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ShinnyGroove (Forum Supporter)
ShinnyGroove (Forum Supporter) HalfDork
8/3/21 3:58 p.m.

I know this is a zombie thread that just came back to the top, but I've been thinking about it.  Keith's post about power/grip ratio and how it changes the racing line "feels" right but I can't conceptualize why.  So thinking like an engineer, what are the boundary conditions?

Boundary condition #1:  All grip, no power.

This one seems pretty simple.  Assume you are in a soapbox derby car with rubber tires and brakes but no motor, and you have to navigate a race turn.  The fastest line through the turn will always be max cornering speed on the "geometric" line- constant turn radius, no early or late apex.  It's fastest because it's the shortest distance around the corner, and it maintains the largest turn radius possible without ever requiring a tighter radius that would scrub speed.

Boundary condition #2: All power, no grip.

Harder to conceptualize, because there has to be some amount of grip or you can't turn at all.  In fact, you can't have power without grip either, so this boundary doesn't really exist.  So the boundary condition here is a car that has exactly the amount of power needed to break the tires loose at all RPM's. 

Based on boundary condition #1, we know that any deviation from the geometric line is going to cost time by either covering more distance or scrubbing speed.  The friction circle dictates that the tires only have so much grip; any grip you use to accelerate will come at the expense of lateral grip and thus cornering speed.  By deviating from the geometric line, you are decelerating to make a tighter turn somewhere, so you can make a looser turn somewhere else to accelerate.  The bet you're making is that the magnitude of acceleration is greater than the magnitude of the deceleration.  This seems like it violates some fundamental law of nature to me; the total amount of grip is finite, and is the only limiting factor for corner speed AND acceleration.

In real life nothing is linear or perfect (least of all drivers) and accommodations need to be made.  However, this logic still strongly suggests to me that the comment "every car is a momentum car" is correct in principle.  Somebody tell me what I'm missing?

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
8/3/21 5:11 p.m.

You're assuming that a lower power/grip car has enough acceleration available to exhaust available traction :) You're also forgetting what happens after the corner.

With the high power/grip car, the earlier you can accelerate the more speed you'll have down the straight. If you can give up 3 mph mid-corner by taking a less-than-ideal line, but gain 5 mph by the time you get to corner exit, you benefit. So it's worth squaring off the corner somewhat in that case, you're setting yourself up to apply that traction longitudinally as soon as possible. Your total time in the corner may be longer but your time on the straight will be less.

Same thing happens to some extent on corner entry.

I'm lucky to have access to very similar cars with radically different power/grip ratios. I definitely have to adjust my lines to get the best lap times, and it's more obvious on some cars than others. In one case - a chicane exit - it's worth taking a line that goes diagonally across the track in the high power/grip because I can get on the power sooner and longer. In a car with the same suspension and tires but half the power, I take a more arcing line to maintain as much speed as possible down the short straight. My line through the chicane is almost identical because there's no way to deploy the power anyway.

Of course, it's not black and white. As noted, it's relative. And as soon as you're trying to deal with another car mid-corner, the rules change :)

codrus (Forum Supporter)
codrus (Forum Supporter) GRM+ Memberand PowerDork
8/3/21 5:32 p.m.

The other thing you're missing is that by taking a larger radius you're travelling a longer distance.  Yes, you're travelling at a higher speed for that distance, but it's not necessarily enough to make up for the time.  The wider corner radius gives you a higher exit speed at the cost of more time in the corner, the idea being that the higher exit speed will translate into less time spent in the next straight.  The longer the following straight the more this tradeoff is worth it, which is why "hugging the inside" at autox is sometimes worthwhile when there's little-to-no straight after a corner.

The other thing that affects this tradeoff is the amount of power you have in the car.  The more spare horsepower you have the less you need those bonus mph on exit, so with enough power it becomes worthwhile to take a smaller radius through the corner in order to minimize the time spent cornering and get on the power sooner.

One argument for the definition of a "momentum car" is one where you're very rarely (never?) making a choice to sacrifice exit speed to favor earlier throttle application.

 

Tom1200
Tom1200 SuperDork
8/3/21 7:34 p.m.

In reply to codrus (Forum Supporter) :

I've always differentiated the two this way.

If you are focusing on getting the maximum grip for all four tires at once, you are in a momentum car.

If you are focusing on foward bite / getting the car up on the rear tires you are not in a momentum car.

rustomatic
rustomatic Reader
10/7/21 11:38 a.m.

Okay, try this one:  Find a kart rental that has the standard B-S L0206 which makes sub-10 hp.  You can have a good time with that, especially on a tighter track, or on one that's opened up enough to where you barely brake at all (momentum, yo).  Then, see if somebody will let you drive a kart with 50 hp.

See how using those pedals makes you feel now?  The difference is massive, and reasonably, many would rather stick with the sub-10 hp kart . . .

Being reasonable severely limits the availability of fun.

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
10/7/21 12:15 p.m.
codrus (Forum Supporter) said:

The other thing you're missing is that by taking a larger radius you're travelling a longer distance.  Yes, you're travelling at a higher speed for that distance, but it's not necessarily enough to make up for the time.  The wider corner radius gives you a higher exit speed at the cost of more time in the corner, the idea being that the higher exit speed will translate into less time spent in the next straight.  The longer the following straight the more this tradeoff is worth it, which is why "hugging the inside" at autox is sometimes worthwhile when there's little-to-no straight after a corner.

The other thing that affects this tradeoff is the amount of power you have in the car.  The more spare horsepower you have the less you need those bonus mph on exit, so with enough power it becomes worthwhile to take a smaller radius through the corner in order to minimize the time spent cornering and get on the power sooner.

One argument for the definition of a "momentum car" is one where you're very rarely (never?) making a choice to sacrifice exit speed to favor earlier throttle application.

 

 

A larger radius requires less maintenance power.

I recall many times taking corners in the widest possible arc in my Quantum at WOT and still slowing down from 75 to 65 by corner's end, because cornering took away more speed than I could maintain.

I know I am not a good driver, because I'm sure there was a way to maximize corner exit speed, and I couldn't seem to find it.

malibuguy
malibuguy GRM+ Memberand HalfDork
10/8/21 7:15 a.m.

I just got my 1nz swap into a Tercel done.  Granted it makes less HP then the motor that it replaced which was heavily turbocharged (106 vs 175)...yet going thru a 4speed automatic.

However with the swap along with the motor came the matching 4.31 final drive 5speed that came mated with it in Scions.

Compared to my other Tercel which has a mild turbo set up and 3.72s its nearly as quick in the 40-70 sprint that I like to measure cars HP in on a local level road.

A lot of people have already said to me "I cant wait for you to boost it" referencing the swapped Tercel.  And I loosely had that planned in the back of my head.  But after driving it some.  Its like spot on as is.  I still need to refine the car, replace the clutch and some other do-dads.  The car is not built for competition (tho I will probably take it to the drags and autoX eventually for fun) its primary purpose is a mountain therapy car.

Beer Baron
Beer Baron MegaDork
1/13/22 10:01 a.m.

On the road, every car goes effectively the same speed.

On the track, fast cars train slow drivers.

Shaun
Shaun Dork
1/13/22 12:18 p.m.
frenchyd said:
rustomatic said:

One metric that has been left out (somewhat) is the one that includes style of track.  Some tracks do not really reward horsepower as much as others.  From what I've been told, as an example, Road America has a straight that literally does not end.  From what I've seen, NOLA has something similar, depending on how they've configured it for the day.  From experience, both Laguna Seca and Sonoma can make you feel pretty happy with 350 HP (which is basically low power these days), depending on your skill (and grip) level.  As for my future track, I somewhat fear Road Atlanta, as I'm not sure if my car's brakes are up to task; grip and power will be sufficient, however.

On that note, who here has figured out how to make up time with brakes?  I did a fairly recent ride-along with a friend in a basically stock (Koni struts and track day pads) first-gen RX7 (not remotely poweful), and he did very well in not being passed by far, far faster cars on a track that favored higher horsepower (Thunderhill).  Granted, he would have killed for an LS swap, given less  

Brakes can be used in many ways. Not just to aide a fast lap. But also to gain position. The easiest to understand is if you have similar braking power you can position yourself to the inside as he swings wide to get his best corner speed. You can drive inside him and as you get your nose in front of his then you brake.  Naturally that will carry you deeper into the corner , off line. Now you are in front of him and he cannot accelerate. Yes your lap time will suffer but you have position.  
Another move is the outside pass position yourself to the outside. You are taking away his usual line slowing his corner speed making him brake harder and sooner. 
 

There are variations of both and counter moves 

Just remember position is king. 

Max agrees!!

MrFancypants
MrFancypants HalfDork
1/13/22 3:20 p.m.

As a semi-competent amateur HPDE warrior my view on this is pretty simple and comes down to $/event.

Aside from certain modified or exotic cars big horsepower tends to come with extra weight, big horsepower and extra weight leads to more heat, extra weight and more heat needs more brake and more tire, more brake and more tire means more $$$ per session.

Because of that I prefer sub 3000 lb cars for track days to maximize my track time per dollar spent, and typically these cars don't exceed 250 hp. I suppose these are regarded as "momentum" cars.

Big horsepower on the street can be fun because ripping it up to or just over freeway speeds doesn't cost so much extra that it's a problem. Buy the right car and it might not even be too far off the fuel efficiency of smaller lighter cars that it's even noticeable at the pump. But for a track junkie the costs can really add up.

trucke
trucke SuperDork
1/13/22 5:06 p.m.

Low HP cars can be a hoot when setup properly.  You can hoon all day under the speed limits and have a fantastic time!  Now I have a Civic Type R to be competitive in autocross and having to learn to manage a bunch of new and different variables. 

This year my oldest daughter wants to autocross so we will co-drive my older autocross toy.........

 

docwyte
docwyte PowerDork
1/13/22 5:11 p.m.

Low HP cars at a DE/Track event suck.  #1 I'm simply not entertained by them.  Going slow, is well, slow and not fun.  #2  It's simply not a fun time constantly watching your mirrors and sticking your arm out the window giving the pass sign.

It's one thing to be racing with a group of cars that all have the same HP/LBS ratio, it's another situation entirely to have 1/2 the HP of everyone else in the group.  That gets really old, really, really fast.

Tom1200
Tom1200 UltraDork
1/13/22 6:40 p.m.

In reply to docwyte :

Although I've replied in this thread ad nauseam I will add some comments; If you're not entertained by low power cars I get it, my son drove his Lexus at his first track day rather than my Datsun. 

Being in a low powered car doesn't mean you are going slow. 

At a track day earlier this month I was asked to run in the advanced group (we need to free up space in other groups). The Datsun currently has the back up motor which makes 80whp. The event allowed point by anywhere; this was not a big deal.  You simply point someone by when it works for you.............it's not a race so you are not holding up any leaders. It's pretty easy to see when and where someone is going to catch up to you.

If you are in an intermediate to advanced driver you should able to drive the car on the limit and keep track of what's behind you (my personal opinion).

I'm only commenting because there seems to be a widely held belief that if you bring a slow car your going to spend your whole day trying to stay out of the way...........which is not accurate.

 

 

  

RWP
RWP New Reader
1/13/22 7:05 p.m.

Great article and very accurate.  Momentum cars I've had and liked include 88 MR2, 99 Miata, 82 Scirocco (82 hp), 91 Mazda Protege.  All handled and stopped above their class.  140 was the most HP of any of them.  Just bought an 86 944....<150 HP.

maj75 (Forum Supporter)
maj75 (Forum Supporter) Dork
1/13/22 8:21 p.m.
Tom1200 said:

In reply to docwyte 

I'm only commenting because there seems to be a widely held belief that if you bring a slow car your going to spend your whole day trying to stay out of the way...........which is not accurate.

Only true if the drivers in the fast cars suck.  And even if the suck, they will still be ahead of you because they pull away on every straight.  Given equal or better drivers, you will be the slow car giving point by's all day and that's the truth.

Tom1200
Tom1200 UltraDork
1/13/22 8:52 p.m.
maj75 (Forum Supporter) said:
Tom1200 said:

In reply to docwyte 

I'm only commenting because there seems to be a widely held belief that if you bring a slow car your going to spend your whole day trying to stay out of the way...........which is not accurate.

Only true if the drivers in the fast cars suck.  And even if the suck, they will still be ahead of you because they pull away on every straight.  Given equal or better drivers, you will be the slow car giving point by's all day and that's the truth.

Yes, but point bys are not that big of a deal. People act like you have to pull over to the side of the track.

Spec Miata guys give each other hand signals mid corner all the time..........pointing someone by is the exact same thing.

Again pointing people by isn't a big deal, no clue why people think it is.

​​​​​​

 

frenchyd
frenchyd UltimaDork
1/13/22 10:10 p.m.

In reply to Tom1200 :

Point by's suck if you don't have anybody to dice with.   I can have a lot of fun pointing others by if I can beat them in corners. Or there are others about my speed.  
 But if I'm tail end Charlie and All I'm doing is driving it's no fun at all.   
      Maybe a newbie can get satisfaction out of doing by himself if his lap times improve. Or he can sort out the car etc. but wheel to wheel racing isn't like track days. 
   By the way that applies to fast cars too.  Running away from the field is boring.  It's not really racing. It's just driving fast 

MrFancypants
MrFancypants HalfDork
1/13/22 10:20 p.m.
maj75 (Forum Supporter) said:
Tom1200 said:

In reply to docwyte 

I'm only commenting because there seems to be a widely held belief that if you bring a slow car your going to spend your whole day trying to stay out of the way...........which is not accurate.

Only true if the drivers in the fast cars suck.  And even if the suck, they will still be ahead of you because they pull away on every straight.  Given equal or better drivers, you will be the slow car giving point by's all day and that's the truth.

I've been the guy in the fastest car doing the fastest times on track and I've been the guy in one of the slower cars on track. You know what's more frustrating than giving point bys? Spending most of the session behind slower cars waiting for your point by.

I'll happily enjoy watching a Corvette or GT-R scream off into the horizon from behind the wheel of a stock Miata after I point them by, it's a lot more fun than being never being able to get a fast lap in.

Tom1200
Tom1200 UltraDork
1/13/22 10:24 p.m.

In reply to frenchyd :

Track days are not racing. Their sole purpose is to simply allow one to drive their car faster than they could on the road. 

I have made the statement that I'd be perfectly content to drive on an empty track as I enjoy driving fast.

I enjoy wheel to wheel racing more but I still enjoy driving fast, much in the same way I enjoy riding my dirt bike. I ride fast enough to be fun but slow enough to come back in one piece.

350z247
350z247 Reader
1/14/22 8:21 a.m.

In reply to Tom1200 :

Nah, I'm with the others on this one. Spending the entire sessions giving point-bys to Corvettes and Camaros really ruins the session for me. I don't need to be the fastest car on track, but I definitely don't want to be the slowest.

docwyte
docwyte PowerDork
1/14/22 10:47 a.m.

In reply to Tom1200 :

Sorry but that's inaccurate.  In the high run groups, having say a miata, with 1/4 to 1/5th the HP of everyone else makes you a rolling chicane.  I don't care how good of a driver you are, because everyone else in that group is pretty damn good too.

Since those groups tend to have open passing, you constantly have to be scanning your mirrors for the traffic come up on you like a cruise missile, figuring out where to let them by and pointing them by.

Again, and again and again for the entire session.  Nope.  Not fun, not in the least.  Overcoming a certain amount of a HP deficit with skill can be fun but when you're just so vastly out gunned and everyone in the group is a pretty damn good driver, that's no longer fun

Tom1200
Tom1200 UltraDork
1/14/22 3:17 p.m.

In reply to docwyte :

So here is where I'm at; I can 4 wheel drift my car from entrance to exit and wave people past at the same time.....so for me it's not a big deal. I'm not alone in this. Watch in car video of Spec Miata guys, they're sliding through corners and using hand signals. For a decent driver it's not that hard.

Additionally if you put a little bit of forethought into passing, this goes for the fastest and slowest cars in the group, it makes it even less of an issue.

Also note I typically run in the intermediate group which is a better fit but running in the advanced group wasn't a big deal.

I'm not advocating people run out and get a car as slow as my Datsun or run a slow car in the advanced group. I'm just saying with the right attitude and some planning it's not a big deal. 

 

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
1/14/22 3:41 p.m.

In reply to 350z247 :

I never minded giving point by to people in Vipers and 911s but it was frustrating as all hell when they would want a point by and then baby their cars around the corners where passing isn't allowed.

Maybe suck it up for a lap and then I'll walk away from you enough that you won't come up to me at the end of the straight.

AnthonyGS (Forum Supporter)
AnthonyGS (Forum Supporter) GRM+ Memberand SuperDork
1/14/22 4:54 p.m.

Worry about everyone in a faster car and not wanting a slower car costs $$$ and takes a lot of fun out of motor sports.  
 

In the early 00s, I saw a spec Miata keeping up with a Viper GTS at Portland Intl.  Sure tye Vioer win the front straight but every lap the Miata would catch up again before the straight.  
 

On any track day, there will be a variety of capabilities car and driver wise.  You can either adapt, or you cannot.  Attitude is a big part of that.  

350z247
350z247 Reader
1/14/22 6:01 p.m.

In reply to AnthonyGS (Forum Supporter) :

Can we get a proof read and some grammar? That first sentence is a doozy.

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