before I landed at your house! But for everyone else, I did just pull hard to extract the cowl ducts:

dont know if you all can see it, but that is a really old pencil that was stuck in one of the cowls. I broke it while removing it.

I think it may have been used to kill the roach that was also dead in the duct, but then wasn't extractable haha.

Is that Zex fuel rail spacer for Nissan or Ford?

Slam it together and start testing. Had I tested more, I woulda hit higher at the challenge. 

Also, as this is now a bodied formula car, please use the patina/rusty body. Leave the good one for a good build/resto.

IIRC, the Fmod that has won Solo Nationals the last two years is a swing-arm chassis...

I'm not saying don't change it, because you know my plan was to convert to a 4-link, but I just want to point out that this slope is steep and quite slippery.

I do like that your proposed engine already has a flat pan on it. Much better for car applications

Woody said:

Won't you cook the back of your head and neck on the headers if you mount the engine that far forward?

Probably. There's still quite a bit of space, but I did worry about that for a moment. No long haired drivers please.

Run_Away said:

Wicked! How long until the clean body shows up?

 

For engine placement, I agree that there's not much difference in spung vs unspung weight when your suspension barely exists. Can you scale the car with driver with engine in each position and go from there?

The clean body is due in about a month, maybe sooner. Joe was hoping to bring it this week but had some higher priority stuff come up at last minute.

I can weight the car with the engine in both places - and then some quick math would tell me the functional difference. But, since I plan to extend the chassis a few inches this may be thrown out the window anyway.

Nitrous stuff isn't even mine, so I don't actually know!

Dusterbd13-michael said:

Slam it together and start testing. Had I tested more, I woulda hit higher at the challenge. 

 

Also, as this is now a bodied formula car, please use the patina/rusty body. Leave the good one for a good build/resto.

Probably good advice on both fronts - I'm not making any decisions on the body until I see both with mine own eyes however.

Ok, here's a couple more pics. I've been staring at this most of the day. Basically the same positions but with headers installed. I was able to remove a minor bracket which means surgery required for option 2 is much much less than I thought.

Option 1: 

Option 2:

For no more than as a  theoretical mind stretch, if you mounted the motor with the sprocket  in line with the rear suspension pivot you wouldn't need asystem to take up the slack. 

Robbie said:

Woody said:

Won't you cook the back of your head and neck on the headers if you mount the engine that far forward?

Probably. There's still quite a bit of space, but I did worry about that for a moment. No long haired drivers please.

Robbie said:

Nitrous stuff isn't even mine, so I don't actually know!

That's the kind of thing I tell my wife.

Me, "Oh, that nitrous kit, that's my friend Rodger's nitrous kit."

My wife, "you don't even have a friend named Rodger."