A Spare Tire That Fits Under the Load Floor

[kjechel]

Here is a Spare Tire & Wheel Combo that fits in the spare tire well without raising the load floor.
NOTE: THIS WILL FIT ON THE REAR ONLY, IT DOES NOT CLEAR THE FRONT BRAKE CALIPER
Here's what it looks like in the spare tire well

and on the car

The wheel is BMW P/N 36116758778. It is 17x4 ET18.

I purchased mine from www.ecstuning.com/b-genuine-bmw-parts/black-steel-emergency-wheel/36116758778. It bolts right up and clears the rear caliper by about 3/8". It's meant to be used with conical seat lug nuts, but our spherical seat OEM lug nuts will work with some limitations (see Post #29 for more about Lug Nuts to use with this wheel).

The centerbore is 72.6 mm, so you may want to use a centering ring. The wheel itself weighs 16.4 pounds

The tire is Continental sContact 115/95R17, 4.4" wide x 25.6" diameter. I purchased mine from Tire Rack. It looks very small, but the diameter matches our OEM tires well and it's rated for 1521 pounds load capacity, which is plenty adequate for the rear of the ITS. This size was used as the spare for some Porsche Boxsters. The wheel & tire together weigh 28.2 pounds.

The unique thing about this wheel & tire combo is that it is only 5 inches high when laid down flat on the floor. That's exactly the right height to support the load floor without any foam blocks. A wider tire will bulge out more (past the rim edges) which may raise the load floor a bit.

I found that to position the tire so its surface is parallel to the load floor, it needs to be supported at the front edge by a 1.5" thick spacer (I'm simulating this with a wooden 2x2).

See post #3 below for how I secured it to the floor of the spare tire well.

See post #9 below for a scissor jack and lug wrench that fits under the wheel & tire.

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[vexingv]

Great work putting that together. I was considering poaching something from BMW as well as up until recently most of their wheels used the 5x120 bolt pattern. I guess if I wanted to I could use an 18 inch wheel from my current BMW, but then it doesn't fit under the floor and neither does the Modern Spare kit.

 

[kjechel]

How the Spare Tire is secured in the Spare Tire Well

Materials Required:

Spare Tire Mounting Screw [www.amazon.com/dp/B0CL91ZPLG]
Mounting Plate (pkg of 4, only 1 needed) [www.amazon.com/dp/B0BR7WGXB4]
Mending Plate 4.7"x 3" [www.national-hardware.com/p/1184-mending-plates] or similar steel plate
Qty 4 6 x 20mm Hex Head Flange Screws (can substitute Hex Head Bolts + Washers)
Qty 4 6mm Serrated Flange Nuts (can substitute standard Nuts + Washers)
Qty 1 8 x 25mm Button Head Cap Screw
Qty 2 8mm Nuts
Qty 1 8 x 24mm Coupling Nut (longer is OK)
Loctite, Red preferred, Blue is OK
Silicone Sealant

Assembly Steps:
Unbolt Heat Shield above Mufflers (5 bolts) and move aside - no need to completely remove.
Drill out holes in Mounting Plate to .250" diameter (to fit 6mm Screws).
Drill .250" diameter holes in Mending Plate to match Mounting Plate (per photo above).
Assemble onto Mounting Plate: 8mm Button Head Cap Screw, 2 8mm Nuts & 8mm Coupling Nut in that order (as shown below) using Loctite*.

Drill .250" diameter holes in floor of spare tire well to match Mounting Plate.

Apply generous amounts of Silicone Sealant around holes on both the top and underside of the spare tire well floor.
Attach Mounting Plate to floor of spare tire well using 6 x 20mm Hex Head Flange Screws with Mending Plate and 6mm Nuts on underside of spare tire well.

*Notes:
1. The two 8mm Nuts raise the Coupling Nut up to allow proper engagement of the Spare Tire Mounting Screw.
2. If you use a longer Coupling Nut you may want to eliminate one or both 8mm Nuts below it.
3. If you use a wider Spare Tire, you may need raise the Coupler Nut upwards and use a longer 8mm Button Head Cap Screw.
4. Loctite (Thread Locker) is recommended to prevent the Coupling Nut from unscrewing from the Mounting Plate when the Spare Tire Mounting Screw is unscrewed to remove the Spare Tire.

 

[vexingv]

Again, great write up and instructions. Curious, without any mounting setup like what you've done, would the spare wheel/tire shift around too much just by itself? Personally, I'd be reluctant to drill through the bottom of the floor...

(Maybe add blankets or some other types of padding/cushion)

 

[kjechel]

Again, great write up and instructions. Curious, without any mounting setup like what you've done, would the spare wheel/tire shift around too much just by itself? Personally, I'd be reluctant to drill through the bottom of the floor...

(Maybe add blankets or some other types of padding/cushion)

Side-to-side and front-to-back I think it would be fine without a mount, however in the event of a rollover I'd prefer to have it secured.

 

[Frenzal]

Nice solution!

Better than the RL spare tire I run as the tray will be flat!

But I would not drill my floor like that here in the rust belt... Would still do what I did to secure the RL spare!

 

[Nickp15]

Wow. Amazing write up. Definitely going with this solution. Probably pad as well like Frenzal advised.

 

[crepr12]

Did I miss were you keep your jack/lug nuts wrench?

 

[kjechel]

Did I miss were you keep your jack/lug nuts wrench?

You beat me to the punch. I received this scissor jack today from www.amazon.com/dp/B0CDVY62DQ. It seems to work well and fits under the wheel & tire in the orientation shown.

The telescoping lug wrench from www.amazon.com/dp/B00RF95TDC works well and fits as shown because of the wooden spacer (described previously to orient wheel & tire parallel to the load floor). Towels or padding may be used as required to suppress rattles.

 

[Padre Dave]

I purchased new wheels and tires for my car and simply kept one of them as my spare, full size if you will. I bought a jack and wrench from Amazon and have them sitting under the tire. I don’t have it secured, and have never noticed it producing sound from being in there. I hear you about the roll over safety but since it is only my wife and me, it is less of a concern….I try really hard to temper my driving style to keep the wheels on the ground and not scratch up the top with a rollover <snark>.

 

[neuronbob]

Thank you for this. I live in Cleveland (ie Rust Belt), so drilling the floor is a definite “no”, but otherwise, having a spare for road trips is worth the purchase.

 

[Blek le Roc]

Interesting.

So, how many inches of trunk height does this save compared to the Modern Spare wheel + tire option that will fit the front wheel?

 

[kjechel]

Interesting.

So, how many inches of trunk height does this save compared to the Modern Spare wheel + tire option that will fit the front wheel?

I asked the folks at Modern Spare what the overall width of their tire & wheel package is and they told me "roughly 6 inches". I don't know the precision or accuracy of that measurement, but assuming it's exactly 6 inches, that's one inch thicker than the wheel and tire package that I have proposed, so I'd expect the load floor to be raised one inch. It would be better if someone who has the Modern Spare wheel & tire could tell us their results.

 

[Blek le Roc]

Thanks for the info.

I'm trying to decide if losing the space is worth not having to play musical chairs with the wheels if I get a flat in front.

 

[kjechel]

Thanks for the info.

I'm trying to decide if losing the space is worth not having to play musical chairs with the wheels if I get a flat in front.

Some may not agree, but I don't think it's a good idea to drive with a different diameter tire (Modern Spare is 24.9" diameter) in the front. It would be like constantly driving in a 338 foot radius circle, so it wears on the LSD.

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