A F1 test driver and an ‘every day Joe’ put a kart through it’s paces to figure out how much slower they will both go with 44lbs (20kg) added to the kart vs their baseline times with no weight added - spoiler: not even half as much as they thought it would. https://www.youtube.com/watch?v=TgQ2EsFheiA
UPDATE: fair points from the comments that the tests being held indoors, and with rental karts, still leaves open questions about outdoor tracks with ‘real’ racing karts.
Interesting. I had always heard it was like a tenth per 10lbs roughly. In the case of Jack Aiken, that seems accurate.
my personal theory is that in that setting, with a really slick surface and ultra hard tire, those results are sku’d quite a bit because the weight is going to add grip where it’s desperately lacking.
We see it every week racing the same spec kart in Masters trim vs. Senior trim (20 lb difference) and the gap every week on a short course is about .5 seconds…tracks like New Castle where it’s an even longer track the 20 lbs can add up to over a second per lap difference.
Rental karts are a different beast all together (particularly heavy and not fine tuned). For normal karts (25-30hp 160kg mark) it’s around .05 per kilo. Sometimes can fluctuate depending on conditions.
These times were set on same day, same tyres and reflect the usual difference.
Rotax 177kg lap record Alex Rowley 45.31 secs
Rotax Max 162kg lap record Edward Brand 44.18
15kg heavier and 1.13 slower. So around .05ish per kilo
Yeah, I personally thought it was silly to use rental cars for this video, but I understand why they did it.
It would be fun to see this test done with real karts. I’m thinking the lap time differences between light and heavy loads would be a curve, with the curve getting steeper as the speed increases. A parabola. I know when I raced Mc’s, the lap time for the pole position in light and heavy, as I remember, were quite a bit different than the lap times in this video.
cheaper and relatable for the audience is probably the main reason. They also organise a rental race each year for WTF1
If it wasn’t that big of a time gap we wouldn’t have medium and heavy weight classes. I’ve seen the top guys at national events run a medium and heavy class. Literally the same kart just adding 15-20 lbs to it and running slower lap times. The only variable is the weight and it does affect the lap times.
I don’t know… Fast lap in medium at Quincy was ran out of the draft at heavy weight, so there’s that…
I think the ultimate answer is that you can’t overcome physics. A heavier kart/ driver will be slower.
The question that is more interesting is “but by how much?” This is where it seems we are all unclear, as a lot of it depends on factors beyond this, namely things that try as we might, we can only make somewhat consistent - the track conditions, parity among engines, and of course ‘skill’ of a driver.
To really understand anything meaningful about weight vs. pace, you’d ideally want the largest data set possible. A couple people have reference relative pace between different categories, but is this enough to make a full determination? Perhaps at one track the larger set of data gives us one conclusion, at another it yields a different result. Another factor to consider is the disparity between two different engine package situations - would time difference be the same gap between a light group and heavy group of 206’ers, and similarly weight difference Rotax racers or Shifter kart racers? Just looking at your local series’ lap times could give you a hint, but it’s still a smaller data set as there are so many variables that are hard to eliminate.
It’s a frustrating question to really nail down, but one id be very curious if with a nationwide data set a more meaningful conclusion could be met.
Generally a light driver will beat a heavier driver.
I am ahem several kilos heavier than most of my competitors when I race and I regularly beat those who are lighter - some even have had to weigh up. I am not fighting for wins, far from it, but a decent karter who can adapt to the kart that is under them does add a lot.
Within certain limits, of course, I’d consider weight balance/distribution to be more important than total weight.
Its not scientific but one of the TAG snrs moved to TAG heavy at the final rounds of the ICP Cup. So same track (he moved Sat to Sun), same tires, same engine, just 35lbs of lead (assuming he met the weight in TAG). 2 tenths slower from the final the day before to qualifying the day after.
Comparing to someone doing times with a tenth of his on the Saturday, he was 4 tenths slower then he’d be expected to be (compared to the TAG snr) in the pre-final.
The local track is ~600m and reasonably technical. The difference between Rotax Light and Rotax Heavy is about 1second. 20kg MAW difference is all.
There’s no blanket answer across the board. I think the closest you’ll get is comparing the same track, tires, track condition (grip level) and kart weight.
Naturally 20lbs on a 360lb kart is a larger percentage vs a 400lb kart too.
Average speed comes into it too with karts because the aero is so poor. In road racing for example weight.
The true answer is “it depends” 
A follow up which I think might satisfy a lot of people. Based on this video from “Col”
The calculations he came up with there for a national level “TAG restricted Heavy” vs “TAG restricted Light” there is roughly .07 seconds of difference in lap times for every 5 pounds. He does add that these amounts could change between different classes, but it’s at least an honest look at how real karts might respond to weight differences.
Add one other variable to the ‘weight vs lap times’ equation; where was the weight added and what did it do to the balance/driveability of the kart. Thinking back I remember when I added 20’ish pounds to my kart (HPV powered at the time), and my times actually improved vs when it was lighter. 75% of the weight I added was behind the seat (e.g., heavily rearward bias), so clearly I had my setup off in the first place (e.g., too much weight forward vs back when it was lighter). Nonetheless, where weight is added and how it affects the setup/driveability would definitely affect the outcome of lap time differences between lighter vs heavier.
Edit: even though what I say above is true and possibly worth some mental exercises, I also think there’s a lot of value in comparing times of top light vs heavy classes since it can be fairly reliably assumed that those guys have their weight distribution figured out well.
So I am in kind of a weird situation here and I wanted to try and get some input on it.
Next year i’ll be 15 so right on that line between Jr and Sr. Originally I was planning on just sticking with Jr for another year because that is where all my friends are racing but it is looking like i’m gonna end up being close to 10ish pounds over the minimum weight for the class. How much time do you think this will actual relate to and would it be better to just try and move to Sr now and put some ballast on the kart?
I’m wondering, how many in your junior class have the same problem, or, maybe they’re racing at 4, 5, 6 pounds over. I know I never went racing right on the class limit.
My rule of thumb is that 5 lbs doesn’t really hurt you, 10 lbs starts to.