Belly Tank Build Diary

[Mike Brown]

I then disconnected the shocks and raised the chassis with a floor jack until the rubber springs were no longer in contact with their perches.  This gave me a total unsprung weight of 521 pounds.  This gives me a rear chassis weight of 974 pounds to use in my calculations for rear coil springs.  My next choice will be to buy bare shocks to install in the original shock mounting positions and add coil springs or to do coil overs behind the rear axle.  I still have more research to do. 

[Rex Schimmer]

Looks like a nice winter project, Mike. Good traction requires fairly soft springs and shock settings. I would think you should probably use shocks that are adjustable in both compression and rebound and start with a pretty low spring rate, somewhere in the 150-200 lb/inch. This is assuming that the spring/shock is directly connected to the axle. Have fun.

Rex

[Interested Observer]

Looking at the data, one would have a few questions/comments

1   Except for coming off the line, no apparent rpm spikes?so why ?pedaling? in 1st and 2nd?
2   Spike at 2-3 shift may have just been a harsh shift, or marginal clutch capacity.
3   Spikes at 49 and 54 seconds may well have been a bumps on the track.
4   Given above, why the concern about rear suspension unless too stiff for the bumps?
5   Did you or how did you level the load cells?
6   Weight distribution ? do you run without water, oil, driver?

[Mike Brown]

Looking at the data, one would have a few questions/comments

1   Except for coming off the line, no apparent rpm spikes?so why ?pedaling? in 1st and 2nd?
2   Spike at 2-3 shift may have just been a harsh shift, or marginal clutch capacity.
3   Spikes at 49 and 54 seconds may well have been a bumps on the track.
4   Given above, why the concern about rear suspension unless too stiff for the bumps?
5   Did you or how did you level the load cells?
6   Weight distribution ? do you run without water, oil, driver?

1)  Pedaling the car is due to the chassis squirming from side to side as I am looking for traction and not wanting to overpower the tires resulting in spinning the car. 
2)  The 2-3 spike was most likely rolling back on the throttle too hard while releasing the clutch.  The clutch is a dual disk dual mass clutch which has not exhibited any issues of slipping. 
3)  The other spikes I would attribute to bumps on the track, that is what I would like to alleviate. 
4)  I believe that the rear suspension is too stiff and too dampened for the bumps in the track. 
5)  That portion of my garage floor is level with about 1/8" as verified by a level on a 2x4 which is sufficient for the need to select a spring rate. 
6)  Weighing of the car was done with the driver in the cockpit, without cooling water and fuel which is about 100 pounds.  Again accurate enough to help with selecting a spring rate. 

[Mike Brown]

Knowledge is a wonderful thing.  While at the last ECTA event two gentlemen came up and were very interested in the car.  After I spilled my guts about what I knew Gerry Clark proceeded to tell me that he was the head engineer for the intake system on the LSA engine.  A very modest guy that really knew this engine inside and out.  I sent him an email regarding my winter project and here was his reply:

"With your described symptoms, I think I agree with the QA1 guy based on what you described.  Less spring and damping should allow better traction as a first step.  Please note the Eaton blower engine should be very linear on power delivery,  (Power Potential = Airflow = Blower displacement x drive ratio x engine RPM).  But the act of pedaling the car can make the 'dumb' bypass open, ditching boost but the blower is still spinning at full tilt so when you reopen the throttle you might hit the bypass vacuum/pressure threshold pretty hard and even with an easy touch on the pedal getting back in you might have 'all the onions'.  I do not know your control mechanization and would scrutinize the bypass position and throttle position (And post throttle vs post blower pressures) during runs if possible."

This reflects how hard the car hits when coming back on the throttle after shifts or when "pedaling" the car.  I believe that any improvements in traction will result in both smoother power delivery and higher speeds as pedaling this blower motor is not a good thing. 

It is great that Gerry would take the time to respond to my inquiry, I am thankful. 

[JimL]

I worked with Jerry Magnuson and TRD on a "bypass problem" with Eaton supercharger kits (back in 1997-98).  I found the transition from "spinning blower in vacuum" to "throttle open" would buck and/or stumble the application of throttle.  Jerry's system had a vacuum line controlling the bypass valve, and by installing an .022" restriction in the vacuum line, the "reboost" was ramped in over about 800 milliseconds, which the ECU could handle easily.

When the blower is spinning, and you close the throttle, it acts as a "booster" vacuum pump in addition to the engine capacity.  Reopening the throttle can drive the measured airflow beyond the capability of the ECU (momentarily.)  Using a portable oscilloscope, I was seeing a 5 volt circuit exceed 5.24 volts (where 4.9 volt was the maximum useable signal in the software).

It all reminds me of the "method" needed to test supersonic airplane design.  It is impossible to blow air supersonic with a fan, but if you put a big enough vaccum tank behind it, atmospheric air will easily go supersonic into that big "hole".  A blower in front of a vacuum pump, becomes part of the vacuum tank when you come off the throttle.  Crazy stuff happens when you reopen that throttle.

If yours has that small vacuum line controlling the bypass, you might try a .023" wire welder tip to see if it helps your throttle control.  I don't actually know what your control system is, of course, but thought I could share my ancient history "memory".  We wound up installing that little restrictor in thousands of kits and ended the complaints.

[Mike Brown]

I worked with Jerry Magnuson and TRD on a "bypass problem" with Eaton supercharger kits (back in 1997-98).  I found the transition from "spinning blower in vacuum" to "throttle open" would buck and/or stumble the application of throttle.  Jerry's system had a vacuum line controlling the bypass valve, and by installing an .022" restriction in the vacuum line, the "reboost" was ramped in over about 800 milliseconds, which the ECU could handle easily.

When the blower is spinning, and you close the throttle, it acts as a "booster" vacuum pump in addition to the engine capacity.  Reopening the throttle can drive the measured airflow beyond the capability of the ECU (momentarily.)  Using a portable oscilloscope, I was seeing a 5 volt circuit exceed 5.24 volts (where 4.9 volt was the maximum useable signal in the software).

It all reminds me of the "method" needed to test supersonic airplane design.  It is impossible to blow air supersonic with a fan, but if you put a big enough vaccum tank behind it, atmospheric air will easily go supersonic into that big "hole".  A blower in front of a vacuum pump, becomes part of the vacuum tank when you come off the throttle.  Crazy stuff happens when you reopen that throttle.

If yours has that small vacuum line controlling the bypass, you might try a .023" wire welder tip to see if it helps your throttle control.  I don't actually know what your control system is, of course, but thought I could share my ancient history "memory".  We wound up installing that little restrictor in thousands of kits and ended the complaints.

Thank you very much for that suggestion.  My blower bypass valve is no longer controlled by the ECM, I simply plumbed the blower bypass valve dashpot to the intake after the blower.  The other side of the dashpot is vented (through a filter) to atmosphere.  I was contemplating using "bucking" pressure on this side of the dashpot by installing a needle valve that I could adjust to dampen the response of the bypass valve.  An .023" jet will give me a starting point.  I am always amazed by the "been there done that" help that pops up on this forum. 

[Lemming Motors]

Hi Mike, your spring discussion is very timely for me and I would like to explore my (deep lack of) understanding here if you don't mind. I setup my Lakester (still working on it, honest) with best guess springs and its harder then a very hard thing - my 240lb mass cannot bounce it even a smidge on the rears. I don't like what I did first iteration and with more bits and pieces attached it has become more obvious regarding a better position for the coil overs so I need to place an order for both longer shocks to get the mounting points reasonable and a pair of softer springs.

You noted 521 lbs un-sprung so 453lb sprung weight (226 each side) and Rex suggested 150 - 200 lb in springs. Do you have any idea what the rubber ones you have are rated at (noting the  assumption they are too firm) and, with 260lb on a 200lb spring  so about 1 and 1/8 of an inch deflection when the weight engages the spring to its settled ride height i.e. the sprung weight is 1.13x the spring rating so it will deflect 1.13" on static load?

[Mike Brown]

Hi Mike, your spring discussion is very timely for me and I would like to explore my (deep lack of) understanding here if you don't mind. I setup my Lakester (still working on it, honest) with best guess springs and its harder then a very hard thing - my 240lb mass cannot bounce it even a smidge on the rears. I don't like what I did first iteration and with more bits and pieces attached it has become more obvious regarding a better position for the coil overs so I need to place an order for both longer shocks to get the mounting points reasonable and a pair of softer springs.

You noted 521 lbs un-sprung so 453lb sprung weight (226 each side) and Rex suggested 150 - 200 lb in springs. Do you have any idea what the rubber ones you have are rated at (noting the  assumption they are too firm) and, with 260lb on a 200lb spring  so about 1 and 1/8 of an inch deflection when the weight engages the spring to its settled ride height i.e. the sprung weight is 1.13x the spring rating so it will deflect 1.13" on static load?

I have attached a graph for the rubber springs that I used on my belly tank.  It is cold here in Ohio but I did notice that my springs did not return to their unloaded height after the weight was lifted from them which is troubling.  I have ordered 160 pound/inch QA1 springs that are 8" tall.  For my preferred ride height I have approximately 5" between perches.  So if this works as I understand it with 487 pounds (1/2 sprung weight of 974 pounds) per side the springs will compress about 3" (487lbs/160lbs per inch) which will get me to the correct ride height of 5" (8"-3").  I hope this helps.  /quote]

[Lemming Motors]

Thanks Mike
My logic was okayish but I clearly misread your weight info.

So far I have only got total weight and haven't disengaged the rear to measure unsprung weight. Currently doing some fibre glassing to create bespoke air plenums for the inlet side of things.

[Mike Brown]

After much planning I ordered 8 inch tall QA1 coil springs from Jegs.  The compression rate is 160 pounds per inch.  With my unsprung weight they should have collapsed about 3" giving me the original ride height of 5 inches.  After installation they measured 4-3/4 inches which was very close.  The springs are designed for use with coil-over shocks but worked well in my application.  The OD is 2-5/8 inches with an ID of just over 1-7/8 inches.  I machined blocks of aluminum 1 inch tall by 1-7/8 inches in diameter.  These were used to keep each end of the spring centered on its perch.  The top blocks have a 3/4-16 thread and the bottom blocks have a 1/2" clearance hole for mounting.  After the shocks were disconnected I raised the chassis up with my hoist to install the springs.  With the shocks disconnected the rear of the car is now very compliant.  The plan is to install QA1 double adjustable shocks which should provide a really wide range of dampening.  No modifications were required to the chassis to switch from rubber springs to the coil springs. 

[Mike Brown]

Inspection of the rubber spring parts revealed that in addition to being oversprung the rear suspension was going solid with the nuts inside the rubber springs impacting the aluminum ride height adjuster nuts.  I don't know if this was happening during a run or while being trailered but this was not good.  The imprint of the nut is obvious in the photo of the adjuster. 

[Mike Brown]

While I had the rear suspension apart I removed the driveshaft to inspect the universal joints.  All was good so I cleaned it up and gave it a quick coat of paint. 

[Mike Brown]

The new shocks from QA1 arrived today.  The only modification required was to narrow bottom mount from 3-3/8" to 2".  I also did this to the original Monroe shocks.  Wow, with the shocks installed you can really tell a difference in the dampening just by shaking the rear of the car up and down.  I am very much looking forward to testing this new rear suspension, probably first on a quarter mile dragstrip.  Installed photo coming soon. 

[Mike Brown]

The new shocks are larger in diameter than the original shocks from Monroe.  When I removed the shocks I noticed some rub marks on the shock bodies.  They were apparently rubbing on the frame.  I don't know if this occurred with large movements of the suspension (possibly on the trailer) or when making sharp turns.  With the slicks and no differential turning is a challenge.  I relocated the bottom of the shocks inward about an 1" to increase clearance.  This necessitated making clearance on the upper stiffening webs of the rear end.  I wanted to do a nice job but did not want to disassemble the entire back end of the car, so I came up with a plan to do it in situation.  I started with a hole saw.  I added an extension so the upper shock mount would help guide the hole saw.  Since the pilot bit would not be engaging any material, I added a piece of rod that would allow me to steady the hole with my hand.  A photo of the hole saw assembly is attached.