# Why does more weight on the tire reduce the grip?

Hi,
Could someone explain this in physics?

P=FA, so grip should increase as more force is applied. My off-the-wall guess is that if weight goes up enough, it also lends to lateral g loading to offset the traction.

He’s right in that it’s not linear, but the grip curve slowly flattens out, it doesn’t go down (unless the tire blows out).

For any tire there is an optimal weight to grip ratio, in that additional weight doesn’t add more grip as quickly. Maybe that’s what he’s trying to get across, although incorrectly shown on a graph.

But he starts the video by “Moving seat to the rear but get more front grip.” I think he does mean grip will drop if the weight exceeds some point.

But at the beginning of the video,he said " Moved the seat back, but got more front grip."

He didn’t actually get more “front grip”, he got relatively “less grip” in the rear because the extra weight started to make the rear tire twist.

Essentially, the extra weight in the rear exceeded the tire’s optimal amount of force it could deal with before it deforms past its normal gripping state.

Before setting the seat to the rear, the extra weight was put on the front tire which deforms the front tires and got “less grip” in the front. It sounds reasonable, too.

Two things about karting that make it a bit counter-intuitive if you’re used to (or reading about) car phsyics:

Karts are significantly “Over tired”. That’s to say they have a lot more grip potential from the tires compared to their weight\mass.

In karting, “Grip” is often conflated with rear load transfer\lifting of the rear wheel.

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I confess, I haven’t watched the video yet, but I can’t resist a good weight distribution disscussion.

I understand karts are their own animal when it come to vehicle physics, but some of this might still apply… I spent a alot of time working with some pretty savvy race engineers on big cars, and this topic came up a lot. I had one finally explain it to me in a way that made more sense. (This guy was annoyingly anal about fore/aft weight distribution, Indycar on ovals we would regularly make changes as small as 2 tenths of a percent front axle weight). Anyways, he explained to me that you have to look at it from the tire’s point of view. With less weight on the front end, the tire’s available grip has to overcome less mass to institute the chassis’ change in direction. This is overly simplistic, but it’s sorta, kinda like when you plop that big bag of dog chow underneath your shopping cart over front wheels, it gets harder to turn/ wants to go straight, take it off, it’s easier.

There is lots of nuance to this and one large caveat… For what I said above to be true, the chassis needs to be close to its weight distribution happy spot; usually the prescribed range from the manufacturer or your own numbers from testing. If you exceed that tuning window in either direction, you can have the exact opposite handling effect as to what you are after.

I’m new and still learning karts, but in my mind at least, some of this thinking might transfer. Love to hear others thoughts on it.

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My \$0.02 on this is:

Weight distribution can influence chassis behavior and weight transfer
Driver inputs dynamically influence weight transfer
Weight transfer directly influences vertical load on tires
Vertical load on a tire influences how that tire contact patch interacts with the track surface

As far as tire/track interaction, goes, I think of it as a continuum ranging from
Essentially no traction (wheel off the track or sliding on ice)
to Interlock (the tire surface interlocking with the imperfections in the track surface
to Friction (a heavily loaded tire scrubbing across the track surface)

Along this continuum, between Interlock and Friction there is a sweet spot; an optimal combination of Interlock and Friction called Stiction, which coincides with optimal tire loading and optimal slip angle generation.

Anyway; bla, bla, bla. There is more (from a driver not engineer perspective) about traction, the 2D and 3D traction circle, etc. here.

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How much deformation can a tire handle before traction begins to diminish? I’m guessing it’s simply a matter of how said deformation distorts the tread surface.

Sort of like rear engine Porsches. Lots of down force on rear tires, so lots of rear grip. Until the lateral force of the heavy engine overwhelms the lateral grip of the rear tires and, SURPRISE, snap oversteer!

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This looks to me like a classic slip angle curve. The more lateral force you generate from steering through a turn the more grip/slip angle increases up to a peak value. What’s interesting at the peak slip angle is that the steering goes light yet continues to steer properly. The curve displayed in the video is really a quantification of the slip angle generated. Typically for a bias ply tire the maximum slip angle is around 4 degrees (which is the difference between the steer angle and the actual direction of travel). On a radial tire the peak slip angle increases to about 7 degrees. The difference in construction between bias ply (rigid sides and soft flat surface) versus radial hard flat surface and flexible sides enables the added slip angle while maintaining grip. With different compounds of either bias ply or radial tires, the skip angle curve will have a similar profile. It will simply ramp at different rates and may peak at slightly different values but not significantly different peaks. Of course the assumption here is that we’re discussing dry surfaces and “normal” temperature ranges. Tire technology for rain and ice conditions takes into consideration other factors that prevail at those conditions.

Although karters cannot practically measure slip angle directly, one can use math channels and calculate steering angles on your MyChron5. The Race Studio analysis can then display the data. The information is embedded in the friction circles but not as intuitive as when calculated as a steering angle. It becomes a continuous trend that can be directly compared with acceleration and braking points. Any quick direction changes in steering within a corner indicate the driver is sawing on the steering and depending on whether its going into the corner or coming out of the corner let you and the driver discuss what really was going on and get on the same page with fixing that.

These numbers seem reversed? All of the research I have done indicates that the optimal slip angle for bias ply tires is greater than radial tires due to the softer sidewall construction of BP vs radial tires.

I believe the steering goes light about 1-3 degrees (depending on tire design) before optimal slip angle is achieved. This is the self aligning torque peak, which the driver can use to predict when peak slip angle will occur in a turn. In car racing, it seems pretty common that inexperienced drivers confuse feeling the self aligning torque peak with the actual slip angle peak, so they feel like they are at ‘the limit’ but they are actually multiple seconds off the pace because they are not achieving optimal slip angles. This doesn’t seem to be as much of an issue in karting because the build/release of cornering energy on kart tracks happens much more quickly than on car racing tracks, so the peaks seem to happen almost as one instead of as two distinct peaks.

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Warren, my experience with kart tires is limited to bias ply tires (that’s all we run). Car racing series do use radials. My empirical data suggests bias ply tires have rigid sidewalls and run much rougher than radials on automobiles/trucks. The tire research information I found described the engineering of bias ply vs radial tires allows for more flexible sidewalls on radials. Having said that, I can’t explain your reversed numbers because its your research.

On the self-aligning torque in car racing I can’t speak from experience on that. From a kart perspective, the “torque” or heaviness of the steering builds due to the chassis jacking created through the front-end settings of Ackerman settings, Camber, Caster and Scrub Radius. The moment the inside of the turn rear wheel unloads, the kart is now rolling the corner on three wheels (two fronts and one rear) and thus less steering input strength is required while maintaining the same direction. That does feel lighter. If the driver is sawing on the wheel too much for his liking the reduction of caster and scrub radius helps maintain the jacking strength input at a more gradual or even input however it comes with less inside turn rear wheel unloading and thus more front end push or understeer. I strive for a good balance for the driver’s benefit and consistency. To be clear this driver mechanic discussion is also informed by the steering trend analysis we do. It helps to either correct or reinforce (with data) the drivers perception of the kart behaviour.

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Is it possible to look at tire wear to determine amount of weight on a tire?

You can’t really put a number on what a corner weight is just by looking at a tire.

The tire wear is a culmination of all the kart’s handling characteristics, driver inputs, and every other variable on the kart/track condition, so there are literally ALL the factors to consider when looking at the west pattern.