diffuser question '88 reynard

[reisertracing]

For those of you who have this fine car, what are your views, opinions and experiences with a larger diffuser on the '88 reynard.

Have heard mixed reviews, some questions:

1. What effect does the larger diffuser have on rear ride height?

2. What diffuser do you use or reccomend?

3. Ease of application

4. balance of handling

5. effect on straight line speed

6. vanes or not

7. special issues related to attachment

8. Do you have one for sale?

9. settings on rear springs/ shocks with diffuser, i.e, stiffen rear springs? wing angles, effect on rear wing.

10. what did you notice when driving after applying new diffuser?

Thanks, Johnny R.

[FC63F]

I almost wasn't going to weigh in on the diffuser issue. Partly because there are tons of opinions but few facts.

A. Diffusers do work in that downforce is generated which can be helpful

B. I have seen very fast guys driving both short and long diffusers in 87-90 Reynards. Exampples of fast are Steve Myers with an 89 SF who has the full averill wing set ups and the short Averill diffuser - Ran 1:29's on the MO pro course last week end. Jim Roberts runs a long duffuser set up and he is also very fast. I could provide many more examples but all we have proved is there is no significant enherent disadvantage. About all you can say is fast guys have diffusers

C. Several years ago Steve Myers and I owned a Crossle 63F that as delivered had no diffuser. Car was a bit pitchy as the wings started to work and we noticed that that the Van Diemens all had diffusers and they were fast and we were slow. So we deduced that we needed one of those. Averill build an aluminum diffuser to fit the car that borrowed from two ideas - one was to manage the air under the car and the other was to manage the air at the rear tires. We had read some SAE studies that suggested that managing the air on top of the diffuser was more critical in flat bottem cars. So that is what we built. Well the first time out the thing ripped out the mounts within a few laps. Keith looked at the diffuser and said "well we know it is doing something" - guess we need to strengthen the mounts. And we did and went out the second time a tore them out again. Hmmmm, this thing is really doing something and Keith re-engineered the mounts and the third time out we decided to take pictures to see what the something might be. The diffuser was distorting but the impact on the car was awsome - falt much more balanced and planted to the ground - net we could drive faster. Using the aluminum prototype which we stiffened, etc, we then built a composit diffuser with better mounts, stiffer construction and a bit longer than the first design. Car was great, rain races showed good flow etc and the cars balance was much better and we were able to take out some of the rear wing which allowed more speed at the end of the straights (car had Averill Biplane system) Car went through the corners better and was less nervous. Diffuser 3 was then built with improved lighter composite construction and was about 12 ounces and twice as stiff. So we can say that having a diffuser is better than no diffuser froma handling standpoint and also enabled running a lower angle of attack on the rear wing system.

Improvements we were considering - - the first three generations ended at the rear axle - and had a relatively gentle slope to it at about 8 degrees. We were close to building anew diffuser that would be about 4 inches longer as an experiment and I was going to re- engineer the side skirts (replacable) based upon reading some the the data form Simon McBeath. I had thought about veins etc but I have no real data to show me how to place them and what is goodness -- no wind tunnel on my budget. Sold the car this year and now I am running a 90 SF. - to your questions

1. Do not know - I think ride height has an effect on the diffuser performance - I am not sure there is am impact on ride height based upon length

2. I use the standard Averill Ultra diffuser. Car was set up for his piece when I bought it - low on my list of investments in optimizing the car. In my case, I changed the entire suspension package to Keith's standard 87-90 Reynard set up -- that cost me springs, a rear sway bar and a pair of new high pressure gas over oil Koni doubles. What is important about this prioritiztion is setting up mechanical grip and handling comes first and then mess with diffusers, trays ect.

3. What ever you have -- There are good parts from Averill, Pennon, Gyrodynamics and others. I cannot provide a fact base for choosing one part over another -- no one to my knowledge has any fact based data for analysis. The perofrmance differences between the parts available is probably very small = the key is to have one

4. Bolt right on - 6 small fasteners into a L bracket - add two stays at the rear and your done

5. Balance of Handling... Not sure what you are asking - I believe based upon my experinece with the Crossle that the balance of the car is improved. Force is generated closer to the CG

6 On the Crossle - no discernable change in straight line speed over no diffuser but the ability to trim out the rear wing and then get a higher end of straight speed without compromising the corner speeds

7. Vanes or Not - what is the objective, what is goodness and how do we know that goodness is happening. No data

8. Robust - the things do work

9. In general you are creating downforce as an element of aero and in some cases improving the performance of other elements of the wing system (pumping action etc), you have to be careful not to pin the rear suspension with the combined downforce. Have caused that -- part of that can be solved with stiffer springs but actually I would trim out thre rear wing and reduce drag as at least part of the solution

10 see all of the above - Also buy both Simon McBeath books and set up your spread sheets and analysis tools -- His equations can allow you to find starting points in aero set up which may not be absolutely correct but will be directionally correct and then you can adjust from a reasonably fact based baseline. Plus -- fun to read

Not sure I answered the question -- but here you go

David

Reynard90SF

[reisertracing]

Thanks alot,
yes, you answered my questions.

[Westroc]

John I have been more than satisfied with the Pennon diffusers I have bought. They are maybe a little pricey @ 1500 but they work real good are bolt on and have the vanes underneath. Fairly sturdy as you can spin over a curb and not cream all the vanes off. The leading edges are a little suspect for going thru grass and the pea gravel but what do you want if the driver makes a mistake right? Ask me I know. Yes I have spun and I almost guage my performance by overstepping the limit on a weekend. DO NOT solid mont the diffuser with ferrous struts. Use wires which will "move" if a curb comes up. I also have 3 original Reynard diffusers and could sell one. Since my customer is about to finally pay me the balance of the car and they came w/ that car I would have to know soon. I don't know what they are worth. Whatever Averill or International says new and what half it?? They are worth a TON OF TIME and add alot to driver confidence which to me is worth the most.








































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[Purple Frog]

I think David pretty much said it all.

You don't need a diffuser on the straights. OBTW, it works best on the straights. Your goal is to maximize it's function in the corner. Keeping it as parallel (left to right) to the track as possible is important. So roll is the enemy, sort of speak. At turn-in you don't want the car laying over, thus lifting the inside edge of the diffuser and dumping unwanted air into the vacumn you so carefully designed. So, cars are stiffer to stop the roll.

Think of a suction cup on a piece of wet glass. You can slide it fairly easily around on the plane of the glass. But it's fairly hard to lift away from the glass. Now lift one edge of the suction cup, letting air into the vacumn, and presto zappo, suction gone. Think of race cars with diffusers as suction cups on wet glass.

Usually the closer the outside rails of the diffuser are to the ground, the better. So, those car with stiffer springs can now also employ lower ride heights. Thus keeping those outside edges low, better to block air being pumped off the rear tires from ruining the vacumn.

All designs are different in little details. Some experience disturbed air coming in from the sides, so vanes are set in a few inches to keep that disturbed air from migrating even farther inward into the vacumn. Somes cars, because of rub strips, bellhousings, etc., have disturbed air in the center section of the diffuser. In that case the vanes keep that turbulence from spreading out wider in the vacumn section of the diffuser. Thus the reason for vanes.

[FC63F]

Purple Frog,

Most cogent explanation for vanes I have ever heard. As a side note, much of our efforts on the Crossle was about managing the air on top of the diffuser and the air moving between the rear tires and the bodywork. We added vanes on top of the diffuser to manage flow and that seemed to work based upon a limited amount of tufting and oil spotting observations. Our view was this was a type of wing in effect. We werw inspired by the Katz book and the McBeath articales at the time.

David

Reynard 90SF

[LAJ]

A subject after my own heart!!
Yes you need to have a Diffuser!!
You need to start thinking of keeping the air out from under the car as much as getting it out from under the car.Air has mass and wieght so when you desturb it (drag) ,it slows you down.Your car has width and pushes air around and over it(high pressure) causing the air to want to move to the low pressure area under the car.The farther forward you start managing the air the more balanced the downforce(center of pressure) will be.I have also seen most CFCs not running a tail section that would clean up the air as it gets to the rear wing.
Mike Bingham's Reynard at Mid-Ohio had a very nice(1 made into a 3piece tail )tail section that cleaned up the air nice as it one goes over the diffuser and gets to the rear wing!!
Pennon's latest diffusser is so good a lot of newer Van Dieman Diffusers are being taken off and Pennons being put on!! Sounds like Jim (WestRoc) Roberts has a pretty good deal to get you started quick and easy!!

Lee
http://metroevents.com/jordaneng/index.htm