Decision chart for Jass Performance ND seat lowering adapters:

If you have ended up here you must be in need to lower the seat of you MX-5 Miata ND or Fiat Spider 124. The good news is, there are options 33mm to 58mm with fixed adapters or rails, the bad – you are spoiled with choice.

All measurements are in the metric system, a conversion table is available here. And please consider this document as work in progress. It, as the products will be updated based on your feedback.

We would need to scroll through some theory to set expectations right. So – Ergonomics it is first:

Ergonomics

It is important to start with this general theory as it will set common language and the correct expectations.

Let use the largest man (LM) as a term for the largest possible person to fit in the car and the smallest woman (SW) as a term for the smallest person to fit in the car. These are btw. terms from the automotive industry.

And to put things in perspective – let’s use MX-5 and say we want to have 160cm woman and 200cm man fit in the car. This is a difference of 40cm, which would spread approximately as 20cm legs and 20cm upper body.

Below is an example of just a regular car seat adjustment (not an MX-5).

It shows how, independently from the seat sliding, car manufacturers are trying to fit the smallest woman and the biggest man in the same car adjusting the seat adjustment kinematics to address the very different body proportions.

Lowering the seat in a regular car has the following aspects:

- The lowering of the seat is not just parallel to the floor.

- It “pivots” around the font end

- The rear is purposely lowered far more than the front

- The incline/rake/angle of the base is as a result more aggressive.

- The backrest travel is affected too – it allows further forward travel than in the lower position

Why is the seat now lowered parallel to the floor?

The smallest woman is heaving seating position similar to sitting on a dining table chair, this allows higher view point but also reaching the pedals with the relatively shorter legs.

The way to fit the largest man in the same car the smallest woman just got it is to … bend him wherever possible and as much as possible torso and knees.

What is needed to fit the largest man in the same seat?

- The rear needs to go as low as possible to allow for the upper body, which cannot be bend, to fit below the car roof

- Given the limited length to the front, the legs need to be bend

- The backrest ergonomics needs to be adjusted to accommodate for this difference of 20cm of the upper body

The same criteria apply for a sports car or MX5/Spider in our case if the purpose of lowering of the seat is ergonomics The following photos are showing the Jass Performance lowering concept in MX-5 Miata ND:

As the motivation is not always rational or functional few approaches to choosing the right lowering (without being too much) will be offered.

Choosing the right adapters

There are different motivations and respectively approaches to estimating the correct adapters for your application/body proportions. We will start with the one below and further details will be provided based on your choices.

Do you rather need in your ND:

Ø Comfort

Ø Racing

Ø Fit

Looking for comfort

Your height is estimated based on your choice between 175cm and 189cm. You are still able to fit and drive and ND but in a compromised position with the head room and mostly with the front and rear sight as of the rear view mirror.

The correct option is the 48mm drop adapters and the fine drop adjustment kit is a must in this case. The 48mm directly would be too much and would create discomfort, raising the knees too high.  The adjustment kit allows to drop the to maximum but if needed to adjust for comfort. Lowering the front of the seat base by disconnecting the height adjustment rack is a must in this case as well.

The fixed adapters offer two additional advantages at the price of fixed seat position (if you would be the only driver). They offer the better position for the not-so-tall drivers (for which the regular ergonomics would work) – as the front of the seat base is lowered with additional 15mm vs. the version with the rails. Which allows a person with shorter legs to comfortably reach the pedals. Not to forget the second advantage – they are cheaper.

Need to fit with a helmet

Your height is estimated based on your body proportions between 175cm and 195cm (it is simply not possible to put larger upper body than 100cm). Using a helmet basically puts in you the category long torso (your torso + 50mm helmet) and this results in disproportionally shorter legs. The shorter legs require less seat rake or maximum seat base drop at the front.

The correct option here are the fixed adapters with optional fine drop adjustment kit. They allow for a maximum drop if really needed but the front of the seat base is lowered considerably as well.

The only correct option if rails are required is the 48mm drop version, again with optional fine adjustment kit and detached height adjustment rack.

Further measurement to create headroom is detaching the tensioners from the backrest. This provides additional side support as well for a car on the track.

Need to fit at all

You must be (depending on your body proportions) above 193 to be struggling to drive the ND at all – you won’t be able to sit up and you won’t see the road ahead or behind as of the rear-view mirror. And you will benefit from all other measures for really tall drivers listed below.

The correct option is the maximum drop in this case. I would recommend the rails over the fixed adapters as of the additional drop in furthest rear most position, the ease of installation and the adjustability as you might find further forward position of the base allows more headroom, as the body is leaning/stretching back at longer distance.

You definitely need to check the following section with topics and solutions concerning you.

Additional Challenges (and solutions) for tallest of tall

Backrest

As the above calculations revealed, the backrest is set up for the average between the smallest woman and the largest man, which sets is at best 50-75mm below the expected position for a tall man.

Luckily the ND/124 Spider seats allow relatively easy adjustment of this issue and we are providing the needed kit for this.

Headroom

The obvious and first impression. It simply is dangerous to attempt to drive a car without seeing the road.

I am sharing here all my tricks as the NDs are my daily drivers and each morning I am driving my little daughter to school. Mornings are bad, as the body of a tall man is additional 1-1.5cm taller and this is noticeable.

Considering the cabin space in an ND I consider myself marginal in terms of height due to my body proportions.

The “me”:

With a height of 196-197cm (depending on when during the day it is measured) I have higher upper body or nearly 100cm.  As unfortunate for me this is, if I had proportional legs, I would be measuring exactly 2 meters. Which makes me believe the below data is helpful for drivers up to 2 meters.

The “she (car)”:

They are both soft tops - 2019 Signature Edition and 2020 100^th anniversary with leather seats.

When standard there is approximately 92cm headroom. In the lowest possible position, we have 98cm.

The maths:

Even with the seat lowered I am missing at least 2cm to fit it, comfort is a different subject. It feels like more.

The solution(s):

There is combination of solutions and even after I cannot confirm there is a “comfortable fit”. It is not a proper fit and it is not comfortable.

1. The backrest is raised, so that the side supports are raised higher and proportional to my body. There is dedicated product for it on my website.

2. The side supports are defoamed from the inside, as my back is proportionally large (although I am by no means big, just rather tall).

3. The tensioner at the back is detached, means the body now leans deep between the side supports. This is a crucial moment, as I will explain below.

The combination of these actions is allowing the upper body to lean back (curve) and thus “win” approximately 2-3cm in height towards a upper straight position, as forced by the OEM backrest. It is a must for a tall person but would help if you are racing with helmet too. The side supports are nice add on feature.

As the above measures are still not allowing it for me to fit in the car or see the road, there is additional one, which does it for me, especially in the mornings. Common understanding and sense say – the seat should be far at the back for a tall person. I would like to have it far at the back, but this reduced my headroom. What I mean is, that with the seat positioned three clicks forward, the backrest is angled and allows the following:

- Being more inclined /diagonal/, the upper body is provided more distance to the roof, than the straight up shortest distance.

- The incline allows me to fit with my head further back, helping both headroom and road visibility (because of the rear-view mirror)

- Because of the better stretched upper body the ergonomics of the raised backseat work noticeably better in terms of support and comfort. I feel planted and held withing the seat.

The photos below show the position I am preferring (but also able to only use)

Legroom

If the body is having longer legs the above measures should be reversed. The seat can get its full back travel and the legs, being bend up, can be supported in their comfortable position with the base height adjustment.

Front View

The front view is the main issue and danger when driving the car. Approximately a third of the front few is available due to the combination of low roof and (huge) rear view mirror.

Going lower in the seat partly helps with the front sight but to see the road a unnatural bend (forward or side) position is adapted from a very tall driver as the rear view mirror is certainly preventing seeing traffic lights, road signs or people on the street.

There is a belief that the mirror can be lifted up with both joints to resolve this, but this is far from reality as the limitations will be pointed in the next topic. It is an obstruction in its upper most position.

The solutions here?

Smaller mirror would make a good change. Or relocating the rear view mirror into a screen with rearview camera would work best – providing the full front sight unobstructed.

Rear View

A picture says more than thousand words – 58mm adapters (lowest possible), rearview mirror in the upper most position.

The Summary:

- This is the best you would get from the front view

- The rear view is impossible if you don’t tilt your head down. Simply because with the mirror up most and the head so high the visibility through the lower rear windows is approximately 4 meters behind the car at most.

The Reality:

- You need to tilt your head down to see the rear traffic. Or to see road signs ahead.

The Verdict:

- The rear view mirror is having inappropriate dimensions. Or position.

The Solution:

- I have now installed rearview camera and screen and removed completely the rear view mirror.

Pedals

There is no major issue, compared to other cars with the pedals on LHD cars. But there is general fault with RHD cars to the point you could get blisters driving in short distance only.

The following gives a 3D overview of the situation in a RHD car:

https://scaniverse.com/scan/aylcje6hi7dq4ayr

And the following picture the situation with the depressed clutch pedal:

The collision occurs each time the clutch is depressed with the steering rod and the pedal, which is rather straight and not deep to allow space for the toe. The shoe size is only 46 EU (or UK 11).

This might not be much of an issue for many if the legs are not this limited as with a tall person. I have only one possible position to fit and it is defined between the steering wheel (as my knee is interfering with it) and the pedal travel.

Now, this is still a project or better said a *struggle* in progress for me. The options are pretty limited and again the combination of many small steps is causing a positive change but not a solution.

The measures that provide ease are:

- Removing the car mats – this brings the shoe lower and further away front he collusion with the rod

- Using shoes with lower heels – again – brings the foot lower and further away from the steering rod

- Raising the steering wheel as high as possible to allow (unnatural) leg travel upwards, instead of backwards

- I would assume smaller steering wheel might help as well

Each of them wont change much but the combination of all makes it … less painful let’s say.

Additional information

Conversion table mm/cm to inches

https://www.thecalculatorsite.com/conversions/common/cm-to-feet-inches.php