LocostUSA.com

Learning how to build Lotus Seven replicas...together!
It is currently January 22, 2018, 2:36 am

All times are UTC - 5 hours [ DST ]




Post new topic Reply to topic  [ 8 posts ] 
Author Message
PostPosted: June 12, 2011, 8:15 pm 
Offline
Toyotaphobe
User avatar

Joined: April 5, 2008, 2:25 am
Posts: 4533
Location: Fort Worth, Texas
After just about an overload of racing this weekend (on TV not in real life) something I saw, or several somethings that merged together in my mind, made me begin to wonder if you had a wider track than the front could you use smaller tires & still maintain balanced handling? Presuming you weren't over powering the tires with the engine.

I think it might have been a pic of the new Indy delta shaped car that began this thread of thought, but also it's been the rash of posts where people are trying to find a narrower rear end to fit the Locost.

At LeMans the prototypes are using much wider front tires than ever before, but I know from personal experience that using the widest tire you can fit under the fenders might increase the overall grip level, but if you go too big it also tends to dumb down feel.

So is there a trade off that could be made by simply widening the rear track and keeping the smaller tire size? Something that gives you that razor sharp feel of handling and yet gives you increased limits and a good balanced feel?

I realize that on the traditional Locost there's definitely a limit or else you destroy the looks, but possibly on the bike engined cars the smaller tires might lessen some of the drivetrain/clutch wear. I know on my S2000 narrower tires seem to be easier on the clutch since it's easier to spin the tires which lessens drivetrain shock.

If this is a viable trade off then is there any type of accepted formula for this trade off?

I have nothing in mind this is more on the line of the National Enquirer where enquiring minds want to know.

_________________
mobilito ergo sum
I drive therefore I am

I can explain it to you,
but I can't understand it for you.


Top
 Profile  
Reply with quote  
PostPosted: June 13, 2011, 12:46 am 
Offline
User avatar

Joined: November 7, 2008, 4:48 am
Posts: 1094
Location: snow city - it's wet!
I think your question really breaks down into a number of separate questions depending on case. I'm not going to try and answer everything, but I'll take a shot at a couple of the easier cases.

Would a wider track allow a car with narrower tires to maintain the same level of cornering grip?

Possibly, but it would depend on the change in tire width and the type of tire used. Increasing track width reduces weight transfer during cornering; that would allow each tire to carry a more even portion of the load. A quick formula is (and we still need to separate front weight vs rear weight):

Lateral Weight Transfer (lbs) = ( Lateral Acceleration (g) * Weight (lbs) * Center of Gravity Height (inches) / Track Width (inches)

The weight transfer would be working against the tire's friction curve. I'm nut sure of how street tires trade traction for friction; with race tires, more load (in lbs) usually both increases the total traction available (in lbs) while their coefficient of friction actually reduces slightly under load. However the friction decrease versus load curve is usually gentle enough that more load equals more grip; at least until you exceed the design envelope of the tire. If you are lucky enough to have friction curves available for different size tires (or have access to proper support for the tire), and you've already worked out your weight transfer at each corner, then:

The tire's coefficient of Friction * (Lateral weight transfer for the tire (lbs) + Static weight on the tire (lbs)) = Tire grip needed (lbs) for that wheel when cornering at that specific lateral G

Which would let you work out how wide a tire you needed.

Why are larger front tires becoming more common?

In a the case of using larger front tires, designers are getting better at making race cars balance more closely to 50/50 weight distribution (especially with aero) and the larger front tires also increase the contact patch available to the front brakes, where most of the braking force is generated. A formula for longitudinal weight transfer is:

Longitudinal Weight Transfer (lbs) = Acceleration (or Deceleration) (g) * (( Weight (lbs) * CG Height (inches) ) / Wheelbase (inches) )

Again, you would need to know the tire's friction curves to size it correctly. Talking with a knowledgeable support engineer for the tire you plan to run can generally produce the information required.

_________________
.. in the world


Last edited by erioshi on June 13, 2011, 1:08 am, edited 2 times in total.

Top
 Profile  
Reply with quote  
PostPosted: June 13, 2011, 12:58 am 
Offline
Toyotaphobe
User avatar

Joined: April 5, 2008, 2:25 am
Posts: 4533
Location: Fort Worth, Texas
At LeMans the wider front tires are helping increase total grip. The new Delta shaped Indy car has such a narrow front track I speculate they will be running pretty large tires.

The more I thought about the Delta shaped Indy car the more it made me wonder if the extra wide rear track (as opposed to the very narrow front track) allowed narrower and cheaper/more common rear tires to do the cornering and balance work that would take wider tires on a car with a more traditional front and rear track that are very nearly the same.

It just made me wonder if doing it that way might not balance things out, improve total cornering ability and yet still leave the vehicle feeling more nimble than just stuffing more tire under it. It also seemed that might open up more options for rear ends, albeit on something with a little less traditional 7 shape.


Like I say, just a mental exercise brought on by racing overload.

_________________
mobilito ergo sum
I drive therefore I am

I can explain it to you,
but I can't understand it for you.


Top
 Profile  
Reply with quote  
PostPosted: June 13, 2011, 1:05 am 
Offline
User avatar

Joined: November 7, 2008, 4:48 am
Posts: 1094
Location: snow city - it's wet!
carguy123 wrote:
At LeMans the wider front tires are helping increase total grip. [snip]

Exactly, wider front tires mean more total contact tire patch is available, and thus more weight and aero load can be placed at the front to increase cornering speeds and front braking loads. The rear loads are maintained, and the front end can be made to work harder.

_________________
.. in the world


Top
 Profile  
Reply with quote  
PostPosted: June 13, 2011, 8:58 am 
Offline
User avatar

Joined: November 12, 2008, 6:29 am
Posts: 3426
erioshi wrote:
carguy123 wrote:
At LeMans the wider front tires are helping increase total grip. [snip]

Exactly, wider front tires mean more total contact tire patch is available, and thus more weight and aero load can be placed at the front to increase cornering speeds and front braking loads. The rear loads are maintained, and the front end can be made to work harder.


Audi did it not to improve lap times but to spread the tyre wear so they could go 5 stints and save more time in the pits (less wheel changes). They took weight from the rear to the front to do it both physically and aero (they decreased the rear loads) - that's from a Dunlop tyre tech who was there BTW.

Wider track with narrow'er tyres is a hard one but generally yes it will work out equally in the real world (if not on paper) as most suspension geometry is bad anyway so you will end up around the same overall combined contact patch with a small increase in stability from less and slower lateral weight transfer, stiffer lighter wheels and shorter tyre wall heights (using the same aspect ratio). The car will be a little easier to drive with more feel and this is an advantage in itself for the majority of drivers even if it had ultimately less grip.

Lets not exaggerate this though, maybe a 2" to 3" track width per width reduction and only to a point (say 215 down to 205).


Top
 Profile  
Reply with quote  
PostPosted: June 13, 2011, 2:07 pm 
Offline
User avatar

Joined: November 7, 2008, 4:48 am
Posts: 1094
Location: snow city - it's wet!
Your are right about the changes being very small. I think the last time I looked at this, the difference in adding 4" to the track of a sports racer that weighed approximately 1,000 lb was about 40 lbs more potential cornering force per outside tire. Dropping the center of gravity by 1" produced about 45 lbs more potential cornering force per outside tire while keeping the original track width.

Also worth noting is that increasing track width usually comes with more associated "costs" to performance in other areas than just lowering a car's CG. Usually weight increases slightly because of longer suspension arms and wider fender and/or body work. Also a wider track generally means more frontal area, and thus a bit more overall drag; probably not important for a street or AutoX car, but it could be an issue for a track car.

_________________
.. in the world


Top
 Profile  
Reply with quote  
PostPosted: June 14, 2011, 11:46 am 
Offline
User avatar

Joined: November 12, 2008, 6:29 am
Posts: 3426
erioshi wrote:
Usually weight increases slightly because of longer suspension arms and wider fender and/or body work. Also a wider track generally means more frontal area, and thus a bit more overall drag; probably not important for a street or AutoX car, but it could be an issue for a track car.


Good points, ultra wide tracks are more common now in order to fit larger/wider diffusers under the car for more downforce which gives a greater lap time advantage than the losses.


Top
 Profile  
Reply with quote  
PostPosted: July 1, 2011, 2:15 am 
Offline

Joined: April 16, 2011, 11:05 pm
Posts: 239
Location: Melbourne, Australia.
A very few inches of extra track width may not seem to offer much in the greater dynamic weight transfer scheme of things, but it does have one major advantage.
Longer suspension links become possible.
And that allows either more suspension travel, or much smoother camber curves with less track change.

My own belief is, that with the skinny steel belted radial street tires we mostly use, sudden forced changes in track with vertical suspension movement, forcing the tires to scrub sideways will be far more detrimental than giving up the last 0.5 degree of camber control.


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 8 posts ] 

All times are UTC - 5 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
POWERED_BY