rdb138

iTrader: (1)

I had some pretty strong hood lift maybe 3/4 of a inch on the back of the hood along with it shaking up and down.

I'm guessing all of the below are wise ideas to help fix this issue. Does one of these options solve most of the problem on it's own? (I was thinking about tracking the car again this weekend coming up and only have so much time to make mods to the car.)

• aerocatch hood pins
• hood vents
• add a splitter / under-tray running back to the front axle
• ????
• grow a bigger pair and just sent it <-- Not really an option, I've had a hood fly up on me at highway speeds, prefer to not have that happen again.

More info...
Took the car out for it's first track day this weekend. Saw the lift on the back straight of Road Atlanta at speed over ~120 mph. (Wipers were lifting too, but I can remove them.) Car is lowered to 4.75" at pinch welds and has a hard top on it...so basically zero aero besides what Mazda gave her.

rascal

Of your options, the undertray is the easy option and will help some. Especially if you are running without the factory belly pan.

Once you’ve done that, then if you still have more hood lift than you’d like then the hood vents will definitely solve the issue.

Gee Emm

You haven't taken out any of the strengthening on the underside of the hood have you? I never saw any of that in my racecar, no hood vents but stock radiator ducting (which you need for proper cooling anyway). I did have two bonnet pins at the front, but they aren't going to fix lift/shake at the back.

If you have the factory two stage bonnet release, you should be safe. If you don't, bonnet pins ASAP., followed by the radiator belly pan which you need anyway. Then bonnet vents if big HP and/or hot climate otherwise only as indicated.

rdb138

iTrader: (1)

Thanks Rascal for the reply! I was thinking the same and started making a wooden undertray / very short splitter earlier today.

Thanks Gee Emm for the reply! I've pulled some of the ribs out of the center / underside of the hood so that my engine could fit. (LFX swap) but the hood seems to be flexing more as one big unit and not flexing like it's weak from the ribs removed. Then again, I didn't spend much time diagnosing it all, since I needed to be watching the track more than the hood at 200kph. I do have some make shift radiator ducting in place (on the front side of the radiator). I'm also still using the two stage bonnet / hood release, and it did hold the bonnet down, but I also backed out of the throttle and coasted down half of the back straight all day.

I wasn't sure if the air is coming from the seam between the bonnet and the bumper cover or coming from below the car building up a high pressure area under the bonnet, but I'm leaning more towards its air coming from below the car.

The plan is to do the bonnet / hood vents along with proper ducting of radiator and a proper splitter / air dam, etc. but wanted to do that in the off season. So far, the car seems to be handling temps fine. We aren't crazy hot here, but temps were around 30c this weekend and next weekend we will be at 35c, so vents won't hurt.

For this weekend coming up...I'm gonna try and get some sort of undertray on her and hope that keeps the hood from lifting, but if that doesn't work...vents will be next.

Arca_ex

iTrader: (2)

Undertray/splitter that extends to the centerline of the front wheels, radiator ducting, and hood vents are going to be your best bet.

sixshooter

iTrader: (12)

There should be about 1/4 inch of rake to the body of the car front to rear measured at the pinch welds. If the car is level or nose up it can increase the lift on the hood (and may contribute to instability at high speeds).

rwyatt365

Interesting...I was at RA on 5/5 and saw a GPS speed of 108 on the back straight in my NB1 (ill-fitting fiberglass nose, no undertray, hood ducts, fender vents and aerocatches) and had no issue with my hood lifting at all. So, either I'm a nascent aerodynamic genius, or extremely lucky. I only entered this discussion to check out the replies because I'm going to be getting a hardtop this summer and I intend to start down the whole air-dam/splitter/wing rabbit-hole over the winter and want to be as prepared as I can be for that roller coaster.

thebeerbaron

iTrader: (1)

It seems silly to ask, but have you checked the bolts on your hood hinges? If you’ve had the hood off, maybe you forgot to tighten them fully after aligning the hood. Or maybe your hinges have slop, or aren’t tight to the body.

Might be worth a second to (gently, carefully) try lifting the back of the hood by hand to see if anything is loose at 0mph.

rdb138

iTrader: (1)

Thanks Arca_ex for the response. I agree and I'll be making all of those changes soon (just not in time for this weekends event.)

Didn't think about rake at all. Car is 4.75" all the way around. I'll raise the back end to 5" before this weekend and see if that help at all. Thanks for the info Sixshooter!

Budding aerodynamic genius it is! I'll be joining you down the aero rabbit hole soon enough, possibly this winter too. I never had any hood lift issues with my old Miata (stock power with a splitter that went back to the centerline on it) and this one doesn't seem to have any issue until I'm over 120. Hopefully I'll see you at the track here soon rwyatt. I normally run with JTI and Jzilla.

Silly maybe, but guess who found a hinge bolt that needed half of a turn still? Thank you for reminding me to check the simple stuff first! Unfortunately, tightening it up didn't seem to make a change with the lifting the back of the hood by hand at 0 mph. The hinges do seem to have some bend in them if you push up on the hood after it's at the "top"...Not sure I'd call that slop, since the pushing on the hood like that is basically a huge lever. It doesn't go left to right or anything. Anyway...Here is a video of me lifting the back of the hood a bit. I'm not really using that much force.


And because someone else already asked about ribs being removed. Here is a picture of the bottom of the hood so you can see what I have removed.




FYI...thank you all again for the help! Much appreciated!

rwyatt365

I'm heading into "summer vacation time" (i.e. taking the wife where SHE wants to go instead of doing what I want to do), So I most likely won't be doing many track days until late July / early August. Keep a lookout for a unkempt-looking, PlastiDip color-shifting purple, NB1 with #75 on the doors...that'll be me!

maplewood

iTrader: (2)

If it helps, I went and pulled on my NB1 hood like in your video and it looked about the same. Its a light hood and its pretty easy to get some flex. I don't have any hood lift issues though. I do have a splitter (and before that, the stock under tray), hood vents (and some ribbing removed but different than yours), and aerocatches at the front (although I don't think they'd do anything for lift at the back). I also don't know that I've ever managed 120 mph.

sixshooter

iTrader: (12)

Mine did not lift at 150 miles an hour on Daytona but I have an under tray, a sealed bumper mouth, 1 quarter inch front rake, and hood vents. I wish I felt like driving on the track again.

rdb138

iTrader: (1)

I'll keep an eye out for the Purple People Eater! (FYI...that color looks great.) Enjoy your wife's summer vacation...it's getting too hot out for the track anyway. I'm fairly certain I'm going to be burning up this weekend. I have the Plain Jane white NB1 with a black top. Running 61 right now because it was available, but change numbers quicker than I change underwear tires...so...

Crappy picture, but you get the idea



Thanks maplewood! That does help knowing yours flexes about the same. Looks like you all have the winning formula on needed mods that I was just hoping to get away with not doing until winter. (Frankly, I'm tired of building the car and just want to drive it right now...It's been 3 years in the making.)

150 is FAST...that's promising that an under tray, sealed bumper mouth, rake and hood vents will do it. I need to buy some hood vents.

I find hobbies ebb and flow. Sometimes you need to take a couple years off to enjoy it again. I have the same feeling about one of my hobbies right now too, but I know I'll come back to it in good time.

So I added the 1/4" of rake tonight. Also started working on a splitter, not sure how I want to mount it. It's probably more OCD, but the splitter was more eyeballed when I went to cut it out and nothing is truly square...it's killing me trying to get things lined up.

FYI...the drilled holes are for the steering rack. I was lazy and didn't want to pull out the router / clean up that mess.


I'm thinking I will just cut it off even with the bumper, this way there is no lip. I probably should tape off / seal the empty fog light holes & the holes in the little front lip too...that can't be helping matters either.


In all honesty, I was seriously eyeballing this storage container. It would be less than an hours work to cut her up and make it into an under tray using zip ties. Not sure how well that would really hold up though...



Thank you all again for the help! I'll let you know all of what I end up doing and how well she does after this weekend.

Gee Emm

There is a thread on here where a cement mixer - a square(ish) plastic tub - was used for the radiator outlet ducting.- right dimensions, tough enough to last. I would not be using cable ties, there are plenty of mounting points there, or make your own - nutserts FTW.

I wonder about those gas lifts, and whether the mount on the bonnet (between the hinge and the bonnet) may be preventing the hinge from sitting as intended flush on the bonnet.

You will want that lip, don't cut it off, it generates appreciable downforce. You will need something at the back though, to balance it. I would first build and test the radiator duct (mixer box), that will help with cooling by facilitating the exit of hot air from the radiator/engine bay, that will do more for your cooling than the splitter. The splitter/undertray will generate DF, and help make the car more slippery

You really need to read the aero threads here, you seem to be just parroting the last information someone has provided, with no indication you understand what they are saying, or why what they are running will help YOU with your problem (bonnet lifting)..

Lokiel

I thought aftermarket hood lifts just don't work and are for posers only since they actually increase hood lift at speed because your car was engineered to keep the hood down with the hood flush with the car.
Hood lifters compromise the strength of the hood hinges that are keeping the hood down and the OEM design of the hood underside was to "seal" to the engine bay, not to encourage air flowing out the rear!
Flow will ALWAYS take the path of least resistance.
Adding gaps with hood lifts will encourage air to exit at the rear of the hood instead of underneath the car - no wonder your hood is lifting, it was never designed to allow it.

I did witness some unexpected MX5 hood lifting:

Years ago (around 2014) at Lakeside I remember seeing Matt Davey's SP (Australia-only MX5 version with Garrett 2560 turbocharger from Mazda Australia) and the front edge of the hood was obviously rippling down the straights, the horizontal gap between the hood front edge and bumper was rippling on the top edge.
Matt's car was a track-only SP that was no longer roadworthy due to a crash that damaged the front of the car and no longer had a working OEM front hood latch.
The hood was only held down by two AeroCatch pins and was an un-modified replacement hood (ie. no OEM bracing/reinforcements cut out).
Matt had done some ducting work but nothing as good as being done "these days".
Based on what I witnessed that day, it's a good idea to retain the OEM centre hood latch (ie. don't "add lightness" by removing it when you install hood pins, even on a track car).
I let Matt know this because it wouldn't have been visible to the driver and I believe he subsequently fabricated a working OEM front hood latch.

Gee Emm

Lokiel, I think they are a different 'hood lift'. The OP has the gas struts that open the hood up and hold it in place, see photo in post 9. Not the spacer type that creates a gap between hood and firewall.

Lokiel

^
My bad, please carry on.

rdb138

iTrader: (1)

Hi Gee Emm...

I appreciate all the help from everyone. Thank you again for replying.

I guess I wasn't very clear in my very first ask. I asked if any one of the items would fix most of the hood lift issues by itself, because I had less then a handful of nights after work to fix the issue before my next track day. I apologize if I'm coming across as a newbie asking for help and not taking it. I agree with you all on the fixes you all have been recommending, I plan to do them and do them correctly, I just didn't / don't have time to do it all in the one week (now really 2 nights) between track days.

I believe after reading peoples responses and other aero threads / cooling thread, that the under tray will be my biggest help to keep air from pushing up into the engine bay and causing the high pressure under the hood / causing it to lift. Thus why I started working on that item. I believe the 2nd biggest item to help would be hood vents. I still need to purchase those.

Another option...and the one I will most likely end up doing again...is letting the car coast on the front and back straight and don't push the car.

Comments on your suggestions:

1. Cement mixer box - I have read that thread in the past, it's what was making me think of cutting up the plastic tote to use as a temporary under tray. How the cement mixer box was used in the tread was to seal up the front of the radiator box, which would improve air flow through the radiator into the engine bay. I know this would improve cooling, but I don't see this improving my hood lift. I didn't have any cooling issues with the car last weekend and I had already sealed off the front of the radiator with a bunch of foam until I build a proper box out of aluminum.

2. Gas Lifts - I could pull those off quickly enough to test. That's a simple test that I will try. I don't think this is the issue, because the hood sits nice and flush (see picture above with the hood down) They also aren't that strong and I believe they push the hood / hinge "back" when the hood is down vs pushing up like it does when open.

3. Front Lip -- I plan on cutting it off because I don't have any rear aero to balance the car out. I do need aero, but this is a brand new build and I want to finally do some track days with it. I'm concentrating on driving the car some and to be frank, safety items is what will be done next. (drop floor & halo seat (vs aluminum on the floor) to be installed in July, I decided to hire someone to weld that in, vs trusting my welding skills. I'm also seriously considering having a cage put in at the same time. I don't feel the roll bar is enough for the car anymore.) Aero to improve times is not a priority right now.

4. Zip ties for plastic under tray -- Again, this is a temp solution, not a permanent one. I agree, zip ties aren't good long term, but I think they would work fine for a weekend to hold some plastic together / in place. My other option at the moment is fabricating mounts for the wooden splitter / under tray and figuring out how to mount it to the car. This takes a lot more time then some zip ties, especially since I have a different subframe then the car came stock, so I don't have those attachment points anymore.

Again, I definitely appreciate the help, and wasn't trying to come across as a parrot without a clue. I never thought about the rake possibly helping with the air under the hood. (For all I know it does nothing) but it was a quick change that should help the car either way, that I hadn't thought about. I was looking for a temporary solution that I could do during the weeknights and probably should have done a better job of explaining that I wanted something that I could do quickly based upon the time I had.

sonofthehill

iTrader: (1)

Maybe it's just me, but the gap between the hood and bumper seems huge. I must admit, I usually see a little lift at the front corners of the hood above 118 or so.

Gee Emm

Thanks for the post RDB, appreciate you are under the hammer! I'll get out of your hair, and let you get on with it - good luck on getting it ready for the weekend, and for the event.