My Green Skoda Fabia (VW Polo) R3 CRTDI - finally smashed the 3L / 80MPG barrier

[California98Civic]

Originally Posted by GreenTDI

... Sealing those gaps will be easier than with the current rims. Just like cleaning! Looking forward to it! ...

You might experiment with leaving the inner third of the holes open to imitate wheel designs that sometimes perform about as well or better than fully covered wheels in wind tunnel tests. More brake cooling would be a side benefit, but the main purpose of retaining a small opening seems to be reducing wheel well ventilation drag. No guarantee it works better for you, and no real precise way to test whether it works better for your car, given our equipment. But if you like the look better...

[GreenTDI]

Originally Posted by California98Civic

You might experiment with leaving the inner third of the holes open to imitate wheel designs that sometimes perform about as well or better than fully covered wheels in wind tunnel tests. More brake cooling would be a side benefit, but the main purpose of retaining a small opening seems to be reducing wheel well ventilation drag. No guarantee it works better for you, and no real precise way to test whether it works better for your car, given our equipment. But if you like the look better...

I recently discovered that the brakes are mainly cooled by a guided airflow underneath the car. But that air has to leave the wheel somewhere. Now the wheels are closed with tape, so the airflow seems disrupted and has to go back in the wheel well and find another way out. See the picture below. It's difficult to test this.

[California98Civic]

Originally Posted by GreenTDI

I recently discovered that the brakes are mainly cooled by a guided airflow underneath the car. But that air has to leave the wheel somewhere. Now the wheels are closed with tape, so the airflow seems disrupted and has to go back in the wheel well and find another way out. See the picture below. It's difficult to test this.

Yes. That will be hard to test. You could try tufting, or perhaps the magnehelic gauge technique Julian has been promoting. Or just take your preference. My bet would be that either will be better than stock for total drag. I am sure you do much less aggressively braking than the designers assumed would be commonplace.

[ptitviet]

Hello GreenTDI! Since you are now trying to see very little differences, I think it would be interesting to do some coast down testing. It helps you to get experimental values of rolling and air resistance.

I have use the "old" method but with better tools and found SCd values accurate to 2% to the real value.

Can you show on your computer display the driving speed? Then by recording with a smartphone your screen during the coast down you can get very good accuracy.

[GreenTDI]

Originally Posted by California98Civic

I am sure you do much less aggressively braking than the designers assumed would be commonplace.

The tape came off from itself, so I removed it. The rims are open yet again. A lot of brake dust had already accumulated in the wheel during ~2200 miles, while I really don't brake much. It's a shame that brakes release so much harmful dust.

Outdoor temperatures reaching 30°C and more, I also removed the grill cover.

Had to do a 300 mile trip afterwards with the whole family (+300 lbs with luggage) and achieved a whopping 71,3 MPG or 3.3l/100 km, of which 80% highway, speeding between 60 and 70 mph ...

And that's after removing previous mods. Tire pressure remained @ 40 PSI. That and the warm weather may be the cause of that good result. Of course I haven't used the airco!

[GreenTDI]

Originally Posted by ptitviet

Hello GreenTDI! Since you are now trying to see very little differences, I think it would be interesting to do some coast down testing. It helps you to get experimental values of rolling and air resistance.

I have use the "old" method but with better tools and found SCd values accurate to 2% to the real value.

Can you show on your computer display the driving speed? Then by recording with a smartphone your screen during the coast down you can get very good accuracy.

That's a good and simple method indeed. I'm sure I will try this with my new wheels. I'm not taping my current rims again, it's not a sustainable solution.

[California98Civic]

Originally Posted by GreenTDI

The tape came off from itself, so I removed it. The rims are open yet again. A lot of brake dust had already accumulated in the wheel during ~2200 miles, while I really don't brake much. It's a shame that brakes release so much harmful dust.

Outdoor temperatures reaching 30°C and more, I also removed the grill cover.

Had to do a 300 mile trip afterwards with the whole family (+300 lbs with luggage) and achieved a whopping 71,3 MPG or 3.3l/100 km, of which 80% highway, speeding between 60 and 70 mph ...

And that's after removing previous mods. Tire pressure remained @ 40 PSI. That and the warm weather may be the cause of that good result. Of course I haven't used the airco!

That's so strange... in many thousands of miles, maybe 20,000 I never had any tape come off and not all that much brake dust. I lost all the tape on one wheel when I ran into a giant puddle that also wrecked my distrbutor and almost hydrolocked the engine.

Did you seal the tape in on the inside of the rim? I used two or three coats of clear plastidip. Protects the tape from heat, moisture, and dust... ultralight.

[GreenTDI]

Originally Posted by California98Civic

That's so strange... in many thousands of miles, maybe 20,000 I never had any tape come off and not all that much brake dust. I lost all the tape on one wheel when I ran into a giant puddle that also wrecked my distrbutor and almost hydrolocked the engine.

Did you seal the tape in on the inside of the rim? I used two or three coats of clear plastidip. Protects the tape from heat, moisture, and dust... ultralight.

Mine was taped very simple on the outside, one layer of ultra strong all-wheater tape. It came loose on several places, after rain, dirt and brake dust slowly began to undermine the tape. It gives the impression that I brake like an idiot! But, it could be the effect of the brake duct under the car as seen on the picture I last posted. It pushes air into the wheel and thus water, dirt and brake dust into the tape where it piled up. So far my theory ... which is plausible.

[ptitviet]

By the way your last consumption is just awesome considering the load and the speed!

If you have a lot of brake dust, might be useful to consider if your brake calipers are well greased. When they get old and rusty, they tend not to go back in place and stay against the discs.

[GreenTDI]

Originally Posted by ptitviet

By the way your last consumption is just awesome considering the load and the speed!

It's the 'power' of diesel, the longer the drive, the better the mileage. And thank god those summer temperatures of course It gives me a little hope for my 90 MPG trip (2.6l/100km) and the 70 MPG/tank challenge (or 3.3 l/100km).

About the brakes: there seems to be no fricton at all, the car continues to roll. And I recon you don't get such good mileages with dragging brakes ... But it will be on the checklist