Spare tyre option

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Mick787

Well-known member
Joined
Jul 11, 2018
Messages
56
Our 94AH REX has the wider tyres on the rear. 155/70 R19 on the front 175/60R19 on the rear. I have noticed uneven tyre wear and as a result, with the winter weather approaching here in the UK, it would be wise to fit new rubber. There has been a delay in obtaining a set of 4, the 175's taking the longer time to obtain. What is concerning me is the time taken to obtain tyres, it appears that these are not something you can drive in and have fitted while you wait. I was present when the dealer rang Bridgestone and could hear that this was not someone messing around, it does appear they are harder to get hold of than 'normal' tyres. Thankfully we have not had an incident such as a sudden tyre deflation, it is wear and uneven at that, but at the end of the day, the reality is that the car could be off the road for a while if tyres aren't available. I fully accept that all tyre sizes cant be stocked by everyone.
So, holding a spare rim seems to be the most logical solution, but the rims are different sizes. I very much doubt the rear rim will fit on the front. Can a front rim be safely fitted on the rear or is it the case that a front rim and a rear rim have to be kept in storage?
 
The front tires/wheels do fit on the rear of all i3's. If you decided to get winter tires, the only ones available are the smaller, front size in a square (same all around) setup. THere is a second supplier for winter tires - Nokian, but not for the summer or all-season tires.
 
The Nokian 155 snow tires will absolutely fit on the 5-1/2" wide rear wheels, even though they are not supposed to. I ordered mine from a large tire company and had a local tire shop mount them. I told them the wheels were staggered and that the tires would fit both front and rear. Over a thousand miles on them with some hard cornering and without any problems. Others have done the same.
 
When you put the same tire on a wider wheel, assuming it is within the allowable range, it will fit, however, that does change the OD of the whole assembly. That will change your speedometer and odometer and maybe confuse the ABS logic. THe difference in this case won't be much, but it's recommended to keep all of the wheels' diameters within a max of 4%.
 
JohnKelly said:
The difference in diameter is only .38 of an inch... a very small difference.
Click to expand...

That's about 1.4%...close to the maximum allowable with an all-wheel drive vehicle, but not as big a deal with one axle driving things, which would cut down your allowable 'real' mileage under warranty, give you a bit more torque (not great when the roads are slippery), but probably close enough, most people would never notice. That also means your speedometer will be more conservative...saying you're going that much faster than you really are (there's about a 2mph bias built in, fixed, but the added variance from this is linear with speed, not fixed).

There's a reason why the aspect ratio varies on the OEM tires when the wheel size changes.
 
Are you sure the speedometer reading is taken from the rear wheels? I drive by a speed reader regularly and I always&still read 2mph above what it reads.
 
Not positive of anything, but, the difference in OD is about 1.4%, which, at say 30mph, may not be enough to show up. The 2mph buffer is a constant above the design speed, however that's detected. But, at 30mph, 2mph is 6.67%, so 1.4 isn't that big of a difference (less than 0.5mph). The vehicle is not using GPS for determining speed or odometer, though. That leaves the gear ratio and the tire size. I'm sure it's not a mechanically driven cable, but the detector must get its reference from some rotating device, and changing the OD of the tire/wheel assembly will throw off that actual correlation.
 
Thanks for all the responses. I still don't know whether to buy just one front tyre and keep it as a spare for any position on the car or whether to buy a rear axle specific rim as well. They seem to be harder to get hold of.
 
I putted the Rials X10 with all seasons on my car, I kept the pizza 5" as a spare, I put the 3 others for sale as a spare but not enough i3's out there
to sell them in the near future :) .
 
Isn't it great getting all of this information from others!

While I'm on a roll I may as well post the question, if the diameter is such a small difference between the 155's and the 175's on the car, why were they fitted by BMW in the first place? Is it because of the surface contact patch being that bit bigger to cope with the increased output potential of the bigger battery better on full throttle?

The next questions are........ is there is any effect on the handling if 155's are fitted all round on the 94ah model? Do they tend to step out if 155's are fitted? Was it an optional fit to have the staggered rims chosen on buying the car by its first owner?
 
Mick787 said:
The next questions are........ is there is any effect on the handling if 155's are fitted all round on the 94ah model? Do they tend to step out if 155's are fitted? Was it an optional fit to have the staggered rims chosen on buying the car by its first owner?
Click to expand...

I do not notice ant difference between the 155 all around instead of my pizza wheels that where 175 on the back. I'm a aggressive driver so I
like a bit of drifting but I must say at this moment there was no drifting involved :lol:
 
Mick787 said:
While I'm on a roll I may as well post the question, if the diameter is such a small difference between the 155's and the 175's on the car, why were they fitted by BMW in the first place?
Click to expand...
The 175 tires have a higher load limit. The rear tires support more weight than the front tires, especially the REx model. The rear tires support more weight when an i3 is carrying 4 occupants and some cargo in the rear.

Mick787 said:
Is it because of the surface contact patch being that bit bigger to cope with the increased output potential of the bigger battery better on full throttle?
Click to expand...
The contact patch area would be identical for both 155 and 175 tires because the contact patch area depends only on the tire's inflation pressure and the load that it's supporting. The shape of the contact patch of a 175 tire would be 20 mm wider and slightly shorter around the circumference of the tire (i.e., the tire would not flatten quite as much) compared with a 155 tire.

Mick787 said:
The next questions are........ is there is any effect on the handling if 155's are fitted all round on the 94ah model? Do they tend to step out if 155's are fitted? Was it an optional fit to have the staggered rims chosen on buying the car by its first owner?
Click to expand...
The 94 Ah model is heavier than the original 60 Ah model primarily because of a slightly heavier battery pack. Probably because the REx model is ~10% heavier than the BEV model, 155 tires were not offered by BMW on the rear. Early BEV models were offered with 155 tires on the lightest wheel, the 427 "pizza slice" wheel (number 1 in the image below). I don't know whether 155 tires were offered by BMW on the rear of 94 Ah BEV's. Almost all i3's have staggered wheels and tires as standard regardless of the wheel option.

wheel+options.jpg
 
The trim package chosen somewhat dictated which wheels were standard. The least expensive options have (or had) square, 5" wheels. From what I remember, all REx versions came standard with the staggered set, but the super economy trim might still have come with a square arrangement...didn't follow it closely after I bought mine.

All of the winter tires available are square at a design for the 5" wheel (and 19" diameter). A narrower winter tire tends to cut through things better rather than float on deeper snow and it's easier to push through slush. They don't support 20" winter tires, either, as you typically want a bit more sidewall compliance when you can't always see the actual road surface.
 
So if a user was to fit 155 tyres all round on a 94aH car, as long as the tyre load rating matches or exceeds that of the 175 Ecopia, all should be fine. I note the Blizzak winter tyres that seem to be available have a load rating 84 and the Ecopia 175's have 86, a difference of 30kg each tyre in their maximum capacity.

Do we have any 94aH REX users here who have 19" 155's fitted all round?
 
Last I looked, the OEM BMW winter tires sets are ALL the 155, square sizes. See what your owner's manual says, as it should list acceptable tire choices. Now, might the rear tires wear out a bit faster? Maybe. I rotate mine F->R each season to even that out.
 
Quote, ' See what your owner's manual says, as it should list acceptable tire choices.'.

No it doesn't.

Pages 202 to 213 inclusive only mentions the speed rating, it does mention only using snow chains on 155 size tyres. It loves referring the reader to getting information from a Service Partner of the manufacturer or another qualified service partner or specialist workshop. It also refers to pressures to the tyre sizes being stated on the label fitted to the door.

On page 205 it says 'Recommended makes of tyre' Certain makes of tyre are recommended by the manufacturer for each tyres size. It does not state which, or alternative sizes.

On the same page it says about interchanging front and Rear wheels and it is possible to swap the wheels over in pairs from one axle to the other. Additional information is available from...... you guessed it..... a Service Partner! BUT a wheel change from one axle to another is not permitted in vehicles with different tyre or rim DIMENSIONS on the front and rear axles.

Nothing is stated about tyres in the vehicle technical data section.
 
Update for any users who may be interested......
Having liaised with BMW UK they state that the 155's can be used all round on my car. (I gave them the VIN number so there could be no confusion.) BUT, having been asked specifically about the different load rating for the tyres on the rear axle to that of the front axle, they chose not to make further comment.
I have done a bit more research and it appears the Bridgestone Blizzak 155 LM500 winter tyre is available as an 'XL' option which has a higher load rating of 88. I guess for complete peace of mind, fitting those should leave any user well within tolerance. They are slightly more expensive than the normal Blizzak which has the lower load rating of 84. It appears the stock number for this Bridgestone tyre is 8332 and an internet search using that number will bring it up.
There is also the option of the Nokia WR Snowproof 155/70 R19Q which has a load rating of 88.
 

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