Going for a wheel in excess of 3% of original diameter

[Izza]

Hi Folks,

I own a Vitz 2017 1000cc (yeah the boring KSP130). Wanted to upsize the wheels from stock 14 to 15. Hence, went searching for a good alloy wheel set, and managed to find a set (~ 5kg each). However, was hit by a different challenge. The recommended wheel size to upsize is 175/60 R15, which has a 0.9% increase in the overall diameter. Unfortunately this size is quite rare in SL, and I was not able to find a set till now.

However, 175/65 R15 is widely available in many brands. But, the change of the overall diameter is 3.9% which exceeds the recommended tolerance level which is 3% as I read. I know upsizing will effect fuel figures, that I do not mind a lot. Also the speedometer and odo meter reading glitches are not huge concerns for me. However, being a Vitz 1.0L the acceleration is pretty average now as well, therefore putting additional weight and slowing down the acceleration further is something I would like to avoid if possible.

Couple of questions:
1. If I manage to find a lightweight set of 175/65 R15 tires with low rolling resistance, where the combination of the wheel + tyre weight is approximately similar to what is there currently, would it stil feel slow in acceleration? Anyone who has prior experience?

2. Apart from the fuel consumption, speedometer + odometer readings, any other potential issues which can arise from going above 3% of original diameter?

Thank you.

[iRage]

1.  Although the weight is equal, the rolling diameter is larger so the engine does need to work a little bit harder to make the car move 100m at X speed than the original wheel size. Thus you will feel that the engine is working harder....that puny little 1.0L engine hates to do most is work more. Other than the rolling diameter, another thing that will have a huge impact is the width of the tire itself. you are looking at 6 to 6.5 inches of width as opposed to the 5.5 inches of the stock wheel (thus, more contact)...and will require more effort from the engine to move it.

2. If you go beyond the tolerances...you will notice that the steering will be slightly off or feel wierd...your suspension geometry will be off and even if you align it again, you will notice that components do tend to wear out more because they take on more impact, etc...at wierd angles and stuff. Now...if you are doing a proper suspension upgrade, etc...then you can ofcourse counter-act this...but then we are looking at huge investment in suspension. Also, you might also have turning radius issues...especially if the wheel is wide and large enough to rub.

My question is...WHY do you need 15inch wheels ? Is it for visual effect ? In which case areyou sure you cannot get that visual touch with proper and good 14" wheels ? and maybe even a slightly lower profile on 14s ? Secondly...why do you need a wider tire ? That can actually reduce your performance...can't you stick with 5.5 ? or even stick to like 6 ?

Edited December 27, 2021 by iRage

[Izza]

Thanks @iRage. A proper suspension upgrade was on my mind, but unfortunately need to give priority to some other personal commitments.

Reasons for 15 inch wheels - better visual effect + the increases stability with 175mm wide tires. Vitz actually has quite a reasonable sized wheel well, but the 14 inch stock tires look tiny in those. Lower profile with 14 would make it look even more smaller IMO. However, the stock width is 165mm (6.5"), not 5.5", and increasing to 175mm means the width would become 6.9". I did not think that would make a huge difference. And I don't think rubbing will be an issue here.

[iRage]

41 minutes ago, Izza said:

Thanks @iRage. A proper suspension upgrade was on my mind, but unfortunately need to give priority to some other personal commitments.

Reasons for 15 inch wheels - better visual effect + the increases stability with 175mm wide tires. Vitz actually has quite a reasonable sized wheel well, but the 14 inch stock tires look tiny in those. Lower profile with 14 would make it look even more smaller IMO. However, the stock width is 165mm (6.5"), not 5.5", and increasing to 175mm means the width would become 6.9". I did not think that would make a huge difference. And I don't think rubbing will be an issue here.

sorry but that is not correct....you need to understand that there is a difference in the width of the tire and the width of the wheel/rim (the difference in the width of the rim and the width of wheel will determine how much of a buldge you have n your wheel as the lip of th tire either seats inwards or outwards to seal against the lip of the rim). If the tire width is 165 that does not mean your contact patch is 165 as well....on a stock wheel with a stock wheel size, I would guess your contact patch is anything from 5.5 inches to 6 inches depending on how heavy your car is and the tire pressure. The 1L and low grade 1.3 came with a 14" 5J wheel, the 1.5 and high grade 1.3 came with a 15inch 5.5J and the RS with a 16inch 6J.

....for a 175 tire, the width of your wheel/rim would ideally have to be a 6J or on average be a 5.5J. So if you are going for a 6J, that is a huge jump in width from the stock 5J. Which in turn does effect your contact patch significantly (for argument sake lets say you go from 6 to 6.5 inch contact)...this is where that fuel efficiency goes towards as now the engine has to work harder, which the 1L does not like to do..but this is not a concern to you, but you might feel a drop in peppyness (or an increase in laziness).

Stability....do not assume that wider wheel size means greater stability. All it does is give a wider contact patch which will put down power better (less slippage when accelerating). With a 1L Vitz, that really does not make that much of a difference. Wider wheel actualy will make the car feel less stable for most drivers because, wider contact patch means the wheels catch on to dips and humps on the road more. So the car tries to run all over the place as the front wheels get steered left and right with the humps and the dips. So wider wheels does not mean increased stability. I have 7J wheels on my Corolla and 8.5J wheels on th Yaris. The Corolla is a handful to drive as I have to make a lot of steering correction as I drive along, which with a standard hydraulic PS is hard as you even feel the increased resistance on the wheels. The Yaris has the same effect but is easier with EPS.

You have not mentioned the most important factor here...the size of the rims; which is why I assumed that you would be going for a wider wheel (I made a mistake thinking that 175 would require a 6.5 when in reality it can be done with a 6 or 5.5. You can have rubbing as it all depends on the width of the rim and the off/in set of the rims. So even if you put 175s and then incease the size to a 15 whilst sticking to the stock offset of 14 rims, you might actually cause rubbing. 

You should also know that what ever wheel and size you go for will eventually affect what you do with your suspension (how much you can lower, strut sizes, etc...).

Now..looking at the catalogs...a 15" 5.5J wheel on a 175/65R15s is the stock size for the 1.3L. Considering that there hardly is any difference in suspension between the 1L and 1.3L; I suppose it would be safe to assume there would not be much of an impact in terms of suspension geometry. What you're left with now is potentially less performance and getting the right offset of wheels so it minimizes rubbing. My instinct is that the Vitz probably has a +39 or +40 offset (because that is what most stock Toyotas with a 5.5J wheel has).

For reference..the following Vitz (1.3) has 15x6J with a +35 offset...so a +39 offset would men the wheel goes 4mm INTO the wheel housing. 

 

[Izza]

Thanks @iRage for the detailed explanation, and pardon my ignorance. Yes, it makes sense. 

My current setup is not actually the stock steelies, they are some Chinese alloys 14" 6J, with +40 offset. And it carries the stock size tires 165/70 R14. The setup weights at around 12 kg (6kg for the wheel and 6kg for the inflated tyre) I measured the contact patch and with 32 PSI it seems it is slightly less than 6".  With this setup, the acceleration/peppiness is ok-ish. And I have driven the same Vitz 1.0L cars with the stock steel wheel setup, and did not note a noticeable drop in acceleration because I have 6" wheels. 

With the new setup alloys are 15" 6.5J with +35 offset. I came across some wheels with 15" 6J but they were actually heavier than the 15" 6.5J ones I have now.

So, I assume it will for sure have a hit on fuel economy + reduction in the peppiness. Plus, the increased width might not be always increase stability. Hmm. Wonder if I should just live with the current setup then.

[iRage]

3 hours ago, Izza said:

With the new setup alloys are 15" 6.5J with +35 offset

Okay....so you are increasing the width of the wheel and pushing out....are you sure there is clearance on the wheel arch side ?

I would be very weary about those Chiense rims. The ones in SL are cheap ones that are not the safest. I am not saying Chinese wheels are bad..there are penty in Japan too..but the ones in SL are the cheaper ones because the SL market is very price sensitive. So think twice about keeping them.

If you want to improve stability,..you should look in to the suspension setup. I am not saying wheels do not play a part....the larger contact patch gives better grip which helps the car to be stable...but it also has its downsides...so it is a composition of things.

Edited December 27, 2021 by iRage

[Izza]

1 hour ago, iRage said:

Okay....so you are increasing the width of the wheel and pushing out....are you sure there is clearance on the wheel arch side ?

Yeah, wheel arch side is ok. I checked that.
 

1 hour ago, iRage said:

If you want to improve stability,..you should look in to the suspension setup. I am not saying wheels do not play a part....the larger contact patch gives better grip which helps the car to be stable...but it also has its downsides...so it is a composition of things.

That was on my mind actually, but unfortunately I can't do that straightaway. What I can do is the alloys + tires, and then maybe front upper and back lower strut bars to start with.

One clarification, even if I go for a proper suspension setup with coilovers/lowering springs, I assume the slowness in acceleration would still be there? Since the original cause of that is the added weight and higher rolling resistance.

[iRage]

19 hours ago, Izza said:

One clarification, even if I go for a proper suspension setup with coilovers/lowering springs, I assume the slowness in acceleration would still be there? Since the original cause of that is the added weight and higher rolling resistance.

sadly yes..the 1l engine is not made for any kind of power...reduction of weight will help but I doubt you will get much weight reduction through suspension change. The wheel change reduction might be a bit too small. If you want to squeez  little bit more power out of that engine, you might want to look at the exhaust and air intake systems and maybe see if anything can be done with the ECU

[Izza]

Thanks @iRage. As always, your inputs are much appreciated. I will go ahead with the upsizing and see what happens. If anything is compromised too drastically, specially the acceleration, will think of reverting to 14" again.

[iRage]

On 12/29/2021 at 1:43 PM, Izza said:

Thanks @iRage. As always, your inputs are much appreciated. I will go ahead with the upsizing and see what happens. If anything is compromised too drastically, specially the acceleration, will think of reverting to 14" again.

I doubt you will feel much of a change under normal driving....at most you will feel a bit of a change in the ride (loer profile means lesser absorbtion of bumps by the tires) and a slightly wierd sensation/difference in the steering

[Izza]

I went ahead and switched to 175/65 R15 and have no complaints so far. No noticeable reduction in acceleration and the car seems much more planted and stable.