On a CB the weight difference, with and without is seven pounds for the blade(just shipped a pair via UPS) and something near that for the support brackets, 14 pounds. I suspect the support brackets could be pared down to just enought to hold the bumper on, 1 pound for both. So 8 pounds is the increase on polar moment. Do you flush fit the CB to the valance? Do you fill in the bottom of the bumper blade to gain some control of that huge turblence-producing gap to the valance? Is there ANY aero benefit at all versus a Sebring style front valance and headlight covers? Smooth is your friend. Bumpy is not.

The Spec Miata national champion two years ago had a large muffler that filled in the back bumper to keep air from getting caught.

I think everything has been covered already by taking a quick glance at all the threads, but why not build a scale model and build a small wind tunnel or put it in an aquarium (water tunnel?)? It would save a lot of the discussion.



Edited 1 time(s). Last edit at 06/25/2012 12:52PM by hoffman900.

Hi all,

Im not too convinced about this CB addition may be beneficial argument, essentially adding a CB to the front even if it was very neatly fitted and weight reduced to a minimum wouldn't change the shape of the front of the car by any great degree. In the end the front of an MGB is pretty terrible aerodynamically speaking, rather like a flying brick actually...

I agree it would split the air flow, maybe tripping more into the rad intake (dependent on design whether this is a good or bad thing), the air diverted down by the CB would cascade down the valance and if there wasn't a rolled edge or better a splitter is could encourage more air under the car, rather then down the sides of the car.

IMO, for what its worth is its better to go with no bumper chrome or rubber (unless you want to dramatically change the front of the car then all bets are off) get a reasonable amount of airflow through the rad (somewhere between 40-60% grill size, approx 10-20% to carbs (ducting or air box or a hybrid (thats what we're penning at the moment) is up for debate) the remaining blanked off. Now the flow through the rad and to the carbs especially if its toward the upper percentages has to be dealt with or it will cause a degree of lift. The way we have tried to deal with it on the GT is funnel it through the inner wings and down the inner surfaces of the front fenders thus making the wheel arches a low pressure area, the rest of the air is (or should be!) funneled out of a gap under the rear edge of the bonnet (the bonnets rear edge is fluted for this purpose).

Now you have to deal with the rest of the airflow over the front of the car...... Sebring headlight covers help, now the valance.... i still think a SCCA air dam is the best way to go (rules permitting if not as deep as you can go) with a bottom rolled edge or splitter to funnel air not under but along the sides of the car, the valance flanks should also be contoured to encourage flow along the sides as much as possible. I believe thats all that can be done to the front (in the rules we have to race within anyway).

Steve brought up some excellent points about under body flow which is the next obvious step and Don about balancing the rear of the car now you've "optimised" the front.

These two really depend on how far you want to take this personally iv seen the gains possible with flat floors and diffusers, we built up a 250cc long circuit gearbox Kart from a chassis that was considered to be too out of date to be competitive with a extensive aero kit including the above and it was stunning it say the least. But with our GT we don't really want to lose all of the MGB character by adding wings and diffusers (i can't believe i just said that my fathers usually the one pushing for origionallity over total all out performance...) The compromise at least for the first year or two will be a adjustable Gurney strip along the back of the roof where the GT boot lid attaches, no diffuser.

Tom

SCCA GT cars here in the US can flat body the chassis, but of course we can't do that in production classes or vintage.

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Not adding flatness... But how much bumpiness can be removed?

Jacking points?

Cross members?

I just took a quick peek at everything. Wow, is this all fragmented.

* I think SVRA calls for a Sebring style valance. Anything bigger will be noticeable. The tough part about racing a popular car is that there will be several at any race weekend... anything that is amiss will stick out.

* I can't imagine any sanctioning body allowing any panel past the rear most part of the radiator.

- SCCA GT rules allow flat bottoms, but only between the firewall and the front part of the rear wheel well opening.

* I'm sure you can go hog wild removing jacking points, etc. That seems reasonable. I'm sure you can go underneath and smooth everything with bondo. That would be a lot of work and for what? However, adding a panel, lowering the floor pan or the such would be flagrantly illegal.

* I'm sure you can fill in the back with careful fuel cell placement. You still have to contend with the jumble of rear suspension pieces and the rear end. Maybe Dave will start selling "aero" track arms

* Best plan of attack, imo, would be a low slung single hoop cage, a soft tonneau cover, tighten all of the body gaps, tighten the gaps in the wheel well, hinge the hood so the rear floats up a little at speed, and only get just enough air into the radiator.

All of this work will be negated by wheel bearings that don't roll well or a slightly dragging brake caliper, etc. FV, FF, SRF and Spec Miata and other spec racers set up their cars using fish scales to measure rolling resistance. Doing aero work and not taking any of that in account is no different than someone buying light weight body panels and yet they themselves are still much overweight.



Edited 4 time(s). Last edit at 06/25/2012 07:11PM by hoffman900.

Dave:

I have a data logger which gives me mph and g at every point on the track.

The straight at Grattan is 3200', only 800' or so shorter than the front straight at Road America. Aero would matter there more than at Blackhawk or Gingerman.

But I spoke with Ken this morning, and we finally rejected the bumper because of the complexity required to fabricate it (I was intending a lightweight specially-fabricated aluminum facsimile with a flush mount at the joint to smooth it out), the difficulty of integrating it with the removable aluminum valance he is making, the difficulty of making it easily removable and thus of comparative testing, and finally the uncertainty about ultimate efficacy. Cd is way too complicated a topic and we are operating on gossamer instinct, which is fine if unsuccessful things don't hurt you and can easily be reversed. Alas not so the bumper.

But I still have the damndest irrational desire to cut a few hood louvers right behind the rad....

Don

Don, I can't think of a track more polar moment sensitive than Grattan.

Right, a great mix of The Tornado rollercoaster, and, for a Formula Vee at least, the Bonneville Salt Flats. If the bumper could be built light enough--a mere air-directing device, not a "bumper"--I would go with the aero. But that is moot now, at least for me.

Does the bumper have to be the original steel bumper with the steel support arms?

A fiberglass replica with aluminum would work... just fine.

It's function isn't bumping?

As far as what tech will or will not bless... I look at the diversity of "modifications" shown on the racecar site... and it is a mish mash. Panels, and devices valances of various shapes, materials and designs.

I keep seeing questions about one item... a bumper, a valance... this really is a conceptual question. If someone asked me; "would a cam make me faster" I'd say, not unless you support it with head work, induction, exhaust and ignition.

No one item is going to make a difference, but a coordinated set of changes that modify the system will have an effect.

Tom wonders if the addition of a bumper would have an effect. My response is "I don't know", but look at the underside of an modern car and there are small tabs to deflect air from the tires. Even at normal roadway speeds the manufactures have verified that small aero devices can have an effect at low speeds. Is a bumper which functions at a splitter going to have an effect at 100+MPH... i think it might.

It's not so much what tech will allow, it's more about what your competitors will allow. If you are fast and have mods not supported by the rule book, you'll have issues with competitors, the friendly ones and the less friendly ones.

Considering that this(vintage) racing is done for fun, not pushing the legality issue is beneficial, especially considering the prize money.

Steve,

I totally agree all modifications have to be thought of in conjunction with the rest of the build.

My concern was not if it will have an effect (I'm certain something that big will) but if that effect will be beneficial. I'm not certain having a splitter because that's what effectively it would be (just a very thick one) positioned where a CB would be could be beneficial. I'm very open to persuasion though!

Tom

On Monday I flew to Budapest… window seat 777. There on the leading edge of a freakin HUGE wing… were 16 surprisingly small vortex generators. I thought… on a wing the size of a football field those VG’s can have a beneficial low speed (150 mph) effect on stall speed, amazing.

When I look at the cars in vintage racing I see so many "additions" which either intentionally or accidentally fracture the rules. I'm not suggesting that anyone turn their car into a winged wonder, but as I've noted before there are cars with deeper front valances... join that parade! With one exception, position the oil cooler inlets differently.

There are cars with radiator cooling shrouds... join that parade, just build the shroud so it delivers an aerodynamic plus in addition to better cooling.

There are cars with "skid plates" I hate to keep saying do what Timmy does, but...

If the rules allow you to discard metal/brackets, then what metal on the undercarriage of an MGB could be dispensed with which might improve the aerodynamics of the car? Could the cross brace at the jacking point be repositioned to the interior and still provide structural support? If it’s legal to add and subtract… why not?

If exhaust is free... why run a round tube? Why not borrow from the Spridget guys and run a tunnel exhaust… a center flat tube (sounds funny) which fills in the transmission tunnel and exits in a more beneficial manner.

If fuel enclosures are free... why not modify the stock later MGB steel enclosure which has a much better shape.

Is it all “fly shit in the pepper”? (Thanks Dave, I’ve used that more than once!)

Could be! Total cost to do everything, a few grand. Benefit… well since it’s aero it’s all going to happen at higher speed… are there any kinks, turns over the brows of hills or high speed turns where the sphincter factor is higher than 9.9 and you’re forced to lift? These changes could bring the sphincter factor back down to 9.5 and allow you to not lift.

Steve,

While monkey see monkey do has it's merits, one has to remember to tread lightly.

A perfect example is Brembo brakes featured on some of those cars on that site. Back in 2009 at Watkins Glen there was some drama between two racers (some of which spilled onto the forums here). While in post incident tech the brakes were noticed and he was dq'ed. Part of the reason was that he was one of the faster guys and they wanted to make an example out of him (SVRA has told other front runners the same). If you run up front at the big vintage events (Watkins Glen, KIC, Gold Cup) expect to be looked at closer.

Even as far as aero is concerned, SVRA gave us a rash of crap for having the passenger headlight open for an oil cooler. They told us "close it up or you're going to Group 8".

I think under body aero can fall into the category "you can remove and smooth things out, but it better be the stock sheet metal and you can't add any panels".

I don't care either way. No one protests in vintage, but it would be a lot of effort to revert back if you got in trouble. I also see in the "tinkerer classes" where people get focused on small details and miss big picture items. At some point you just have to say "hell with it", pull the pin, and hope you threw it far enough. My .02 from the track side.



Edited 9 time(s). Last edit at 06/27/2012 08:35AM by hoffman900.

Steve, one of the fundamentals to govern car prep under SCCA rules, which are the basis for most if not all vintage prep rules, is that you can not make mods not specifically allowed by the rules. Omission in the rules of a desired mod does nor validate it's legality. Vintage orgs tend to follow this premise rather loosely.

Monkey see monkey do will eventually lead to some level of grief.IMO of course!

pull the pin, and hope you threw it far enough... that ones going right next to: fly shit in the pepper.

Although I will add one caveat... it's not monkey see monkey do... when no one is doing it!?!

But I get your point



Edited 1 time(s). Last edit at 06/27/2012 09:36AM by Steve64B.

BTW: Both of the cars had Brembo brake disks... but one had 4 pot Wilwood calipers! Was it the disks or the calipers that caused the DQ?

I can't speak for Bob, I remember the incident, I'm guessing it was Brembo calipers, because Brembo are brand name of otherwise OEM MGB rotor. I try my best to find brembo rotor for my race cars, becuase they never gave any problems,and my supplier carries Brembo rotors for MGBs

Bob, can elaborate more, but SVRA is probably the most proactive rule enforcer in vintage racing, because they have point championships and such, I think I've heard guys talk about them weighing cars, P&Ging cylinders and such, where alot of vintage groups tend to be more laid back, no throphies, no points and such. Most of the the time in vintage it is you and the tech man for that group, but even then sometimes they say one thing and do another, and often times it may come down to your competitiors complaining about you, and then the tech guy having to deal with you. The thing with vintage vs SCCA is SCCA is a nationwide group, and it rules apply everywhere, but with vintage, it comes down to whatever that tech guy says goes with that group. Then there are vintage things that seem to be tolerated that never were part of SCCA 1972 GCR rules, head fairings and Sebring head light cover come to mind.

I think these conversations are neat, but not a deal maker as far getting one to a race class win, you see everyday, and I've seen it for decades, a guy with less tricker car out runs a guy with a tricker car, I think at some point you have to start looking in the mirror as to why one is not winning, If I had bundles to spend right now in racing, it probably would not be on my car, but rather on me, after I witnessed firsthand the improvments Peter Krause made in Fred McConnell with his driver coaching, sure it takes good equipment to win, but after that nomrally the weak link is the loose nut behind the wheel Peter's driver training for exmaple may be the better spent money, than some new fangled part. I think we all think we have god-given talent to drive a race car up front, and sometime in given race with a given set of opponents, we do, but most of us are just not as good as we think we are, self included

I have friend who has ran SCCA HP, forever, Tony Drum, some of you may know him, he like 72 years now, and set a track record this year at Roebling, he's never had a air dam on the front of his car that I can remember, yes he always had good engines and trannys, but I would never considered his cars ultra trick, but his driving talent always got him to the front, and stills does, Randy Canfield is another guy that fills this bill. Trick parts will only get you so far, at some point, you have make up the difference.

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Edited 1 time(s). Last edit at 06/27/2012 01:40PM by Speedracer.

In the early 70's there was a race called the Questar Gran Prix at the recently opened Ontario Motor Speedway.

The promotors had the stars of Indy and F1 racing F5000 cars. The story goes that AJ Foyt was unhappy with his car. After a brief practice he said he'd be back on race day. The organizers asked Jackie Stewart if he would drive the car and figure out what was wrong... AJ was a star and they didn't want him unhappy. Stewart went out and was several seconds quicker. When he came into the pits, he got out of the cars and said: It's fine.

Even the guys who are good, aren't always good.

I think they were vented rotors and Brembo calipers. He had them fixed for the next race and set a new personal best at that track. They didn't make a performance difference and if anything the extra mass slowed him down. You see this with the Miatas as the 1.6L brakes are smaller. On an IT or SM car, the smaller brakes are a performance advantage. They don't have brake fade issues with the smaller rotors, so the lighter rotating mass is a performance advantage.

However, SVRA does allow alternate calipers:


Note: it doesn't say " MUST use stock calipers". This was confirmed by the old head of tech.

SVRA seems to be the most proactive of the groups, but the racing seems to be more serious in nature than the other groups so this is understandable. I haven't been to a VRG event, but I've seen pictures of one of our competitors running Avons that are illegal with SVRA at a VRG event, and pictures of it at a SVRA event a month later with the SVRA legal tires.

The showcase events tend to be more serious in nature and we have been weighed at more race weekends than not. They also check a few other items as well.

Talking about trick cars and good drivers. I've heard people who were there say that about Group 44. There were almost always tricker cars on the grid, but the Group had great drivers.



Edited 5 time(s). Last edit at 06/27/2012 11:55AM by hoffman900.