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My chassis is as stiff as a cooked Farfalle

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My chassis is as stiff as a cooked Farfalle

 
Old 03-04-2019, 12:44 PM
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Default My chassis is as stiff as a cooked Farfalle

Hi, I tried doing a quick search on the forums but did not find much "tips and tricks" on chassis stiffness, besides some rollcage stuff.

I have a 1999NB, and I am having serious issues with chassis stiffness without my hard-top (especially with open roof). While driving open-top, my car is shaking booty like a hooker with a seizure, and my rearview mirror is literally dropping off from the windscreen. Not really nice when I am slaying some vaping Hondaboys and I can't see their sad gay faces in the mirror since it is in the passenger floorwell.

With hard-top most violent shakes are suppressed, but the car still feels a bit flexy under heavy beating (don't really know how to describe it in words). As said, without the hardtop I can feel an enormous difference even in very low speeds (large intense/sharp vibrations and a lot of bouncines), this ******* thing is literally blurring my vision due to "rptprptrptrptrpt" on bad roads. I would say that the issue is propably longitudinal torsional stiffness.

I do not want to put too much extra weight into the car, since I am running stock 1.8 power, so question is: What would be the most weight-efficient way to add torsional stiffness to the miata chassis? I tried looking at some underbody frames etc, (like the one they put into the NBFL's from the factory) but I am a bit sceptical and I would not want to dump hundreds of dogecoins into some junk that will not actually help at all.

Some maybe relevant information: R888R's 15x7et25 with stock suspension (i know it's a bit retarded but I haven't gotten into suspension yet), front strut bar and stiffer engine-mounts. Stock bushings all around (suspension and diff). Hardtop with oem locks.

Any other discussion about chassis stiffness also welcome, I am curious to learn.
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Old 03-04-2019, 01:11 PM
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If you have a roll bar, adding door bars stiffens things up considerably more than underbody bracing from what I have heard. However, you really need a full cage if you want a NA/NB Miata to be "stiff".

That being said, I'd probably address your suspension first. I'd imagine a lot of what you are feeling comes down to the shocks being blown and the bump stops being hard as hockey pucks due to age. On rough roads you blow through your travel and those bump stops transfer everything to the chassis.
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Old 03-04-2019, 01:12 PM
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Suspension helps.

Then probably frame rails. People say a rollbar does noticeable things. I don't buy it.
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Old 03-04-2019, 01:13 PM
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Something is 100% broke.

Find what is broke before trying to stiffen things up.

I also am pretty sure strut bars don't so much on Miatas, since they're 4 link.
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Old 03-04-2019, 02:43 PM
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You didn't happen to go to solid motor mounts or something did you? You mentioned "stiffer" mounts. The rear view mirror buzzing and dropping sounds similar to a car I drove with solid mounts. The entire car rattles and shakes.
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Old 03-04-2019, 02:57 PM
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I'm going to vote with Erat. Sounds to me like there is something broken in your setup. Sounds downright dangerous.
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Old 03-04-2019, 03:00 PM
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Door bars are huge helps and do not interfere with entry. Strut bars are of minimal effectiveness because our cars do not have struts. Stainless frame rails make a noticeable difference.

Check your wheels and also your tires for being out of round. Jack each one up and spin it by hand and inspect for wobble. Even if they have been balanced they may not be round. As a side note to this there is a known issue with using locking lug nuts causing an imbalance. It is possible your tires may either be flat spotted or have a belt separation that will show up in your spin inspection. It is also possible you may have lost one or more wheel weights. A check of balance maybe in order.

If all of this checks out, jack up the back wheels on jack stands and spin the rear tires up using the engine and see if you develop a vibration at any particular speed. Your drive shaft alignment may be out of whack at the ppf and this can induce a vibration.

Report back your findings.

I have a set of stainless frame rails for sale if you decide to go that way.
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Old 03-04-2019, 05:12 PM
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"my car is shaking booty like a hooker with a seizure, and my rearview mirror is literally dropping off from the windscreen" [and] "this ******* thing is literally blurring my vision due to "rptprptrptrptrpt""


That description sounds more like NVH from engine mounts, rather than an issue with chassis stiffness...
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Old 03-05-2019, 04:46 AM
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Originally Posted by dleavitt View Post
If you have a roll bar, adding door bars stiffens things up considerably more than underbody bracing from what I have heard. However, you really need a full cage if you want a NA/NB Miata to be "stiff".

That being said, I'd probably address your suspension first. I'd imagine a lot of what you are feeling comes down to the shocks being blown and the bump stops being hard as hockey pucks due to age. On rough roads you blow through your travel and those bump stops transfer everything to the chassis.
Originally Posted by concealer404 View Post
Suspension helps.

Then probably frame rails. People say a rollbar does noticeable things. I don't buy it.

Suspension kinda makes sense, the shaking for sure was not this violent when I bought the car, and it has seen a lot of beating on and off track after that. However Finnish mandatory yearly technical check last summer did not show any anomalies with the shocks or springs.


Originally Posted by Erat View Post
Something is 100% broke.

Find what is broke before trying to stiffen things up.

I also am pretty sure strut bars don't so much on Miatas, since they're 4 link.

This came to my mind also at some point, I have tried twice to find some broken mounts or bushing without much luck. Any good ideas where I should look at besides the obvious stuff? Are there some weak spots where some spot-welds etc. could give up? Honestly speaking, I think I have also some times heard weird cracks and resonations when accelerating or braking. I always thought that it is just my paranoia trying to get the best of me and I should just ignore it since it is after all an 20-year old convertible with a hard life behind it.


Originally Posted by Swanpuppy View Post
You didn't happen to go to solid motor mounts or something did you? You mentioned "stiffer" mounts. The rear view mirror buzzing and dropping sounds similar to a car I drove with solid mounts. The entire car rattles and shakes.

Nah, I bought these: https://www.ilmotorsport.de/shop/art...807c95&lang=EN

Stiffer for sure than the oems but not solid, I put these to a vice and grabbed them with a pair of big-pliers to wiggle and see if I can notice any difference by hand before putting them in.


Originally Posted by sixshooter View Post
Door bars are huge helps and do not interfere with entry. Strut bars are of minimal effectiveness because our cars do not have struts. Stainless frame rails make a noticeable difference.

Check your wheels and also your tires for being out of round. Jack each one up and spin it by hand and inspect for wobble. Even if they have been balanced they may not be round. As a side note to this there is a known issue with using locking lug nuts causing an imbalance. It is possible your tires may either be flat spotted or have a belt separation that will show up in your spin inspection. It is also possible you may have lost one or more wheel weights. A check of balance maybe in order.

If all of this checks out, jack up the back wheels on jack stands and spin the rear tires up using the engine and see if you develop a vibration at any particular speed. Your drive shaft alignment may be out of whack at the ppf and this can induce a vibration.

Report back your findings.

I have a set of stainless frame rails for sale if you decide to go that way.

This is a nice check-list of stuff, I think I will reserve some time to do this also just to be sure. However what makes me suspect that it is not wheel or "out of balance" vibrations, is that when driving calmly on really good smooth roads (on the Swiss side of the border) the car is absolutely rock solid. On the swiss highways it drives like a train at any speed. I have tried on autobahn also taking it all the way it can go and did not have any noticeable vibrations at any speed.

The frame rails were mentioned couple of times, so I guess these would be nice nevertheless. I live in europe so unless you wanna send them here I think I need to find a seller here.

Also one thing I forgot to mention is that I have the Kenauto-Taikan door bushings. With these I notice also a big difference when closing the door with or without hard-top. If I have hard-top on, the doors are closing and opening easier (still quite stiff). With open roof I really need to give the doors a beating to open or close them.

I will try to get the car on the lift this week to start ticking off boxes, just to make sure that it is not my car that has the problem.
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Old 03-05-2019, 07:56 AM
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I found a noticeable difference after I got frame rails on. Bonus is you can jack the car on them.
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Old 03-05-2019, 10:21 AM
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I almost have to question if your ppf is drastically out of alignment giving you some bad shimmies while driving.
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Old 03-05-2019, 10:38 AM
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Frame rails made a pretty drastic difference in my car. One of the most noticeable improvements I've made. Then I added a roll bar, which again made a dramatic difference even after the frame rails.
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Old 03-05-2019, 11:34 AM
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I did frame rails as well and they made a noticeable difference. I noticed that after installing them, when I jack up the car the whole side of the car lifts and not just a single corner from a floppy chassis. My rollbar helped a great deal too but my boss frog mounts in the usual places as well as on the floor by the seats where the frame rails are so I don't know if my roll bar is also acting as a bridge between frame rails. I do however have no interest in the butterfly brace from FM.
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Old 03-05-2019, 12:30 PM
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Old 03-05-2019, 12:57 PM
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If your 99 is that messed up, something is wrong. My NB1 is a rusty shitbox and it is still way more solid than my NA6.
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Old 03-05-2019, 03:00 PM
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Check to see if the PPF is loose.
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Old 03-05-2019, 03:49 PM
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I'm on cheap coils but otherwise stock NA (read: worse chassis) and mine is smooth even cruising the highway... You need to jack the car up and give everything a good look over.
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Old 03-05-2019, 04:13 PM
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Old 03-05-2019, 06:35 PM
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Here's my $0.02 based on my own experience.

NA/NBs suffer from chassis twist which was addressed in the NC.

My car started life as an MSM so had the stiffest bracing of all the NBs from the factory (a couple of extra underbody braces/shields and a front strut brace).

I initially installed an MX5-Plus Twin Hoop roll bar and that really transformed the car.
See Mazda MX5 NA NB 89 - 05 Roll Bar Twin Hoop Torque Box - they also make a single hoop version.
As soon as I turned right onto the street from the parking lot at low speed, the car felt very different and I noticed it too at the next two roundabouts, the car was MUCH firmer.
Some doubt the effectiveness of a roll bar as a chassis stiffener but the MX5 Plus roll bar with built in torque box essentially completes the "box" of the seatbelt U-brace towers really makes a difference to chassis twist.
Next up I installed FM Frame Rails.

I didn't notice any difference in feel driving-wise (If I did them before the roll bar I suspect I may have noticed something).
I DID notice the difference when I jacked up the car using a scissor jack from one of the corner jack-points.
The other wheel on that same side got airborne MUCH quicker than it normally did while jacking the car up.
This demonstrated that the frame rail DO improve rigidity, even though I couldn't feel it while driving (maybe I would have if I did autocross and really threw the car around hard).
These are a great add-on to protect your frame rails and make it easy to change tyres/pads on track day since you can jack the car up from the middle of the rail.

The next mod was Boss Frog Frog Arms.
I really noticed the difference when turning hard at high speeds, the car felt like it was turning sharper.
Dash vibration is reduced notibly when driving over corrugated surfaces.

I've always had a front brace in the car so can't really comment on what the car feels like without one.
I now have a Carbing 3-point front brace which makes a great place to mount the Master Brace cylinder stopper and oil catch cans to.

My car is extremely stiff with these modz and I need to use at least one wind-up jack stand - if I use 4x pin-stands, the car will rock diagonally across two of them.
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Old 03-05-2019, 10:05 PM
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Something's stuffed on it.

My 99 is flexy enough that my interior light comes on when I pull into my driveway.
And it doesnt wobble/vibrate too badly on rough roads, even with the Tein Flex Z coils (which arent that stiff by coilover standards, but lots stiffer than factory)
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