[DMmotorsport]

No, you do not understand, as you explained yourself...
What i mean is, that hobby drivers (mostly rotax in Asia) will not even see a chenge in laptimes... especially because of there driving skills are a lot more important than a braking system which costs 2 times there original...

Also in shifter class, driving is lot different from your classes.
You brake in to the corner, as late and as hard as possible... with top speeds that can reach 190 and more Km/h you really can miss every gram... cause it is no only about the rotating mas to start turning with will be harder, but also bring it to stop... the same for rims...
the same for flywheels of 4 stroke. the same for a heavy football. or whatever heavy thing you want to get in movement and afterall try to stop it !, AND especially rotating masses which make A LOT of RPM's.

But as i said, when you drive for hobby, of in lower leveled classes, this is not nassacary...
As i am sure the rest of the opponent not even using the proper type of pads...
ALSO because most of karting drivers, don't even know there are more than 4 and even 5 types of brakepads for each system brand...

ciao, Kristof

[Mikko Nassi]

Also in shifter class, driving is lot different from your classes.
You brake in to the corner, as late and as hard as possible... with top speeds that can reach 190 and more Km/h you really can miss every gram... cause it is no only about the rotating mas to start turning with will be harder, but also bring it to stop... the same for rims...

But how does say 0.5kg extra on the brake disc make a big difference to that kart. Are you saying it is because it is rotating? Does 0.5kg on the seat make a big difference compared to having that 0.5kg on the brake disc? You said you found a 8kph difference in top speed from difference weight brake discs.

An axle with a kart going at 110kph only moves at roughly 2,500 rpm (3,500 at 180kph). Not amazingly high.

I agree that for example if you have a kart with a big 2kg clutch vs direct drive with no clutch on the same engine (and overall kart weight is still kept the same), you'll probably see a big difference especially when you're revving to 16,000rpm. But at 2,500rpm and at the size that brake discs are the difference should be almost impossible to measure by looking at top speed or laptime because other variables would render the test pointless. And your equipment would have to be able to measure speed to within 0.01kph because that is likely the accuracy you'd need to find a difference.

[DMmotorsport]

So you think 3500 rpm's for an axle is not much???
I think you underestimate the influance a lot A LOT ! for sure...

not to mention the starting.... where you have to accelerate from 0 to 150... in a few seconds...

To be clear.... clear you back axle of your kart. add 1 wheel on 1 side, a magnesium one... give it a fast turn with you hand, as much as you can... than do the same with the Alu one...
The alu one will take more energy to make same speed (check on alfano/MyChron) AND you will see the Alu one will keep un spinning longer....
SO heavier to start, takes more energy..... AND afterall stopping it, even more !!

It can't be clearer....
also the waight of you kart totaly has been by regulation..
1 grams on the chassis, has less resistence than 100 on the rotating things... as rims, discs, sprockets, ...

gr.

[Mikko Nassi]

I think you underestimate the influance a lot A LOT ! for sure...

I think you overestimate the influence a lot .

1 grams on the chassis, has less resistence than 100 on the rotating things... as rims, discs, sprockets, ...

I'm not sure what you mean by this? Obviously 1 gram on the chassis effects the kart less than an extra 100grams on the disc for example. Or were you meant to say that the other way around? That 1 gram on the disc has more effect than 100 on the chassis?

[DMmotorsport]

Mikko

that 1 used to be 100... so i typt wrong...
My english is not that good...
I think you underestimate the technology of karting...
And for sure i do not think ASIA is ready for this discussion... and ASIA is not ready for
KF or KZ classes too... Even KF is starting to grow now...
I have been there, and for sure i have seen mentality in Asia is tha same as in the late 80's in Belgium or anywhere in Europe...

And i think you try to put my tips and uppion to the side as you have a good reason for that...
Isnt'it?

gr, Kristof
DMmotorsport

[BrianZ]

exciting thread! i dont know which one is right... but i also dont think that top speed can change much from difference weight brakes. if the difference was that big everybody would use the lighter ones.

[JohnKing6]

Mikko

that 1 used to be 100... so i typt wrong...
My english is not that good...
I think you underestimate the technology of karting...
And for sure i do not think ASIA is ready for this discussion... and ASIA is not ready for
KF or KZ classes too... Even KF is starting to grow now...
I have been there, and for sure i have seen mentality in Asia is tha same as in the late 80's in Belgium or anywhere in Europe...

And i think you try to put my tips and uppion to the side as you have a good reason for that...
Isnt'it?

gr, Kristof
DMmotorsport

Kristof,

ASIA is a big place and there are some very successful KF series held here. The fact that some smaller countries like Malaysia don't have a nation KF series is simply due to the small market and the difficulty of changing over to new regulations. But that doesn't justify insulting every karter in Asia or Malaysia. We regularly have drivers based in South East Asia competing in KF, ROK and Rotax races in Europe and big Asian markets like Japan very successfully.

You seem to be obsessed with dismissing Rotax drivers as amateur hobby drivers. I wonder what Ben Cooper, reigning world Rotax champion and now competing very well in KF, would say about that?

There are a number of people here - and Mikko is one of the best - who understand the technology of karts very well. We are not disputing that rotational mass will make a difference - just disputing the scale that you claim. For top speed tests to be accurate they need to be conducted in a wind tunnel or, if not, the environmental factors have to be taken into account - like windspeed. A drop or increase in the windspeed between 2 runs is far more likely to impact the top speed than a 100 gram change on the rear axle.

[Mikko Nassi]

Yeah and I'm not sure what the mentality of karting in Asia vs. Europe has nothing to do with how much difference a heavier brake disc makes on a kart, or being able to discuss technical matters.

that 1 used to be 100... so i typt wrong...

Ok that makes sense. Yes I agree that 100grams on the brake disc has more of an effect than if that 100grams was on the frame. But the question is how much? You seem to feel that there is a big difference, and you base this on some speed tests.

The good thing is that we can actually use simple math and laws of physics to figure it out a way of quantifying the difference and we can say "X amount of weight on the disc is the same as X amount on the chassis".. or if we wanted to we could figure out for example "200grams of extra weight on the rims is the same as _____grams extra on the chassis".

I checked online for formulas on how to figure it out. It turned out being an easier calculation that I thought it would be.

Here is an example between 2 discs of the same size, but different weight.

The outer diameter of a disc is 194mm, and the inner diameter is 136mm. (Actual specs of a CRG Ven-05 brake disc according to homologation forms).

I don't have the discs to weigh them, but CRG did say that their special lightweight Ven-05 disc weighs the same as their thin Ven-04 disc. I checked the Ven-04 discs' dimensions and it should weigh roughly 2kg (+/- a few hundred grams) in cast iron. Lets say that the Ven-05 "cheaper" disc weighs 4kg, so we'll do a comparison of a 4kg and a 2kg disc, with similar dimensions, and see what kind of an effect that has. Now here we are assuming that the driver is not overweight - he will be able to add 2kg onto the kart if he takes if off on the brake disc - we are talking about rotational mass here, while keeping the total static kart weight the same.

So the heavier disc weighs 4kg. And the lighter disc weighs 2 kg. That is actually quite a big difference, it might be more than the difference in reality. The rotational inertia is what we need to figure out to see the resistance created when being rotated.

The equation to figure out the rotational inertia is:
1/2m(r1^2+r2^2) (http://en.wikipedia.org/wiki/List_of_moments_of_inertia)

The rotational inertia (I) for the discs is:
4kg disc = 2(0.068^2 + 0.097^2) = 0.028 kg-m^2
2kg disc = 1(0.068^2 + 0.097^2) = 0.014 kg-m^2

Wow the 4kg disc has twice the rotational inertia!! But what does that really mean in reality?

To be able to say X grams on the disc is like X grams on the chassis we need to figure out the "equivalent mass" as explained very well on this website: http://www.stephenmason.com/cars/rotationalinertia.html

The interesting thing is that the rotational velocity of a brake disc (or anything else on the axle) of a kart is directly proportional to the velocity of the kart. This means that the result will be valid at all speeds.

I won't re-work everything from the site above, but if you follow along there with these numbers you should understand it:
We'll set the tyre's rolling radius at 0.135m (85cm tyre which is quite typical - rolling radius = circ/pi/2 = 0.135)

So we end up with the equation equivalent mass = m + In^2

n is 1/0.135 = 7.41
I from earlier is 0.028 and 0.014
m are the weights 4kg and 2kg

Solve the equivalent mass for the 2 discs:
4kg + 0.028*7.41^2 = 5.537kg
2kg + 0.014*7.41^2 = 2.768kg

What this means is that with a 4kg disc, if is effectively a 5.54kg disc when accelerating. Or with the 2kg disc it is effectively a 2.77kg disc when accelerating.

Don't forget that the 2kg we saved on the disc was put back on the kart, so the difference between the two karts is effectively:
5.54 - (2+2.77) = 0.77kg.

So if you have a 4kg disc vs. a 2kg disc you can say that using the heavier disc is the same as having 0.77kg added to your seat (extra above the weight limit).

But the difference isn't actually quite that big. This "weight penalty" only applies when you are accelerating, and only when your wheels aren't capable of spinning. When you are traction limited (wheels are able to spin if you give it 100% throttle), the rotational inertia does absolutely nothing - the engine clearly has enough power to overcome all of the rotational inertia and has excess above the grip available from the tyres. It makes no difference if the 2kg is on the disc or on your seat.

So for example with a KZ shifter kart from say 0 to 50kph you might still be able to spin your wheels, so in that range it makes no difference to the acceleration. After that the difference is the same as if you added 0.77kg of weight to the kart. Adding that amount of weight would not be measurable in a change of top speed.

How about under braking? If you are able to lock up the tyres - then you are not being effected by the rotational inertia! Extra weight on the brake disc is just the same as having that weight elsewhere under braking. It also doesn't add weight transfer to the kart when cornering/accelerating/braking, and it doesn't do anything to slow you down when you've already reached top speed if you hit the rev limiter for example. So in fact it's not even "the same as adding 0.77kg" - the effect is a lot less than that! It also doesn't add (small amounts) of resistance to the kart in the same way that led on the seat would (by adding to the friction in bearings etc).

So once you take away the parts where you could spin the wheels, and the braking areas, then you are left with the bit where the rotational mass will make a (tiny) difference. So overall laptime difference simply couldn't be able to be measured even with a 2kg vs 4kg disc because other variables would make a larger difference. This also means that the more power you have available - the less the rotational inertia effects the kart (so a KZ1 for example will see less improvement than a Rotax Max from the same reduction in brake disc weight).

If the discs were 2kg vs 3kg then the difference would drop down to around 350 grams. With a 3kg disc vs. a 3.5kg disc this is down to under 200grams (and remember, only for a part of the lap). You can quite easily plug in the numbers for different weight discs (and other components such as rims, tyres, etc.) in the website to test things out.

The closer the component is to the axle - the less rotational inertia it has. So the same weight savings in hubs for example will have a lot less effect than the brake disc, and rims will also have less of an effect than the brake disc. Weight saving on the axle will have even less effect . The most effect would actually be from a lighter tyre.

I learned a lot thanks to this thread. I always thought the difference wouldn't be very big, but now I can actually calculate the difference to an extent and understand it more!

An easy way to see how little effect rotating mass has is when you start your engine on the stand. If you rev it on the stand from 0 to 10,000rpm it will happen in an instant - on the track it will take lot longer. On the stand the engine overcomes all of the rotational inertia on the rear axle, whereas on the track the engine has to overcome the rotational inertia + the inertia of the total mass of the kart (and wind resistance etc). The inertia of the total mass of the kart is obviously massive compared to even all of the rotational inertia on the rear axle. You could try to find the lowest power 60cc engine you can find and it would still be able to rotate the axle with all the wheels etc. on it very easily.

I may not have all the calculations correct in this post because I'm not exactly a mathematician (anyone please correct anything you see that is wrong), but they should be thereabouts.

[JohnKing6]

Jeez Mikko, ... and I thought you were being lazy today when I didn't see you at Sepang for practice

[gp8]

You've got your math pretty close Mikko - but a little lateral thought about the degree of variance of rotational mass vs power applied will have you realise that the difference / benefits of low rotational mass is most significant in low powered classes. Yes that's right - cadets. All you need is an inertia dyno and you will the see quite dramatic changes for a comer cadet and very much less significant changes for a KF or even Rotax if you make the same changes to rotational inertia (heavier rims, heavier brake dsks etc)

We found that rotational mass changes on the comer were actually measurable in lap times. We are unable to detect the changes in even jnr rotax as the changes are within driver variance / inconsistency.

The 0.1sec you may possibly gain per lap may well be material if you are doing times within 0.1sec off pole consistently. If you are 0.4 off the changes aren't really material - you will still be 0.3 sec / lap off pole, so you should be spending your money on driving (where the 0.3 sec is) tuition and not fancy lightweight gizmos and brakes where there is only 0.1sec.

Best brakes we ever had on a kart? Custom made metal matrix ceramic disk (just 4.5mm thick) with EBC green pads in a TonyKart EVO setup on a little Rocky EXP2 cadet with a Comer engine. The ceramic chassis bearings, bimetallic axles and composite sprockets all helped - although we did have to use aluminium rims to have sufficient stiffness to make the crappy rental grade tyres we had to use work at all...

[JohnKing6]

You've got your math pretty close Mikko - but a little lateral thought about the degree of variance of rotational mass vs power applied will have you realise that the difference / benefits of low rotational mass is most significant in low powered classes.

I think Mikko and I both said that early in thread - but you have to fight your way thru a lot of bs to find it.

so you should be spending your money on driving (where the 0.3 sec is) tuition and not fancy lightweight gizmos and brakes where there is only 0.1sec.

Yep, I'm amazed at how much money some parents are prepared to spend on equipment and engine tuning but then expect their child to improve magically without any help. When I watch my cadets practice and see the lines that some of the other kids take in their super tuned, brand new, expensive karts, I always wonder why the parents don't just give me 1/10th of what they pay their engine tuner (to make the kart .1 faster for the next race only) to make their kids go 1 sec faster - permanently ... and the answer is that it's harder work and there's too many people who say "buy this chassis (normally Tonykart) and your son will be instantly faster" - you know the normal bullshit - and it's not as appealing as someone who says that we'll try and teach them to drive properly and if they get it, they'll be faster.

[Jules]

double wow...

Wow 1 - Mikko, maybe you could have taken engineering......from an engineer, I can say you have the persistency.

Wow 2 - this is indeed getting VIOLENT as John said it!

Good read, this thread!

[Mikko Nassi]

Jules I've been interested with this rotational mass question for a while now, this thread finally got me to really analyse it .

I just noticed a big mistake in my previous post - I miscalculated the approximate weight of the discs - which looking at it now is quite obvious.

I used diameter rather than radius when calculating the weight of the disc - the weights should be a lot less (will have to actually weigh the discs to be accurate).

The comparison should've been more like a 0.5kg disc (VEN-05 expensive) vs a 1kg disc. Not a 2kg vs 4kg disc.

Should've been:
1kg + 0.007*7.41^2 = 1.385
0.5kg + 0.0035*7.41^2 = 0.693

1.385 - 1.193 = 0.192kg, or 192 grams.

So when we use a 0.5kg disc vs a 1kg disc (and add the 0.5kg back to the kart), we are left with an effective mass difference of only 192 grams. Again this only effects the kart when accelerating.

Would you notice a difference in laptimes if you took a 200ml bottle of water with you in your kart.. and only had it with you when accelerating?

You're better off finding a way to make sure you're never 100grams over the weight limit when you finish a race . That 100grams over the weight limit will effect you more than a 0.5kg difference in brake disc weight.

Then again on a 100 kart grid with a tenth or two separating the top 50 you probably could do with a thousandth or two that the lighter disc might give you. All else being equal 1 thousandth is worth everything. The psychological effect of thinking that you've saved a heap of rotational inertia would probably help most drivers more than that though.

You've got your math pretty close Mikko - but a little lateral thought about the degree of variance of rotational mass vs power applied will have you realise that the difference / benefits of low rotational mass is most significant in low powered classes.

Yeah the less power the less the same difference effects you. Using the above calculation with a 1kg disc vs. a 0.5kg disc - the difference is still exactly the same though - it will feel like 192 grams was added to the kart when it is accelerating (and only when accelerating).

200grams added will definitely be felt easier on a cadet kart weighing ~110kg and only 8hp to push it along than in a 30hp+ kart weighing 160kg+ already. The cadet kart also accelerates for a longer time, doesn't have many corners where it would spin its wheels out of, and has an overall longer lap so laptime difference would be easier to find. But even so I think when you change that brake disc the way you align the axle with the bearings will have a more significant difference to laptimes - as will the change in track conditions while you where changing out the disc.

All you need is an inertia dyno and you will the see quite dramatic changes for a comer cadet and very much less significant changes for a KF or even Rotax if you make the same changes to rotational inertia (heavier rims, heavier brake dsks etc)

On that dyno don't forget that you're not taking into account the weight you now have to add back on to the kart before you head out to the track.

When talking about the brake disc only Cadet brake discs are already fairly light and they're smaller in diameter as well which means they already have a fair bit less rotational inertia than larger discs.

Using a standard Intrepid Maky brake disc as an example (4mm thick, 160mm outer diameter, 104mm inner diameter) which should weigh roughly 0.35kg, even if we fitted the kart with a brake disc from the future that weighs 0grams, the effective mass reduction would be less than 90 grams.

Now that's assuming you're using a Mojo D2 sized tyre on the back - with a smaller diameter tyre the difference is slightly larger.

The 0.1sec you may possibly gain per lap may well be material if you are doing times within 0.1sec off pole consistently.

Yeah that's very true. I would kill for 0.1 sec . But I also know I won't find it from a brake disc.

I did a search and found some mentions of a test that Karting Magazine did. They kept adding brake discs to the axle of the kart - and took the weight off of the seat as they added them to the axle. They added a total of about (depending on which forum post you believe) 7 brake discs - and STILL didn't notice a difference in laptime.

Using our 1kg brake disc, they would've added about 2.5kg of effective mass with the 7 discs (again it only effects under accel). Which quite understandably would be quite difficult to catch on laptime.


Using Kristof's example a way to realise just how little rotational mass effects the kart:

Take the chain off the kart, put the kart on the stand. Now rotate the axle. Not very difficult to get it spinning right(if you have the axle aligned)? You just overcame all of the rotational inertia on the rear axle + all of the friction from the bearings. With you little finger .

Another visual example: it is quite common with cadet karts on the pre-grid for the mechanic to lift the rear of the kart slightly in the air - and have the driver floor it and really rev out the engine (to clear the engine out of oil). Notice how when the mechanic drops the kart back on the ground the rear axle stops almost immediately? That was all of the rotational inertia on the axle being stopped in an instant, without even pressing the brakes.

Same thing when you rev the engine on the stand - first you overcame all the rotational inertia very quickly when revving the engine to 10,000rpm, and then in an instant with a dab on the brakes took all that inertia out.

Here's an interesting point: When the karting world moved from 40mm axles to 50mm axles, the rotational inertia of the rear axle actually went up! Along with it the rotational inertia of the hubs, sprocket carriers, brake disc carrier, and rims all also went up! Funny how nobody was worried about that slowing them down .

[gp8]

Quote: But I also know I won't find it from a brake disc.

err...well we did..kind of. In addition to a reduction in rotational mass (the MMC disk was an enormous 150gm lighter ), the disk / pad combo provided much better feel and modulation than the standard piece of steel sheet the Rocky standard brake was cut out of. The result was repeatable too.

Could we ever separate out the effect of rotational mass from the effect of driver confidence? Don't know how you ever could.

So whilst we loved our blinged up rear axle, and it was the great envy of many, the actual cost benefit was something like this:-
1. MMC disk / EBC green pads in OTK EVO calipers = -AUD$600 and a lap time reduction of about 0.1 sec
2. Ceramic bearings = -AUD$800, and maybe they needed less maintenance?
3. Bi-metallic axle = -AUD$250, but also a lap time reduction of about 0.2 sec in the wet or a very green track and about a 0.2 lap time blow out if the track gripped up or dried out - your throw of the dice!
4. Driver training = -AUD$200 / day and a lap time reduction of about 0.3sec from each of the intensive days undertaken.

Simply put - driver training clearly has the most significant impact of all in reducing rotational inertia

[David Goldman]

So ive read this very interesting thread pretty thoroughly and my question is Mikko, you are saying that a driving lesson would have the largest impact on a driver going faster, but lets say that driver is already at the top of the line (here in the us it would be stars of karting) but is just that tiny little bit off the pole. Giving that driver a lesson would just be a waste of time, they are already at the top and clearly have a lot of driving skill and knowledge, so wouldnt them getting a lighter brake disc make sense? And if the weight was so important for the rear axle, why wouldnt companies make carbon fiber hubs and brake disc carriers and sprocket carriers? Plus another thing to think about is that we are talking about the weight of the brake disc, but that is assuming that EVERYTHING is the exact same. Some brakes use different carriers, who says that one brake system used the exact same weight and shape carrier? On a straightaway test track, you could put more aerodynamic bodywork on and i feel at a speed of 170 kph that would make more of a difference than a lighter brake dics.