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Fladiver64
Fladiver64 Reader
1/13/20 9:29 a.m.

Since you have guys with many challenge builds under there belts telling you to put it together and test, they are probably right. Sometimes I am not any good at listening to good advice.

My concern would be the very little suspension movement available with the puck/chassis flex system, given the not so smooth challenge course we run on. 

I would go with the forward engine position and modify the rear to form a swing arm suspension with the pivot about at the front sprocket. Then use the rear shock from the motorcycle donor mounted to the rear of the engine to a brace off the new swing arm. So basically you are re creating motorcycle rear suspension just expanding this to include the rear axle instead of a single rear tire.

Since it looks like the axle is in a fixed position and that is normally how you would adjust chain tension on a motor cycle, I would reverse that idea. Make you motor mounts with slots and have a bolt adjustment system so that you can use the engine mount as a chain adjustment. 

The other thing I would be looking at is you said the wheel base needs to be lengthened, where does this need to take place? If you put the drivers seat in the stock location of the Datsun body it looks to me like the rear tires are all ready too far back to center in the wheel wells. Now this is judged from pictures and my perspective could be off, but if you are going to modify the chassis then this should be taken into account at the same time.

 

Robbie
Robbie MegaDork
1/13/20 11:54 a.m.

In reply to Fladiver64 :

You are right on many fronts. The engine mount currently is setup as a slider. There are two crossbars that slide on the swingarm frame and two very large bolts to move the sliders forward and back. So chain tension is set by sliding the engine forward and back, and then theroetically everything is on one frame so if the wheels move so does the engine (so there is no change in chain length with suspension movement).

Also, ideally the lengthinging of the chassis has to happen at the front. The rear is already too long like you've noticed, and roadsters have big long noses because of the front engine setup.

Robbie
Robbie MegaDork
1/13/20 12:04 p.m.

Here's my big sticking point currently:

Rules say:

Five point roll bars meeting nhra 4:10 and arm restraints are mandatory in all open top cars running a 13.49 or faster or running non-dot tires. (Paraphrased slightly).

Additionally the rules say that cars must meet the safety regs for the duration of the event, so I don't think it's kosher for me to run non-dot tires for autox unless I fix the bar situation. While this rule is probably annoying for miatas running slicks, they can still just get a7s that are dot legal.

Unless I'm putting on different wheels in order to run dot tires, I think this means I need a rollbar. (The current tube is 1 and 1/4, not 1 and 3/4 like nhra regs require).

Anyone read it differently? I actually might email Tom on this one because I think the non-dot autox tires for open top cars will either be not enforced or a big disappointment to a lot of contenders.

Stampie
Stampie UltimaDork
1/13/20 12:08 p.m.

In reply to Robbie :

I was wondering about this for you also. I was more worried about the drags but didn't know you couldn't get DOT tires for the autocross. I read it like you read it. 

 

Edit. I remember Tom bringing up the DOT tire rule when he and I were discussing my 26 Model T build so I think he'll be enforcing it. 

SVreX
SVreX MegaDork
1/13/20 12:29 p.m.

I read it the same. That roll bar and associated bracing is a long way from from the NHRA specs. 

Any chance you could get it inspected by an NHRA inspector for a cage for a kart?

Karts are not really an NHRA thing... (thinking out loud)

AWSX1686
AWSX1686 UltraDork
1/13/20 12:34 p.m.
Robbie said:

Option 2:

Get some chain, or some string or something to mock up the chain. In this position, it looks like the chain would hit on the rectangle bar/cross member in between the engine and rear sprocket. 

SVreX
SVreX MegaDork
1/13/20 12:34 p.m.

Looks like drag racing karts are a thing in the UK:

Road and Track article

NONE of the karts in that video would meet NHRA specs. The roll bars are all too small in diameter, too low above the driver’s helmet, and the bracing is too small in diameter and too low on the roll bar. 

We’re missing something. 

Robbie
Robbie MegaDork
1/13/20 12:39 p.m.

The rules don't really define "open top". What if I have a hardtop (removable fiberglass)?

Seems fishy to me, of course.

Robbie
Robbie MegaDork
1/13/20 12:45 p.m.

Actually, the rule really is confusing because an nhra cage may only require 1 5/8 tubing for the main hoop, meaning that even if you have a cage and are nhra certified to run 9s that you still don't meet the letter of this law.

We all sort of assume that cage > bar, but that isn't how this is written.

Robbie
Robbie MegaDork
1/13/20 12:47 p.m.
AWSX1686 said:
Robbie said:

Option 2:

Get some chain, or some string or something to mock up the chain. In this position, it looks like the chain would hit on the rectangle bar/cross member in between the engine and rear sprocket. 

I did check that, and it's not nearly as bad as it looks. The front sprocket is actually pretty high up on the motor.

Robbie
Robbie MegaDork
1/13/20 12:50 p.m.

Lol, there are 10 inch DOT legal golf cart tires....

Maybe I just run on those.

Fladiver64
Fladiver64 Reader
1/13/20 12:58 p.m.

Inspired by someones post of reverse drag racing, how about mounting the body on the chassis backwards. Then the long hood would be covering the rear engine. Then you just need a team name/story/theme to give a reason to the backwards car. 

 

Indy-Guy
Indy-Guy PowerDork
1/13/20 1:35 p.m.
Fladiver64 said:

Inspired by someones post of reverse drag racing, how about mounting the body on the chassis backwards. Then the long hood would be covering the rear engine. Then you just need a team name/story/theme to give a reason to the backwards car. 

 

Brilliant!

 

Similar to this:

Stampie
Stampie UltimaDork
1/13/20 1:57 p.m.

In reply to SVreX :

The UK isn't our "National" hot rod association. 

Stampie
Stampie UltimaDork
1/13/20 1:59 p.m.

In reply to Robbie :

5 point is 1.75 but a full cage is 1.625. 

SVreX
SVreX MegaDork
1/13/20 2:34 p.m.
Stampie said:

In reply to SVreX :

The UK isn't our "National" hot rod association. 

Yup

Robbie
Robbie MegaDork
1/13/20 2:49 p.m.
Stampie said:

In reply to Robbie :

5 point is 1.75 but a full cage is 1.625. 

That's exactly what I'm saying. If someone has a car with a full cage and non-dot tires they are in violation of the rule.

SVreX
SVreX MegaDork
1/13/20 2:55 p.m.

Jr dragsters have much different tubing size requirements. 

I gotta assume there is a different NHRA standard for karts. 

They don’t require roll bars for motorcycles. 

 

SVreX
SVreX MegaDork
1/13/20 3:21 p.m.
Robbie said:
Stampie said:

In reply to Robbie :

5 point is 1.75 but a full cage is 1.625. 

That's exactly what I'm saying. If someone has a car with a full cage and non-dot tires they are in violation of the rule.

I disagree, Robbie. 

The rule says cars must meet the NHRA’s general safety regulations. NHRA general safety regulations include cage guidelines. 

The rules go on to try to capture one part of the NHRA safety regs (13.49 verts), and open the door for this event to non-DOT tires.  That means non-DOT tires can run on the autocross with rollover protection.

Thats not an exclusion for cages and non-DOTs. It’s emphasizing that yes, fast Miatas need NHRA compliant rollover protection, and yes you can use non DOT tires (a change), but you will have to have rollover protection (regardless of speed).

For you, that means either have DOT legal tires, or add NHRA legal rollover protection (roll bar or cage).  But it doesn’t necessarily mean a 1 3/4” roll bar (dependent on NHRA rules for karts)

Robbie
Robbie MegaDork
1/13/20 4:03 p.m.

In reply to SVreX :

The rules say "except where otherwise stated, follow general nhra regs", then they go on to say "the following will be required: ... Roll bar ...".

That seems "otherwise stated" to me.

I think your interpretation is based on what we all assume the rules should say, not what they actually do.

But regardless, I think I'm doing a roll bar just to make this all easy.

gumby
gumby HalfDork
1/13/20 5:25 p.m.
Robbie said:

In reply to SVreX :

I think your interpretation is based on what we all assume the rules should say, not what they actually do.

But regardless, I think I'm doing a roll bar just to make this all easy.

IMO Robbie, you are reading the Challenge rules correctly.
Go over sec. 4:10 carefully. Don't add any more weight than required. I am all for safety, but the 1¾" weight factor relative to this build was the reason I mentioned NHRA rollbars when you picked the car up. I totally overlooked the non-DOT tire clause in the Challenge rules.

Alternately, I also think you may have a reasonable leg to stand on if you wanted to send a request to Tom about tweaking the rule to read that the 4:10 rollbar is the minimum requirement. Then you could dig deeper into sec. 4:11 and build your rollover protection like a rear engine funny car. But, you hafta reverse hinge the body!

SVreX
SVreX MegaDork
1/13/20 5:30 p.m.
Robbie said:

In reply to SVreX :

The rules say "except where otherwise stated, follow general nhra regs", then they go on to say "the following will be required: ... Roll bar ...".

That seems "otherwise stated" to me.

I think your interpretation is based on what we all assume the rules should say, not what they actually do.

But regardless, I think I'm doing a roll bar just to make this all easy.

Not exactly. 

I’ll admit, you are partly correct (regarding my interpretation).  But my response is not my opinion, but what I’m quite certain Tom will say if asked.  Because that’s what he expressed at the time he wrote them. 

I suggest you ask him for clarification. 

SVreX
SVreX MegaDork
1/13/20 5:41 p.m.

I think I’m lost. 

Are you saying that because the car is an open top with non-DOT tires that it has to have a roll bar that’s 1 3/4”, even if it’s incorporated into a cage?

That sounds off. 

GoLucky
GoLucky Reader
1/13/20 6:55 p.m.

I don't know nothing to do with the safety rules but a challenge budget, autocross-centric funny car sounds pretty awesome. 

Robbie
Robbie MegaDork
1/13/20 8:27 p.m.
GoLucky said:

I don't know nothing to do with the safety rules but a challenge budget, autocross-centric funny car sounds pretty awesome. 

Agreed! 

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