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chicane67
08-16-2004, 11:53 AM
Ralph L
Moderator
Posts: 3251
(5/17/04 7:25 pm)
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Technical Discussion - Track Width
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How about a nice friendly techincal discussion concerning track width, and also wheel width? This thread could go in either suspension, or wheels and tires, but I'd like to focus more on the "tuning" of race suspensions in relation to track width, and tire size, instead of tire fitment

Just how important is track width when designing a "race car"? For instance...let's just say someone was building a purpose built first or second gen Camaro, if they were to run 315's up front and 315's out back with stock flares, but with the tires tucked up under the car, how much more of a benefit would they see if they went with 345's or larger slicks all around with 2 or 3" flares thus increasing the track width by atleast 2 or 3"? Basically how much of a difference would you notice both on the track and in raw numbers such as 200 and 300 foot skid pad data, and slalom.

Also, when do wide tires start becoming detrimental? At what point on a race car do you say to yourself, okay...this is just getting TOO wide? I know they now make 400 series 18" race slicks, and it alwasy seems as if the mentality (atleast the way I think) is that wider is better...why not run 400's all the way around with some huge flares, and narrowed frame/back halved car...why would you want to stop at say 335 series tires up front, or 315's? I think that GT1 cars like the C5-R run 385's out back, but I'm not too sure of the front tire size.

Also, if fitment wasn't an issue, would it be better to run the same size tires all the way around, or a larger width tire out back?

Experience racers, fabricators, suspension designers, anyone, feel free to add opinions and thoughts.

Thanks for your time!

Ralph
Project Fantom Expected Completion - May 2005
My Tahoe - "Black Mamba"
Rendering of Project Fantom

Edited by: Ralph L at: 5/17/04 7:26 pm

walapus
Registered User
Posts: 193
(5/18/04 9:11 am)
Reply suspension layout
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The school of thought I was given was that you work your way in. One of the first things selected is your tire and wheel size, next is your track. It's not supposed to be an after thought. Weight transfer is DIRECTLY related to it, and since you can't do hardly anything without knowing your weight transfer, track width is a variable which must be defined early on.

Note that you also cannot calculate weight transfer without cornering loads, and in order to know those you must know what your tire can support (coefficient of friction), and hence tires come before anything else.

As for tires being too wide: I think there are diminishing returns, but I don't have a solid argument beyond my intuition. I keep thinking about the differences in what's going on at the inner and outer edge of the tire, how it will be harder and harder to maintain even wear and how heat cycling could prematurely damage the tire, but I'm totally blowing smoke and I'd like to hear what others have to say.

Mean 69
Registered User
Posts: 22
(5/18/04 9:58 am)
Reply Good question!
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Without question, the tires are the most important thing with regard to handling. We go through all of this fuss with the suspension in efforts to keep the tires vertical to the pavement, in all situations. This is where they make their most grip. Of course, because the car rolls, bumps, crests, skids, and all other issues, the suspension is really hard to get right.

You will definitely reach a point of diminishing returns by going to a wider and wider tire, but darn it, for our packages, we will probably never get there. Granted, a 400 mm wide tire is an interesting thing to think about, but really not practical.

The wider a car is, the less weight transfer (laterally). This is an easy thing to visualize, and really easy to calculate too. So why not just make the darned thing as wide as possible? Well, one reason is that the wider you make a car, the harder time it will have with really tight turns, again it all comes back to a compromise. Think about the F1 cars, they have to do it all, straight line, hairpins, and everything in between. Looking from above, the track width on those cars is actually pretty modest. For them, a huge motivation is aerodynamics, the resistance to air goes as the square of frontal area, basically.

Weight is another consideration, again, not too huge an issue with our already far-less-than-optimized cars, but tires are 100% unpsrung weight. The heavier the weight, the slower the natural reaction to bumps, etc. Keeping the tires on the ground at all times is really important to maximizing traction.

All that said, a 3000 lb. plus sedan with a front engine will benefit from more front tire. I can't see that in performance handling application that a 315 tire on the front for our cars would be anything but really good. Tucking it under the fenders is a challenge for sure. I attended a NASA race a few weeks back at Cal Speedway, and there was one car there that really impressed me in the American Iron Extreme series. It was a 65 Mustang, 315's on all corners, independant race team. The car took the pole on Saturday, against the likes of the Grigg's racing team, Maximum Motorsports, etc.

Mark

katz
Unregistered User
(5/18/04 10:50 am)
Reply re
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Mean 69 already said half of what I was about to say, but here it goes...

When it comes to racecar design, track width is usually limited by rules. I was under the impression that most designers push the track width as far out as the rule allows, but on high speed vehicles running on fast tracks, increased frontal area definetely becomes a major issue. On a related note, rolling tires on open-wheel cars have HUGE aero drag.

Although I have a decent theoretical understanding of susp. geometry and dynamics, I must admit that I have very limited real-world experience since I've never had an opportunity to race on track myself. So it's hard to say what kind of actual effects and benefits can be expected from increased track. However, I can quantify theoretical benefit on a skidpad.

I'll use Art's '55 Chevy as an example. It's not b/c I'm trying to advertise, but b/c I know the exact dimensions of this particular vehicle and its performance limit. The car's front tires broke loose at 0.935G on a 200-ft skidpad. The car has 57.5" front track width with sprung-mass CG height of 24.35" (22" overall, including unsprung mass). It weighed 3500 lbs w/ driver with 51.5% over the nose. With all stereo equipment, A/C and all other stuff, it's a pig now. Anyway, at the limit of adhesion, front weight transfer is 683.2 lb, with outside tire carrying 1575.7 lb and inside tire 209.3 lb.

Now let's increase the front track by 3" to 60.5" with everything else being equal. At the same lateral acceleration level, front weight transfer decreases to 649.3 lb (1541.8 lb outside, 243.2 lb inside). Since I know that the tires we used can maintain lateral acceleration until front weight transfer reaches 683.2 lb on this particular vehicle, new lateral acceleration limit can be reverse calculated, assuming the rear tires will be able to hold up. Limit increases to 0.989G. It's a significant increase.

Again, this is just theoretical number. As G number increases, body roll angle also increases. Slight camber change will affect the acutal traction limit of tires. You'll have to increase roll resistance to maintain the same body roll angle. You could increase camber curve so that outside tire will remain at the same camber at increased body roll, but then you're likely to loose some traction on inside tire. It's pretty complex.

As for the tire width, it's limited by the rules also (track width and allowable body modification will dictate the max size). If ultimate traction is the only concern, wider the better. But you'll have to take other things into consideration, like increased drag for example. Straight-line stability will suffer (especially on front tires). Wider tires are more sensitive to camber so you'll have to alter camber curve, which typically decreases straight stability further. Fuel consumption increases as a result of increased rolling mass and resistance (more pit stops). On a bumpy track, suspension wonít be able to react as fast due to increased unsprung mass. Like Mean 69 said, keeping the tires on ground is extremely important. The list goes on and on. These may not be big concerns on smooth short tracks, but itís a different story on fast tracks. It also depends on vehicle setup. I can see a point of using wide rear tires on high-hp, solid rear axle cars since solid axle does a decent job of maintaining wheel camber, and axle can be narrowed by using neg. offset rims to save some unsprung weight. But then again, you could go too wide. As the axle gets narrowed, spring track also gets narrowed (think of pro streeters with coil overs 15" apart). Dampening efficience decreases when it really needs to be increased for heavier tires. IRS is very tricky. Finding a good compromise of camber curve is very difficult. It really cannot be narrowed like solid axle since you really should have very small scrub radius, preferably on negative side (thatís why you see flat-faced rims on racecars in Le Man GT- classes, and you see deep-dish rear wheels on Trans Am cars). All the while, you'll also have to think about roll center stability or migration, bumpsteer curve, anti-squat, side scrub, etc. Itís very involved and honestly, it gives me headache. One of these days I'd like to design IRS, but only when I have lots of time to think.




davidpozzi
Moderator
Posts: 1119
(5/18/04 2:15 pm)
Reply
Re: re
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Wider track width can achieve a result similar to lowering the Center of Gravity height, -less weight transfer allows the inside of turn tires to carry more of the load, generate more total traction for the car.

Multiply tire vert load by the tire perf curve, = available cornering traction generated by the tire. The outer tire is heavily loaded and gains less traction per lb of load added, the inner tire is on a very high traction to load part of it's perf curve and will generate much more traction per lb of vert load, so you gain more total cornering force.

A wider tire is almost ALLWAYS BETTER for cornering. A wider tire can have a softer compound and gain traction from added surface area in contact with the pavement. I'm referring to tracks where you corner hard, not something like a Daytona Speedway with restrictor plate racing. Once you are cornering at full throttle and not sliding anymore, there is not going to be a gain from going wider.

Autocross cars can be hurt by excess width. The car has to make a sharper radius turn if it is a little wider. Longer wheelbase is a dis-advantage too, it requires a sharper turn of the front wheels. The rear wheels track over to the inside and you have to stay away from the cones more with the front wheels to clear the rear wheels.

F1 has a track width limit to try and limit cornering speeds, but they still corner amazingly! F1 CG's are BELOW THE SPINDLE HEIGHT!
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: 5/18/04 2:24 pm

Ralph L
Moderator
Posts: 3257
(5/18/04 3:38 pm)
Reply
WOW!
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Thanks for the responses, that is EXACTLY what kind of feedback I was hoping to get!!

So, like everything else in life, it's all a compromise, but if maximum traction and cornering speed is your ULTIMATE goal, a LOW center of Gravity, and WIDE tires front and rear(with a suspension designed AROUND the tires) would be optimal.

So just for fun, add/modify/remove items from this checklist that you would try to incorporate in your optimal all out-street-legal race car

I'm thinkin':
- Channeled frame to reduce COG
- Engine/Tranny as low as possible, along with all accessories to reduce COG
- 345's all the way around
- 3" or so fender flares to increase track width
- Modified Camber curve to compensate/take advantage of large front and rear tires

What are some other ways to lower COG?

Thanks again for the responses, a good technical discussion is always beneficial to all, I think .
Ralph
Project Fantom Expected Completion - May 2005
My Tahoe - "Black Mamba"
Rendering of Project Fantom

katz
Unregistered User
(5/18/04 4:45 pm)
Reply re
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I wouldn't use 345s on front, but that's just me.

To lower CG height...

- glass or carbon exterior parts...hood, bumpers, fenders, door skins (assuming you'll have a cage), cowl panel, trunk lid, etc. Acid-dipped body shell.

- Lexan glass. Permanent-mount quarter glasses.

- light weight interior parts...seats, steering column (stock 1st gen column is damn heavy), fab'ed dashboard, etc. No carpet & sound deadner, stereo, backseats, A/C.

- Battery mounted in rear seat area...this is better than mounting it in trunk even though weight distribution number will be "less desirable". Concentrating the mass closer to vehicle center is better than achieving theoretical 50/50 distribution. In a similar sense, you can fabricate a firewall behind the front seats and move the fuel cell to ahead of rear axle.

- light weight driver...on second thought, a beer-belly'ed couch potato may result in lower CG than a guy who works out regularly, but I have no scientific proof.

There are lots of other stuff you can do, like dash mounted rear view mirror instead of hanging it from roof, floor-mount pedals and M/Cs, trimming drip rails, late-model plastic door handles, Oval-tube exhaust pipes mounted low, etc. Every little bit counts.

Ralph L
Moderator
Posts: 3261
(5/18/04 5:30 pm)
Reply
Re: re
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Katz,
345's do seem a bit extreme up front...how about a 315/345 FR/RR combo? I like the rear tires a bit bigger anyways...

I'm doing a few of the things you mentioned on Fantom, and a few years down the road on my next project, I will incorporate more, if not all of these ideas and then some. That car will be a hardcore racer, Fantom is more of a weekend racer/street machine...

But on Fantom..I'll be using a glass hood, and a fab'd aluminum spoiler...Gotta go with regular glass since it's going to be more a street car then a track car...I got rid of the bumpers all-together. Deleting the back seat and A/C as well...still torn on a stereo...Steering column will be gone, some of the dash will be fab'd alum. but not full custom..don't have the resources close enought to me to get that done yet...got me thinking about the back seat battery, though...I may have to do that, I'll have to see how it plays out further down the road.

Thanks for the comments!
Ralph
Project Fantom Expected Completion - May 2005
My Tahoe - "Black Mamba"
Rendering of Project Fantom

jeffandre
Registered User
Posts: 666
(5/19/04 7:38 am)
Reply Re: Katz
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Could one use the Optima inline 6v batteries and place one on each side of the rear seat area? I was thinking of putting one on each side of the rear seat cushions under/inside the armrest area...
Jeff Andre'

katz
Unregistered User
(5/19/04 10:27 am)
Reply re
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Ralph,
315/345 sounds fine, but like I said, it's JMO. 345/345 may work well depending on setup and situation.

Define the primary use of your racecar first - there are many kinds of racing (autocross, road racing, open road racing, etc), then build it to suit your needs. My car, for example, will be built primarily for open road racing and dry lakes, so I'm sticking with good o'l 275/315 for pavement and skinny GY LSR tires for salt and mud flats. I'd even go skinner depending on finished vehicle weight (due to concerns of load rating of rims & tires).

Jeff,
That's a pretty damn good idea. I've seen street rodders mount those two 1/2 batteries in trunk just so they can hide them behind trick upholstery, but I never thought of using them that way.

I don't know what kind of car you're working on, but depending on the height or thickness of your backseat cushion, you may be able to lay them on their sides and mount them right next to the tunnel. That will move the mass further towards the vehicle centerline and still keep the backseats completely intact. I'd say go for it!

perkidelic
Registered User
Posts: 549
(5/19/04 6:11 pm)
Reply wow
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Great thread Ralphy!


Quote:
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Steering column will be gone
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You won't need any of this advice then - didn't know you were such a dedicated straightliner j/k

I'd like to get these guys in a conference room for a week and just inhale the knowledge and wisdom

perk - todd's hot rods

jeffandre
Registered User
Posts: 669
(5/20/04 6:40 pm)
Reply Re: re
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Katz,
71 Camaro, so not sure if I can fit the two batteries in the interior, but I will strongly consider placing them low int he seat backrest area. Maybe directly behind the rear seat partion (in the trunk), but that would put them higher than I would like though. Maybe I can stuff them somewhere as close to the center of the vehicle while keeping them low. I figure the relatively thin Optimas could be mounted under the car possibly, in some type of light enclosure (I am 99% certain that I will be using the same type of side exhaust the Baldwin Motion Camaros had).
Jeff Andre'

katz
Unregistered User
(5/24/04 4:43 pm)
Reply re
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Let us know how it works out.

Mean 69
Registered User
Posts: 28
(5/24/04 8:41 pm)
Reply More Importantly...
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No question, distributing the weight and keeping it centered as close as possible to the CG of the car is important, but let's face it. Only so much we can do. Let's accept the fact that weight transfer happens, and design the suspension to deal with that fact.

I really love Herb Adams' opening sections, the message is that the tires are the most important aspect to handling, and that they make the most grip when they are perpendicular to the pavement, period. You can't prevent weight transfer, it is a matter of acceleration. What you can do is design a system that deals with this undeniable fact, and wring out the compromises that come along for the ride. Heck, if our cars were really light, most of all of this geometry stuff wouldn't matter.

Keep the tires perpendicular, and not loaded beyond their inherent traction limit.

I really like this thread, let's keep it going with some more good tech!

Mark

fiorano
01-17-2008, 03:25 PM
witha narrow track car can the same be accomlished concerning wieght>?

at 2000 lbs and 52" track-solid axle?
can i pull the G's i need?
r better put can this be made to work competitively:hmm: