22" wheel

[Tony]

Since the desire for bigger and bigger wheels is rife, what consequences do you feel could evolve? Geometry is obvious but for now lets concentrate on another area.

 

If you fitted 22" wheels......like this one...

 

 

And the Technician was holding it hostage do you think the brake discs need to be bigger or smaller?

 

Clue: Rolling radius/ Kinetic resistance.... for debate.

[Adie]

I think the technician is looking very friendly with that large wheel. He didn't take it home with him did he

[discount tyres dan]

as far as the technical side of things go id say smaller, because the friction level of the huge tyre will be a lot higher so braking is aided there also if the rolling radius is longer then the wheel will be spinning slower than a small one at the same speed.

 

but

 

how silly would a smaller disc look behind that rim, so in truth id go for a larger disc.

[Tony]

I think the technician is looking very friendly with that large wheel. He didn't take it home with him did he

He insisted.... wims thoughts go out for the tyre familiy and all the barrow tyre children..

 

as far as the technical side of things go id say smaller, because the friction level of the huge tyre will be a lot higher so braking is aided there also if the rolling radius is longer then the wheel will be spinning slower than a small one at the same speed.

 

but

 

how silly would a smaller disc look behind that rim, so in truth id go for a larger disc.

 

So a mechanical conundum.. The bigger wheel has less RPM denying friction at the disc, this suggests a smaller disc but the brake area would be less... If then you decide a bigger disc the friction would be less... so would the compressive force at the pads resolve the problem?

[discount tyres dan]

so what if you went for just an up-rated calliper.. maybe brembo and a set of drilled, grooved disks and some red stuff pads

[Tony]

so what if you went for just an up-rated calliper.. maybe brembo and a set of drilled, grooved disks and some red stuff pads

Well indeed Dan.. wise words... size is an issue i think but lets team wim add their thoughts.

[Mark H.]

As the wheel weighs more will that effect it?

 

I take it that it will effect handling more as well as the suspension it is connected to (which has to control the extra weight on the suspension.)

 

On another note the red stuff pads i had installed on the front do slow the car down somewhat quick when asked to, especially if they are slightly warm.

[Tony]

As the wheel weighs more will that effect it?

 

I take it that it will effect handling more as well as the suspension it is connected to (which has to control the extra weight on the suspension.)

 

On another note the red stuff pads i had installed on the front do slow the car down somewhat quick when asked to, especially if they are slightly warm.

The wheels weight effects the Geometry big time (Another Topic).... Wheel size and rate of disc rotation -ve- temperature is an issue.... Do you feel a large disc with red pads would suit this conversion.

[Mark H.]

You say about the consequences and radius difference, however you'd have to give us an idea of what the previous wheel was. Ie if it was a highish profile 20" it could be a lowish 22" and be similar distance per revoloution, but you are kind of hinting at there being a difference.

 

However, perhaps a step up in the grade of the pad so it is a bit more aggressive at slowing down would be a good idea if it does indeed do less revolutions to cover the distance.

[Tony]

You say about the consequences and radius difference, however you'd have to give us an idea of what the previous wheel was. Ie if it was a highish profile 20" it could be a lowish 22" and be similar distance per revoloution, but you are kind of hinting at there being a difference.

 

However, perhaps a step up in the grade of the pad so it is a bit more aggressive at slowing down would be a good idea.

Good question sorry..... OE size was 19" tyre aspect 255/55 replacement 22" aspect 285/35, oddly enough the RPM does not correlate as a conversion

[Mark H.]

Using an online tyre size calculator the 22" rims + tyres are 5mm smaller rolling radius than OE if its correct.

 

So the new tyres will read slower than the OE ones by a minute amount if i am correct? Thus the rolling radius shouldn't really change much, but would sidewall flex make a difference, more on the OE perhaps .

[Tony]

Using an online tyre size calculator the 22" rims + tyres are 5mm smaller rolling radius than OE if its correct.

 

So the new tyres will read slower than the OE ones by a minute amount if i am correct? Thus the rolling radius shouldn't really change much, but would sidewall flex make a difference, more on the OE perhaps .

You are very attentive Now established the RPM is within 5% where is this conversion disparity... Weight or Rolling resistance due to tyre width, remember the brakes need to apply a Kinetic % through the tyre contact patch, does this suggest the OE disc is suitable since the tyres friction covers a greater area...... or not? remember inertia also owns a valuable consideration.

[Mark H.]

Under weight transfer, eg heavy braking, there would be more weight moved to the front on most cars, so with the OE wheels would the sidewall flex allow more tread to touch the floor?

 

Does 100% tread touch the road surface in general or is it more of 85% of the tread in the middle touches the floor in general. As the 22" tyres probably wont flex much would they have less tread touch the floor under braking? Also different tyres treads seem to have different edges so would some have more grip under braking than others?

 

I dont really know how having heavier rims effects the cars handling, i just recalled that it has more of an effect on the suspension as it has more weight to control with movement up and down so probably new adjustable dampers for rebound etc and a matched set of springs? to get better control of the rim.

 

I dont know if increased wheel and tyre weight requires more braking force from the disk and pads to stop the car over lighter rim and tyre combinations though . Is there more torque (twisting force) creaated from the heavier rim + tyre combo that the brakes have to counter act as well?

[Tony]

Under weight transfer, eg heavy braking, there would be more weight moved to the front on most cars, so with the OE wheels would the sidewall flex allow more tread to touch the floor?

 

Does 100% tread touch the road surface in general or is it more of 85% of the tread in the middle touches the floor in general. As the 22" tyres probably wont flex much would they have less tread touch the floor under braking? Also different tyres treads seem to have different edges so would some have more grip under braking than others?

 

I dont really know how having heavier rims effects the cars handling, i just recalled that it has more of an effect on the suspension as it has more weight to control with movement up and down so probably new adjustable dampers for rebound etc and a matched set of springs? to get better control of the rim.

 

I dont know if increased wheel and tyre weight requires more braking force from the disk and pads to stop the car over lighter rim and tyre combinations though . Is there more torque (twisting force) creaated from the heavier rim + tyre combo that the brakes have to counter act as well?

 

You are correct in most areas here.... Indeed the weight transfer deserves notability and the tyre will have 100% contact but with lateral forces applied, this is how we gain control of the car.

 

Geometry side of things we will skip for now since this is another topic. (To come)

 

Kinetic/inertia is a real balance, the extra wheel diameter reduces RPM so the disc has less opportunity to apply temperature enabling friction.... So it would be wise to assume a larger disc contact area with the ability to stay cool (vented/slotted) combined with low metallic (red) pads and a much higher caliper compression ratio.....How does this read to you?

[Mark H.]

But as the tyres are ultimately the same rolling radius (5mm different), surely the extra wheel diameter makes no difference and the wheels will still spin at the same rate? Bar any difference the tyres flexing makes.

[Tony]

But as the tyres are ultimately the same rolling radius (5mm different), surely the extra wheel diameter makes no difference and the wheels will still spin at the same rate? Bar any difference the tyres flexing makes.

I understand what you are saying... maybe the comparison in wheel size was to close to display my thoughts...

 

I'll try again.... If a car had 14" wheels then had 16" wheels fitted with an incorrect aspect ratio (this happens often) then the discs would be rotating at a lower level than expected to permit the friction to react to the inertia...... but i may be wrong

[star a]

surely the difference would be negligable either way, now if, theoretically, the wheel was changed from a 10" (mini for example) to 20 inch rims (ok bear with me) keeping the original aspect ratio, then the discs would theoretically rotate at more or less half the original rate at any given road speed. so for stock pads this would surely be a good thing as the disc/pads will run cooler, but for race pads, it could possibly be a bad thing as perhaps the discs will not reach optimal temp.

[Mark H.]

Hmm, so say the new wheel combo went 20% more per revoloution than the older one.

 

You could say that the brake would need more stopping power to haul the car to a stop.

 

However if it had the power, then would it stop quicker? as the car would be going 20% faster for the same brake disk rpm's, so if slowing down then the car would be able to knock off an extra 6th of its distance per brake disk revoloution compared to with the smaller wheels would it not?

[Tony]

surely the difference would be negligable either way, now if, theoretically, the wheel was changed from a 10" (mini for example) to 20 inch rims (ok bear with me) keeping the original aspect ratio, then the discs would theoretically rotate at more or less half the original rate at any given road speed. so for stock pads this would surely be a good thing as the disc/pads will run cooler, but for race pads, it could possibly be a bad thing as perhaps the discs will not reach optimal temp.

The OE aspect ratio is the debate here..... what if the 10" to 20" did not respect the aspect conversion?.... This will not be without consequence toward the kinetic calculations at the brakes don't you think.

[jon]

Jumping in a little late which is a shame as I love brake topics.

 

People greatly under-estimate the inertia of a spinning wheel and just how much harder the engine / brakes need to work to start and stop the spining force when you move the weight further out.

 

As an example, an american civic evolution i was following lost 4bhp on the rolling road due to new brakes.

 

The thing that threw the developers was the fact the new discs were actually LIGHTER OVERALL as they had an alloy hub, it's just because they were bigger there was more weight further out. Took them ages to work out where that 4bhp had gone lol.

 

To answer the original question, the car WILL be harder to stop so some sort of uprated brakes will be a smart idea. However for such a small change in this example uprated pads will probably do the job.

 

IMHO ebc redstuff is a poor road choice as they feel quite wooden at low temperatures and do not inspire confidence. Something like a mintex fast road pad is more appropriate, again, just from my personal experience.

[Tony]

Jumping in a little late which is a shame as I love brake topics.

 

People greatly under-estimate the inertia of a spinning wheel and just how much harder the engine / brakes need to work to start and stop the spining force when you move the weight further out.

 

As an example, an american civic evolution i was following lost 4bhp on the rolling road due to new brakes.

 

The thing that threw the developers was the fact the new discs were actually LIGHTER OVERALL as they had an alloy hub, it's just because they were bigger there was more weight further out. Took them ages to work out where that 4bhp had gone lol.

 

To answer the original question, the car WILL be harder to stop so some sort of uprated brakes will be a smart idea. However for such a small change in this example uprated pads will probably do the job.

 

IMHO ebc redstuff is a poor road choice as they feel quite wooden at low temperatures and do not inspire confidence. Something like a mintex fast road pad is more appropriate, again, just from my personal experience.

Taking the fact that wim is a collaboration of knowledge and you have had much experience in a topic that interests you, how do you feel about writing an article covering all your thoughts?

[jon]

Unfortunately my writing skills are on par with my pole vault skills, poor at best

[Tony]

Unfortunately my writing skills are on par with my pole vault skills, poor at best

You underestimate your self mate.....wim will develop this our poles are very 'grippy' when needed

[jon]

One last nugget of information before i go to bed, this is from a engineer at triumph:

 

oh, talking to my boss today about wheel weights, he reckons as a rough guide to saving weight is due to referred inertia if i save 10kg a wheel in dead weight its like saving 20kg in total weight of the car in equivalent performance