Diffuser Design help - Andylaurence

[andylaurence]

I vaguely remember that testing of wheel covers. I shall have a look through my back issues. I've not done any hillclimbs, although Crystal Palace is similar in nature (without the hill).

Road tyres are certainly not the way forwards. On the start line, they'd be cold and generate less grip. Being a harder compound, they would take longer to get up to temperature too. Several manufacturers do very soft compounds for hillclimbs and sprints, notably Avon and Yokohama. These tyres produce good grip from cold and warm up quickly. I can overheat all four tyres in two laps on a test day.

[garygph]

At the chequered flag, the tyres and brakes can usually be touched with a bare hand!

My mistake, I assumed from the above statement that you had very low temperature in the tyre and i know that a cold slick has less grip than a cold high performance road tyre. Did not know about those other tyres you mentioned, I would have cut sipes into a slick first

[andylaurence]

Learning; that's what forums are for. I'm learning lots from you guys and you're learning that soft compound tyres exist the are designed to work from cold.

[wesley123]

Note that by the wheel covers the brake cooling keeps 'contained' within the rim, heating the tires a little bit quicker

[andylaurence]

Yes, that had occurred to me, but given how slowly rubber conducts heat and how much heat is generated from half a dozen stops from ~110-130mph to ~50-70mph in a 550kg (including driver) car, I don't know if that will affect things much. Either way, it throws the balance towards fitting some wheel trims. I'll have a think about how I could do it - an aluminium tube held on by the nut retaining clip and a second clip to attach a flat sheet of carbon should do it.

[machin]

I can't remember where I read it; it might have been in Racecar Engineering, but following that original article in RE someone tried to fit wheel covers to their singleseater for a race but were subsequently disallowed by the scrutineers.... Hillclimbing and sprinting regs are notoriously open... but worth bearing in mind...

I looked at something similar for my Westfield hillclimber a while back, but the speeds are so low in hillclimbing that the mass/inertia vs drag equation didn't come out in their favour... I'm guessing with your higher speeds it might just sway the other way...

The other thing I seem to remember is that on that VW single seater in RE fitting the wheel covers affected the aero balance.... (you'd think it would affect front and rear equally, but I think I remember correctly that it didn't..?)

[wesley123]

I think it is somewhere understandable that is the case. The front brakes are regularely cooled a bit better, certainly in closed wheelers. Here more air is used to cool the front brakes, thus more air stays within the rim/wheel arch than on the rear, reducing downforce on the front.

But back to the design. If you can get these things made cheap and light, it is certainly worth a try. If they weight too much there is more inertia/mass on the wheels like machin pointed out.

And for the rear, it might be an idea to just fill the wheel arch instead of the rim, reduces drag a little better.

[Smokes]

Avon do a large range of compounds as andy will tell you.http://www.avonracing.co.uk/resource-centre/compounds
A treaded tyre will heat up quicker than a slick of same compound from cold due to the tread block movement and smaller surface area, but a treaded tyre give less grip once they get to temperature due less contact area and the block movement meaning it will break traction under lower load than slick of the same compound and load and temp.

Also due to the movement of the blocks it will limit laptime due excessive heating at high loads. Bubble gum is what David Coulthard said when he was showing what happend with a wet F1 tyre on a dry track. Though the tyre is an extreme soft compound designed to be cooled by water

For on road racecars the tyres are oversized I.e wider than the needs to be for the static weight, hench the use of downforce to increase the weight of the car and hence force on the contact area of the tyre f/conctact area =pressure and pressure * rolling co effient friction of the tyre at X temp = friction , there is alot more maths to it than just that as slip angles and carcass stiffness and tyre pressure have to be taken in to account.

[andylaurence]

To give an idea of grip levels, I was pulling 1.35G at the first corner (Quarry) on stone cold tyres at 52mph at Castle Combe earlier this year.

@machin
I have much less power (999cc) and travel at significantly higher speeds at most sprints. I reached >115mph at Castle Combe (Avon Rise & Hammerdown), Silverstone Stowe (both straights) and Anglesey (3 straights) this year. Crystal Palace was my only other venue, where top speed was 72mph. I can imagine that the reduced drag would benefit me greatly for what should be <500g of unsprung weight per corner, but your point about scrutineering is an interesting one as it's obviously a moving aerodynamic part. I'll review the Blue Book.

@wesley123
I have no cooling ducts to the wheels at the front or the rear. There are two intakes above the front splitter, but they only feed the underside of the bodywork and cause drag. I've thought about covering the rear wheels with bodywork, but where the rear clam joins the sidepods makes this impractical, not to mention the effort in removing the rear bodywork to check the wheel nuts are still tight - I do this as a matter of course before each run and often run the car on my own without assistance. The rear clamshell is 1700mm wide and heavy enough that I can't lift it off unassisted. If I get around to making a new body, it will be made in more pieces to make it easier to remove on my own.

[machin]

I don't think there's anything in the blue book that specifically states you can't hvae moveable aero is there? I think its more a case of whether the scrutineers think they're securely attached and whether they're likely to become some sort of projectile weapon! That sort of thing is very much down to the individual scrutineer. I swear this year one time the scrutineer just checked that I have 4 wheels -not even that they were properly attached!

[andylaurence]

I saw a quote by someone about movable aero being allowed. 5.20.10 in Section J states:

5.20.10. Not have skirts, bridging devices or any form of aerodynamic device between the chassis and the ground/track. Any specific part of the car influencing its aerodynamic performance must:

• a) comply with rules relating to coachwork.
• b) be rigidly secured to the entirely sprung part of the vehicle.
• c) remain immobile in relation to the vehicle.

It all boils down to how you define "vehicle", for which there is no Blue Book definition. The wheels are part of the vehicle and the covers would remain immobile relative to those. Brake ducts are acceptable, movable relative to the chassis and their only purpose is aerodynamics. It's a grey area that I could see becoming black if I tried it...

[machin]

Its like F1 all over again isn't it?! When is a part of the wheel not a part of the wheel?!

The old wheels on my Westy had small covers about 200mm in diameter which covered over the wheel nut recesses and made the centre of the wheel flush... undoubtedly they had some aero benefit (not as much as full covers of course).... They were made of alloy (same colour as the wheels) so would be difficult for anyone to argue that they're not part of the wheel, but technically they fall under the same bracket as what is being proposed here; bolted on rotating aerodynamic devices! Like you say; its a grey area....

I was pulling 1.35G at the first corner (Quarry) on stone cold tyres at 52mph at Castle Combe earlier this year.

Off topic for a moment; below is the speed trace from my 1st timed run at castle combe earlier in the year... interesting to note that my Quarry apex speed (about 600 metres into the run) was just over 50mph too, but lateral G-force only 0.86 (this was in a Porsche Boxster running bog standard road tyres), so I must've been taking a significantly wider line through that corner... would be interesting to see your speed trace for comparison...???

[andylaurence]

Grey indeed. Some composite covers would be hard for me to argue, especially as my wheels are off-the-shelf components.

Thanks for sharing your data. My apex speed is very similar and, as you say, is likely down to me taking a tighter line. The GPS-derived lateral G and the readings from the accelerometers look in the same ball-park, so I believe they are reasonably accurate. Certainly enough to be sure I'm not down at <1G. What logger are you using to derive that figure? Is it from GPS or accelerometers? Being a two-lap event, I assume that's the Great Western Sprint which was dry all day, from memory. I suspect part of my difference in line is down to the fact that I was doing 116mph over Avon Rise and this was the first time I'd not braked for the rise, so perhaps I took a tighter line. That said, all my speed data is from GPS. Using RPM and knowing my ratios, I was doing 53mph and that was before the lateral G peaked (only 1G at this point), suggesting that I slowed down too much and accelerated into the bend, at which point I was doing 56mph. This is a common trait for me - I did mention my braking needs work!

Anyway, rather than posting a graph, you might find the raw data more useful, so here it is. GPS data is about 1.4 seconds behind sensor data (RPM & G).

[machin]

Cool, I'll take a look at your data, thanks. Mine is recorded from a Racelogic GPS; speed wise its always been very good -whenever we've had a finish line speed recorded the GPS speed has been bang on. I have no way of verifying the G reading, but for a heavy road car on normal road tyres something around 0.9G is what I would expect. I think you're right -I was lifting going around Avon Rise and then getting right over to the left before coming over to take the apex at Quarry, so plenty of time to take a wide line....

[andylaurence]

The data is from the second timed run at the Dick Mayo Sprint so you can correlate video with data. As you can see, I went straight for the apex. I think this allows me to carry more speed over Avon Rise and I've seen video of drivers in the Top 12 doing something similar, so it must be good! When I finally got the wheel straight, I was doing 85mph, so I don't think the loss in the early-mid corner compromises the exit, it's just whether the extra speed over Avon Rise is worth the lower apex speed. I don't know - I don't have enough lap data from Combe to know.