Technical Discussion - Wheelbase, Page one (from old site)
perkidelic
Registered User
Posts: 550
(5/19/04 6:34 pm)
Reply Technical Discussion - Wheelbase
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I didn't want to hijack Ralph's thread on track width, but some things I read there made me want your expert opinions.
For autocrossing and tighter road courses a short wheelbase is obviously desirable - BUT - is it really an achille's heel for straight-line stability.
I am a quarter-mile junkie so I have always followed the belief that a longer wheelbase is essential for safe, consistent, maximum straight-line performance. Messing with Fieros (93.4"), made me really think about what is possible with a short wheelbase. I like the idea of a short car that can twist around cones and hairpins really easily. The one car that got my attention is the Porsche 911 (92.6"). It doesn't seem to have any stability problems and can run over 200mph relatively safely.
So my questions are:
One
Is a longer wheelbase really that critical and/or beneficial for straight-line performance, or is it more a matter of design and tuning?
Two
How much does the engine/drivetrain layout (f/f, f/r, m/r, r/r) matter?
Three
What would it take to make a short (under 95") wheelbase vehicle as confidence inspiring, safe, and stable in a straight line as it may be in the twisties?
By straight-line performance I am talking about high speed straights on a race track (wouldn't do that on the street now would we), drag strip, and the salt flats.
Thanks in advance,
perk - todd's hot rods
Ralph L
Moderator
Posts: 3269
(5/19/04 8:14 pm)
Reply
Re: Technical Discussion - Wheelbase
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To add onto perk's questions...Is there any formula or "rule" when selecting tire width in relation to the wheelbase? For example, in a compromising situation where tight cornering, straight-line stability, and open road racing are a factor--How would you go about picking optimal width tires? Would say a 315/345 FR/RR tire combo work best on a 104" Wheel base car, but on a smaller car like perk's fiero, (93") wheel base, would you be better off going with a 285/315 combo to yeild simiilar results?
Another good thread!
EDIT: P.S. upon completion of these two technical threads, I will make a copy of them in the FAQ section, stuck to the top of the forum, so as new members can easily find them.
Ralph
Project Fantom Expected Completion - May 2005
My Tahoe - "Black Mamba"
Rendering of Project Fantom
Edited by: Ralph L at: 5/19/04 8:16 pm
davidpozzi
Moderator
Posts: 1120
(5/20/04 12:23 am)
Reply
Re: Technical Discussion - Wheelbase
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For autocross and the tight road courses, I think a shorter wheelbase is better. Mainly because the front wheels will have to turn less to make the turn lowering the front tire lateral load. The longer wheelbase car also has the rear tires trailing inward more, so must either have more oversteer to swing the rear axle away from the inside corner cone, or must steer wider with the front. The shorter wheelbase car "sees" a more open course. Usually a shorter car is lighter too. The shorter car must have a lower center of gravity and a fairly good rear weight bais or the front tires will be overloaded in braking. The car will run into pitch problems sooner. A shorter car usually will have a lower polar moment which helps in a slalom and left/right linked turns.
Look at Go-Karts, they go darn fast but are fairly stable. I think the secret to short wheelbase stability is low center of gravity, and at very high speeds, center of aero pressure is going to have a large effect on stability. I think there is less room for error on a shorter car at high speeds, it would be more touchy on setup of balance.
My lola is 96" wheelbase and is very stable up to the speeds I've had it. I've only had it to 140 mph though, but very often!
Later big block Can-Am cars benefitted from longer wheelbases. I think the higher torque and horsepower of a big block really pushes the limit of shorter wheelbases. My car has over 60% rear weight bias, and it wants to push the front wheels straight when I get on it coming out of a corner. The front end feels very light. working in my favor is the low CG and lots of rubber. The rear weight bias and low CG keeps the front tires from being overloaded when braking.
I guess from this you can see if the CG is kinda high and the wheelbase short, along with high HP to weight ratio, and you can run into trouble.
F 5000 cars were lengthened in the early 70's and they handled better too. On the mid engined cars I'm referring to, the front wheels gain leverage over the rear when the wheelbase is longer, the rear has most of the weight, but the cars, both Can-Am and F5000 were also lengthened in the bellhousing area, so I imagine the CG location either stayed the same or moved slightly forward.
I was quite amazed by the cornering speeds of the Donohue Porsche 917/30, I followed it through turn 2 at Laguna Seca and it was faster than I was by a bunch. I thought of all the turns on the track, that was the one where I could at least keep up! Of course it was driven by a pro.
David
67 RS Camaro, 69 Camaro vint racer, 65 Lola T-70 Can-Am vint racer.
ourworld.compuserve.com/homepages/David_Pozzi/
Edited by: davidpozzi at: 5/20/04 12:28 am
Mean 69
Registered User
Posts: 27
(5/20/04 11:53 am)
Reply Great Stuff!
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Nice stuff, David. Everytime you write about that Lola, I start day-dreaming about how cool it would be to drive it! If I am not mistaken, the 917/30 was the highest power to weight ratio car in the history of sports car racing, I read that it had something like 1100 HP, and I think those wonderful Can Am-ers generally weighed in the 1800 lb range. Incredible. I recall an article that when Donohue was driving in one of the events, that he was so far ahead of everyone, he started to intentionally unbalance the car by adjusting the sway bars to learn what would happen. In a race! I am pretty sure that car is why the Can Am series died, no one could come close to that car. Man, I hope to make it to the vintage event this summer. Will you be driving this year? I'd be honored to help in the pits!!!! Oh and by the way, the Trans Am races ARE fixed. I spoke to a guy at a different vintage race, and he stated that at 'Seca, the Trans Am event is more of a pagent than a race. He was asked to not come back due to aggressive driving (uh, I think that's why they call it "racing").
I can't specifically speak to the wheelbase length issue, but one area of interest is HOW the weight is distributed, in this case longitudinally. It's not hard to determine the center of gravity, but of perhaps more interest is the polar moment of the car. The polar moment speaks to how the weight is distributed along the front to rear axis of the car, and under acceleration, and more importantly in most case, braking, this can have some interesting effects. 'Course, there is only so much we can do about it with the sedans that we are building.
The ultimate goal is to get the weight of the car as close to the actual center of gravity as possible. This effectively minimizes the rotational inertia of the car. Think about a dumbell, with a given amount of weight on the ends. Now think about how hard it is to rotate it, by twisting your wrist. Now imagine taking the same total amount of weight, but make the bar twice as long. It will be alot harder to twist it, or to slow it down.
Again, with our cars, there is only so much we can do. Moving the engine back, and down for that matter, is a neat idea, but actually has a very small impact on CG and distribution. One idea that I liked alot was moving the battery to the trunk, but rather than on the passenger side towards the back, like most of the drag racers tend to do, do it like Steilow did, in the center of the car, towards the passenger compartment. I fully plan on doing that to my 69.
It just seems to me that a moderate wheelbase car can better deal with the weight, in a dynamic sense. I have watched some of the old Ford Anglia converted drag cars, or Vegas, etc, and I would be scared to drive one of them. They are just so erratic under throttle. If you could build a short wheelbase car, and make the CG really low, and everything light, you will have an excellent platform to build on. What is the wheelbase for a 66 Chevy II?
Mark
katz
Unregistered User
(5/20/04 6:49 pm)
Reply re
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First, without getting into any tech discussion, increased stability of long WB is pretty obvious if you drive a 120+" WB fullsize PU at freeway speed. This is mostly due to higher polar moment of a long WB vehicle.
Just like wider track decreases lateral weight transfer, longer WB decreases longitudinal weight transfer. This is what David and Mark are talking about. Front-engine short WB vehicles will have problems unless CG is pretty low. It's not practical to build 100% anti-dive into IFS. Similarly, Rear-engine, IRS'ed short WB vehicles will probably have severe squat problem.
For a given CG height, longer WB also reduces the incline of neutral line so it is easier to get higher anti-squat. This is especially beneficial on IRS-equipped vehicles.
There are other not-so-obvious benefits. For a given shape, longer object has less aero drag coefficient most of the time. It is seen on those streamliners on dry lakes. For the size of typical passenger cars, you can't really make it long enough to realize this benefit. Depending on the body shape, you'll get better Cd if you chop off the back end abruptly. This (and to minimize overhang) is the reason why C5/C6 have really short rear deck. C5 has WB of 104" or something, but its overall length is shorter than C4 which has 98" WB.
Many people seem to be obsessed with the magic 50/50 distribution, but concentrating the mass near CG is far more important. I mentioned in the other thread that battery can be mounted in backseat area (if you don't need backseats). Jeffandre came up with an idea of using two 6V, 3-cell Optimas in parallel, and hiding them under arm rests. I don't know what kind of car he's working on, but it's a neat idea. Depending on seat cushion thickness, you may be able to lay those 1/2 batts on their side and mount them right next to the tunnel. That will move the mass towards vehicle centerline and lower CG at the same time.
I'm not really sure what would it take to make a short car really stable at high speed. On top of my head there are...balanced downforce, minimum lift coefficient, and stable suspension geometry (not aggressive, just stable). Wide tires wouldn't help here. Active suspension really helps, as it allows to change the pitch (or rake) of vehicle as speed changes. This was one of crucial parts to the success of Thrust SSC, but then, we're talking about 200+mph at most instead of 765mph.
'62-'67 Chevy II has 110" WB.
Gotta go. Be back tomorrow (or Monday).
parsonsj
Registered User
Posts: 1616
(5/20/04 10:06 pm)
Reply Most do ...
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Quote:
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'62-'67 Chevy II has 110" WB.
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At least they do from the factory. I know of one with a 110.75 wb ...
jp
gchandler
Unregistered User
(5/20/04 10:20 pm)
Reply Battery
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I plan on mounting my dry cell optima in the firewall as it keeps all of the weight of the battery between the axles and helps to ofset the weight of the driver, as I have removed all of the a/c and heater equipment from my car.
perkidelic
Registered User
Posts: 551
(5/20/04 10:21 pm)
Reply wow again
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I am somewhat familiar with Polar Moment of Inertia and it is actually one of the things that keeps me messing with these freakin Fieros It is also a nice double edged sword, as you guys have pointed out. A 93.4" mid-engined car is a perfect example of this because it definitely rotates very easily on its yaw axis, but can also be very difficult (if not impossible) to catch when it come around too far and too fast.
Aerodynamically loading the front and rear of the chassis was the one thought I had on making a short wheelbase, low polar moment, vehicle more stable at speed. Isn't that what Funny Car, Top Fuelers, Indy cars, and F1 cars do? I know those aren't exactly short wheelbase vehicles but "at a glance" that's what appears to be at work. Especially in F1 where the cars are expected to do everthing well. Isn't this actually artificially creating a high polar moment vehicle for certain situations?
I know big gigantic wings don't fit well with the Pro-Touring theme, but what about active aero treatments - like the automatic pop-up at XXmph rear wings that are becoming commonplace on new production sporting vehicles? Doesn't seem like it would be hard to accomplish and would theoretically increase straight-line stability.
I understand wide tires won't help my stability cause, but I will have to find the best compromise there. For a variety of reason the widest street legal rubber available will be a part of the equation for many of us.
The chopped-off butt aero thing gives me something to think about because some of my designs are pretty well rounded back there.
perk
gchandler
Unregistered User
(5/20/04 10:22 pm)
Reply here is a picture of one we did at the shop
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davidpozzi
Moderator
Posts: 1121
(5/20/04 11:07 pm)
Reply
Re: here is a picture of one we did at the shop
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What's the wheelbase of a Pantera? Aren't C3 vettes 98"?
We have an open road racer locally who runs one.
What is the F/R percent of a SB Fiero-ish car?
I think the 60's and early 70's original 911 porsches had a wheelbase in the 80's, something like 88 inches is what I remember. (found it, 89.8") www.ultimatecarpage.com/f...v&num=1681
I used to be extremely aware of their wheelbase advantage for autocross. The 914 is much longer.
If the rear weight get's much above 60%, I think the handling will suffer. Larger rear tires helps compensate for rear weight bias, but keeping the front inside wheel on the ground becomes more difficult, as seen on many 911 Porsches.
David
67 RS Camaro, 69 Camaro vint racer, 65 Lola T-70 Can-Am vint racer.
ourworld.compuserve.com/homepages/David_Pozzi/
Edited by: davidpozzi at: 5/20/04 11:11 pm
davidpozzi
Moderator
Posts: 1122
(5/20/04 11:39 pm)
Reply
Re: Great Stuff!
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Mean 69,
Good points on wheelbase.
I've got some more things to do to the Lola so may run late this year. The Monterey Historics will feature Ferarri this year and there will be no Can-Am feature. there will be one at Sears Point, AKA Infenion, and at Portland and Willow springs this fall.
I knew the TA Vintage races were "fixed" when George Follmer got the lead, and then got passed!
All Camaro weights Ive seen have more weight on the driver's side without driver on board. The front is usually worse than the rear L/R balance.
Anything you can do to move weight to the passenger side will help RH turn performance. When you remove the battery from the right front, you are further unbalancing the Left/Right front weight balance. It's worth it to move the bat to the rear, but the L/R ft balance will suffer.
David
67 RS Camaro, 69 Camaro vint racer, 65 Lola T-70 Can-Am vint racer.
ourworld.compuserve.com/homepages/David_Pozzi/
perkidelic
Registered User
Posts: 552
(5/21/04 6:40 am)
Reply in this corner...
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Glad you brought corner weights into this discussion.
The best known SBC Fiero setup is a guy who works for GM (engineer). His car's corner weights are:
F(l/r) - 648/624
R(l/r) - 762/783
For a mid-engine car the desired balance is about 45/55% (f/r) and as you can see he is almost right on it. His corner weights are pretty darn close too. His conversion is not typical because it uses a less popular kit that moves the transverse engine and transaxle 2" to the left. His battery was moved to the extreme left side of the engine bay, rather than up front where most performance Fiero guys go with it. I guess they were looking to improve the rear corner weights.
Fieros have the gas tank in the center hump so it doesn't have much effect on f/r, l/r, or corner weights. It is also as low as you can get it in the car. The drawback is it's small (10-12 gallons).
It was tested on GM's Milford Proving Grounds (think I got that right) by a professional test driver and recorded 1.17g on the "pad"!
I have a question - shouldn't corner weights be done with the driver, fuel, etc., on board? What's the point of getting the corner weights perfect then slapping 150-250lbs on the left side of the car? Or am I missing something?
89.4" - geez that's short. That car was probably a blast to drive - light and powerful. 911's are actually high polar moment cars though, which explains some of their stability.
perk
Edited by: perkidelic at: 5/21/04 6:59 am
walapus
Registered User
Posts: 194
(5/21/04 9:10 am)
Reply don't mean to hijack the thread, but
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" Depending on the body shape, you'll get better Cd if you chop off the back end abruptly. This (and to minimize overhang) is the reason why C5/C6 have really short rear deck.
I don't think that is correct, or at very least it is a gross simplification. I have always understood it to be true that the rear is MORE important than the front. For an example of this, think of the most efficient shape: a tear drop. It is relatively blunt on the front end, but the smooth transition at the rear is critical to control seperation, turbulence and hence extremely chaotic low pressure areas which create drag.
Usually, the reason cars are blunt in the rear are to reduce weight and size for handling and economy. Percentage wise, for a street car, it is wiser to sacrifice a little aero in favor of weight and MOI.
The corvette I believe has a lot of subtle accents, such as the lip at edge, underbody air control, etc, that lessen the effect and can reduce the negatives of the abrupt end, but make no mistake they are negatives.
davidpozzi
Moderator
Posts: 1124
(5/22/04 2:48 pm)
Reply
Re: in this corner...
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I have an article on a Fiero buildup by Pontiac. It's a full tube chassis with four cyl engine for road racing. I think it ran IMSA GTU class. (under two liter) Chief engineer was John Callies, aero was done by two pontiac engineers and wind tunnel tested.
The article says: "To limit lateral weight transfer with a rear wheel weight bias, the rear track had to be wider than the front-and the front track had to be compatable with the overall styling theme of the car and because IMSA and SCCA rules prohibit a raddical departure from OEM styling".
The car used a 56" ft track, 63" rear track.
Ft tires 23.5 x 11.5 x16
R tires, 25.5 X 12.5 x 16
Diversified Glass products did the bdywork. 313-373-7575
Ground clearance was 1/4" at full suspension bump.
A 29 gal fuel cell was fitted "in the stock location" Don Allen made the fel cell PN 2M4 503-479-2949
Huffaker made copies of the tube chassis. and built suspension links, aluminum pannels, and sold complete cars.
blueprints are Huffaker part #70-801
Super Bell Axle co made front and rear aluminum uprights. 209-445-1601
Webster gears did special ratios for the trans. 415-388-2728
I couldn't find F/R weights for this car, a V8 would be different anyway.
I read an intreresting passage about mid engine cars needing more rear aero downforce...
Lots of photos but not a lot of tech details or data in this article.
David
67 RS Camaro, 69 Camaro vint racer, 65 Lola T-70 Can-Am vint racer.
ourworld.compuserve.com/homepages/David_Pozzi/
jp455
Registered User
Posts: 292
(5/23/04 4:03 pm)
Reply aero...
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Hi guys,
I think one of the main things we failed to discuss is that the long wheel base stability thing is usually applied to road cars which try to achieve a comfortable ride. The longer wheel base helps cure the bobbing while dealing with bumpy roads which is not the case with a race track. But, race cars tend have longer wheel bases so they can take advantage of underbody aerodynamics. The longer the bottom the more surface area they have to create down force and to manage the air flow out from the back to create a smooth transition so there is less drag. Longer bodies also help make it possible to make a more teardrop shape for the body itself again for the whole drag thing. A sweet example is the moby **** series 911...of course, only when the rules allow it!
manuel scettri
davidpozzi
Moderator
Posts: 1127
(5/23/04 11:04 pm)
Reply
Re: aero...
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I agree that pitch sensitivity is higher with a short wheelbase. A low center of gravity is more important to have if the wheelbase is short.
Imagine running an open road race and hitting normal road bumps at very high speeds in a tall short wheelbase car!
David
67 RS Camaro, 69 Camaro vint racer, 65 Lola T-70 Can-Am vint racer.
ourworld.compuserve.com/homepages/David_Pozzi/
katz
Unregistered User
(5/24/04 4:51 pm)
Reply re
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About aerodynamics stuff...
I got the info from a source that I thought was credible. It didn't make sense to me, but I'm no aerodynamicist so I just took their words.
I need to make one more correction. Even if you built in 100% Anti-'s in suspension, load transfer still takes place. The amount is determined by CG height and WB.
Pontiaddict
Registered User
Posts: 115
(5/24/04 10:32 pm)
Reply --
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This might explain it. A new beetle isn't exactly a teardrop shape though.
www.autospeed.com/cms/A_1...ticle.html
here are some other cars the magazine did some aerodynamic articles on:
Some wind tunnel shots of newer daimler chrysler cars
www.autospeed.com/cms/A_1...ticle.html
newer Subaru impreza and 1991 Lexus L4400
a new rx-7 www.autospeed.com/cms/A_1...ticle.html
This is just some interesting and odd aerodynamic ideas. I stumbled onto it trying to find the first article
www.autospeed.com/cms/A_1...ticle.html
Pontiaddict
Registered User
Posts: 116
(5/24/04 10:33 pm)
Reply Re: --
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www.autospeed.com/cms/A_1...ticle.html
oops forgot the subaru/ lexus article
davidpozzi
Moderator
Posts: 1131
(5/24/04 10:43 pm)
Reply
Re: re
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The rear is very important to aero drag. The long tail approach might yield a lower drag on a LSR car, but if you add in the needed rear aero downforce for road racing, to keep the car from lifting at high speeds, then the shorter tail with spoiler will probably win as a prefferred version.
Once you have the spoiler, there is no point in having more tail section behind it. Notice the high flat rear deck area on the Fiero, a spoiler backs up air and the air presses down on the deck, good for downforce even though the rear window is notched like a C3 vette.
I saw cars running at Muroc and the dust kicked up by some of the old ford roadstersc with vertical radiators was very apparent. there was a cloud of dust right under the front axle.
There is a great aero book by Joseph Katz, "Race Car Aerodynamics: Designing For Speed" I met one of his students at a race and got one of the early copies of this book.
David
67 RS Camaro, 69 Camaro vint racer, 65 Lola T-70 Can-Am vint racer.
ourworld.compuserve.com/homepages/David_Pozzi/
perk
Unregistered User
(5/26/04 7:16 am)
Reply so good
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Man this is great, I'll be back in it as soon as SBC finds a tech guy who isn't on strike to fix my freakin phone line!!!! (at the library now but can't stay long enough to get in this)
perk
