825 replies to this topic

[MPR22]

We became engineers because we couldn't make a living racing.
Except MPR of course who is a real estate development guy who has been learning engineering words to use on this forum
CNJ

Learned a word or two from my father a Chem E and I spent a couple years at The Cockrell School of Engineering at the University of Texas. Alas I was not as interested in studying as I was other campus activities and needed to find a less rigorous major.

[speedengineer]

Learned a word or two from my father a Chem E and I spent a couple years at The Cockrell School of Engineering at the University of Texas. Alas I was not as interested in studying as I was other campus activities and needed to find a less rigorous major.

Would have been funnier if you had responded "Well, I did stay at a Holiday Inn Express last night." 

[RWP80000]

 

 

Steve Scheifler, on 06 Jan 2016 - 5:53 PM, said:

Your estimate of 65% in the hoop sounds high for a 7x15, but still your calculations come up with very low values, even lower than I might have guessed off the cuff. You probably came in too late for the latest flywheel weight debate, but suffice to say that I was on the side doing the calcs to prove that the impact was FAR below common expectations, and this is further support of that. The difference from a reduction of 12lbs at the wheels is lost in the noise. Of course heavier wheels also sightly Impact braking, and the unsprung weight impacts handling, but in terms of forward acceleration the difference is near zero.

Yeah I erred on high side for hoop weight, that way the numbers for sure aren't underestimated.

 

Flywheels.  So the flywheel actually can have significant power absorption.  P=I*a*w.  Depending on which trans gear you're in, the flywheel spins significantly higher rpm than wheel rpm.  5.7 times faster in 3rd gear for example.  Likewise the fylwheel acceleration rate is also 5.7 times higher.  Therefore the flywheel absorbed power (for the a given inertia) matters more by a factor of about 32 for 3rd gear.  It really adds up fast.  Guys that do autocross or standing starts need to pay attention even more so, in first gear the overall drive ratio is 13.5, so the factor is 13.5^2, or 182. Plus the acceleration rate is even higher yet, because in first geat, the vehicle acceleration rate is much higher than it is at greater vehicle speeds.

 

But the power that the flywheel is absorbing isn't going away, per say.  It's being stored as kinetic energy in the flywheel.  So the question is, when you make a very fast shift and drop the clutch out, how much of that stored inertial energy is dumped through the clutch back into the drivetrain providing motive force?  Probably a decent bit.  So, for the flywheel case, maybe or maybe it doesn't matter as much???

 

I may be a little late chiming in here in reference to the above discussions but here is link to some information previously developed that covers some of the flywheel related questions being discussed:

 

https://www.dropbox....ons v4.pdf?dl=0

 

Rich Powers

[Erik Hardy]

^^ I like the flip side paragraph on your link Rich; "Gain .33hp during normal acceleration, lose 33hp during a shift"

 

woooo math stomping on parity words all day long!

[Bench Racer]

Ingenious ideas are always great. View at 51 seconds and after.

 

http://www.msn.com/e...trim/vi-AAgl6I2

[speedengineer]

 

 

I may be a little late chiming in here in reference to the above discussions but here is link to some information previously developed that covers some of the flywheel related questions being discussed:

 

https://www.dropbox....ons v4.pdf?dl=0

 

Rich Powers

 

Very nice!  Thanks

[speedengineer]

Some cage progress the last couple of days, both driver's side and passenger side front hoops are bent and tacked into place.  I forgot how much I hate fabricating these bars, by far they are the most difficult and tedious bars to make on the entire cage.  Well, at least if you want them to fit tightly and maximize head clearance.

 

I am quite happy with how they turned out, particularly the driver's side.  I think it will be very rare to find another miata with this much helmet clearance.  Also, the footing is designed to be curved, which eliminates any sharp corners that could impact your ankle, and makes it very easy to get to the dead pedal.

 

pics:

 

 

[Diller]

Looks great. What kind of welding tip do you use to get a 360* weld behind that bar at the drivers side foot box?

[speedengineer]

Looks great. What kind of welding tip do you use to get a 360* weld behind that bar at the drivers side foot box?

 

Thanks Andrew. 

 

Re welding: 

1. For now everything is just tacked in place. 
2. Next I bend, notch, and tack in place the nascar door bars and the footwell bar that goes to the fire wall. 
3. They then get welded as much as possible where they attach to the front hoop. 
4. Then, I will break the tacks holding the door bars and front hoop to the main hoop and remove the whole front hoop/door bar assembly
5. Finish welding 360 around the joints
6. Reinstall in car and weld tubes to the main hoop, footwell bar to the firewall plate, and the front hoop base plate to the chassis.

[dstevens]

If you get in a pinch where space is limited I use a small profile Profax gun with a gooseneck.  It's only rated at 180 amps but that's plenty for .095 or .120.  In a couple of places on my car I had to use TIG with a gas nozzle and long stickout but that was more due to how I planned the fab than anything else.

[speedengineer]

Windshield bar done. 

 

[DrDomm]

I see you still have the parking brake in there...you gonna leave that?

[speedengineer]

I see you still have the parking brake in there...you gonna leave that?

 

The current plan is to yank it out.  Figured I'd leave it in until I get the rear axle nuts off.  I wish I could justify keeping it.  I never noticed rear brake drag on my last car (which retained the parking brake).  That said, there is likely a reason every fast SM has it removed.  So, not worth the risk of running it.  Plus it will save time when doing rear pad changes!

 

Doubt this would be legal, but has anyone ever seen an SM with a line-lock installed?  This would allow you to still retain parking functionality.

[DrDomm]

The current plan is to yank it out.  Figured I'd leave it in until I get the rear axle nuts off.  I wish I could justify keeping it.  I never noticed rear brake drag on my last car (which retained the parking brake).  That said, there is likely a reason every fast SM has it removed.  So, not worth the risk of running it.  Plus it will save time when doing rear pad changes!

 

My last SM had the parking brake.  Admittedly, I did have it lock up once...prolly my fault after an adjustment.  I think Dave Wheeler might be one of the advocates of leaving it in the car.

 

When you get there...please post photos of how you remove the adjuster from the rear brakes.

[mhiggins10]

The current plan is to yank it out.  Figured I'd leave it in until I get the rear axle nuts off.  I wish I could justify keeping it.  I never noticed rear brake drag on my last car (which retained the parking brake).  That said, there is likely a reason every fast SM has it removed.  So, not worth the risk of running it.  Plus it will save time when doing rear pad changes!

 

Doubt this would be legal, but has anyone ever seen an SM with a line-lock installed?  This would allow you to still retain parking functionality.

What does a line lock get you over just parking it in gear or using a wheel chock?  Would it be used in instances where the car/brakes are hot and you need to park for a moment with the engine running (taking hot tire temps in the pits)?

[speedengineer]

 

When you get there...please post photos of how you remove the adjuster from the rear brakes.

Will do.

 

What does a line lock get you over just parking it in gear or using a wheel chock?  Would it be used in instances where the car/brakes are hot and you need to park for a moment with the engine running (taking hot tire temps in the pits)?

 

I've found the parking brake to come in handy very often.  I'm usually at the track by myself with no crew, so nobody is around to move wheel chocks.  Often times I want the car to be running to warm up the engine, which means I can't leave it in gear.  Or say I'm working on the car by myself, it's sometimes helpful to be able to lock the rear wheels from rotating. 

[Rob Burgoon]

Will do.

 

 

I've found the parking brake to come in handy very often.  I'm usually at the track by myself with no crew, so nobody is around to move wheel chocks.  Often times I want the car to be running to warm up the engine, which means I can't leave it in gear.  Or say I'm working on the car by myself, it's sometimes helpful to be able to lock the rear wheels from rotating. 

 

Throw down a 2x4, drive over it when ready to move.   I'll sometimes do it with a breaker bar on the ground, or if there's a slope, roll up against my trailer ramps

[949Racing]

Throw down a 2x4, drive over it when ready to move.   I'll sometimes do it with a breaker bar on the ground, or if there's a slope, roll up against my trailer ramps

Ditto that. I keep a few short sections of 2x4 in the bed for that purpose.

[Killian]

We had a car get away from us last fall in the paddock and it almost made for an expensive weekend. Now we line our spots next to the trailer with the 2x6s we use to slope the rear trailer door.

[speedengineer]

A smidge of progress tonight. 

 

Footwell bars tacked in place.  Ideally, for both the driver and passenger side, I would locate these bars such that they meet the front hoop down-tube directly opposite where the upper door bar attaches.  However, on the passenger side, you need to be aware that you have to mount the tube low enough so that you can still fit the HVAC blower motor in place.  On the driver's side, you have to mount the bar high enough so that it doesn't interfere with the toe of your left foot when you go for the clutch or dead pedal.  On both sides, you need to make sure you don't cover up the opening for the wiring harness to pass through into the doors, or you'll be trimming sheet metal later. 

 

It is possible to mount these bars so that they go from the front hoop down-tube directly forward, and don't come in laterally inward at all.  I don't like this method, as having a slight inward angle means that the bar can support a larger component of force in a side impact - it can help keep the front hoop in place by reacting forces from the nascar side bars through into the chassis, instead of solely relying on the weld at the base of the front hoop.  Load paths.  Plus, it's easier to fabricate this way.  

 

 

 

So, this happened tonight.  I was planning to wait until after the cage was welded in.  Often you want to be able to mock up your seat when you're building the cage to ensure that the door bars will clear it.  I hadn't been concerned, as I have previously owned a Recaro pro racer hans that I was planning to buy again, and it's narrow so fitment is not an issue.  However, I changed things up, and am buying a Racetech seat now.  Not sure if it will fit as nicely, so I am proceeding with the floor pan modification prior to door bars, to ensure proper fitment. 

 

Ready for Flintstone power! 

 

 

 

Snow on the ground here in MI.  Jealous of those who are already racing!  Looking at Majors schedules today.  Funny how the SE majors season will be completely finished in April, potentially before my car even sees the track for the first time!