Theoretical Speed & Distance Curves

[rich_f]

5 hours ago, OoALEJOoO said:

Correct,  speed-realted losses indeed will affect the entire curve. Many of them are exponential with speed and therefore incurr heavier movement penalties the faster you go. The good thing is that qualitatively the trends on the graph would still be maintained (i.e. every ratio will be affected by speed, and not inherently by ratio). What could change, although I suspect not dramatically, might be the position of cross-over points

I think the effect on the crossover might be more significant. And I think that taller ratios will be more affected by the unmodelled losses than the shorter ones. I'll try to explain why I think this. 

Your current model says that as long as there is some torque (given by the torque curve on the backs of boxes) then the car will keep accelerating until there is none.

Shorter gear ratios decrease wheel torque less than taller ratios, therefore taller ratios have less torque at all speeds than shorter ones.

Since actual acceleration is proportional to the excess force (after losses), taller ratios, which have less wheel torque (force) than shorter ratios at a given motor rpm, will hit their limit at lower rpm than shorter ratios. 

The outcome is that there will be a point where going to a taller ratio results in a lower top speed in addition to lower acceleration than a shorter one (just as @Wooders28 recalls) - something your current graphs don't express. 

So I think there will be a qualitative change as well as a quantitative one with the speed-related losses considered.

[Wooders28]

6 hours ago, OoALEJOoO said:

However, the NiMh does produce the same max-speed as a LiPo.

Not in my experience, 

With lipos having a higher discharge, there's alot less voltage sag, so a nimh voltage drops like a stone, compared to lipo.

 

 

[OoALEJOoO]

1 hour ago, Wooders28 said:

Not in my experience, 

With lipos having a higher discharge, there's alot less voltage sag, so a nimh voltage drops like a stone, compared to lipo.

 

 

I should have added that my statement was based on the following video, where NiMh and LiPos were tested with marginal max-speed results difference (and perhaps related to the fact the LiPo is lighter but that would depend on how long the straight was):

On the other hand, I have also found videos supporting that LiPo does indeed result in higher max-speed than NiMh:

Somewhat conflicting results, but I do tend to trust Cageman's results better (I admit this is somewhat subjective on my end!), which does support your experience with LiPos.

However (here is where it gets interesting), this las video about battery testing makes me conlcude, although without 100% certainty, that perhaps LiPo and NiMh indeed do reach the very similar max-speeds (given a long-enough straight). The idea is that current demand is not very high once the car picks-up speed and all battery types can supply more-or-less the same level of currnet (except NiCad):

The key summary of the last video (screenshot below) is that LiPo can supply a vastly larger current during the high-torque high-power demand during the first half-second on an acceleration run from stand-still. After this 0.5 second all battery types (except NiCad) supply pretty much the same current and therefore should reach similar max-speeds. Maybe more data past 1.0s would give more assurance.

I am basing this on the (small) video evidence and reasoning, which are the tools I have available. More experienced people can prove/disprove the theory. Adding a battery current module into the model could shed some light as well.

 

[Nikko85]

Whenever I look at anything like this these days I remember this fantastic quote

"All models are wrong, but some models are useful"

Any model is full or errors, simplifications and omissions, but that doesn't mean they can't tell us something interesting.

Great thread.

[Wooders28]

1 hour ago, OoALEJOoO said:

Somewhat conflicting results

No idea why the first vid only gets a few mph more, especially running a 10.5t motor. A 10.5t on 2s, should lift the nose under hard acceleration, so not sure if they where, rolling on it or wheel spin?

1 hour ago, OoALEJOoO said:

After this 0.5 second all battery types (except NiCad) supply pretty much the same current

Surprised at that. That rates the nimh at 20C, when most state a discharge for nimh is 3C, with the high performance cells upto 10C.

You'd really need an esc with data logging, like the Castle Mamba X (on my list, but still a bit pricey, even in the 2nd hand market....), to get true, full load readings.

[OoALEJOoO]

On 9/27/2021 at 3:22 PM, rich_f said:

I think the effect on the crossover might be more significant. And I think that taller ratios will be more affected by the unmodelled losses than the shorter ones. I'll try to explain why I think this. 

Your current model says that as long as there is some torque (given by the torque curve on the backs of boxes) then the car will keep accelerating until there is none.

Shorter gear ratios decrease wheel torque less than taller ratios, therefore taller ratios have less torque at all speeds than shorter ones.

Since actual acceleration is proportional to the excess force (after losses), taller ratios, which have less wheel torque (force) than shorter ratios at a given motor rpm, will hit their limit at lower rpm than shorter ratios. 

The outcome is that there will be a point where going to a taller ratio results in a lower top speed in addition to lower acceleration than a shorter one (just as @Wooders28 recalls) - something your current graphs don't express. 

So I think there will be a qualitative change as well as a quantitative one with the speed-related losses considered.

You might be confused by the fact that indeed tall gear ratios have less wheel torque at lower speeds, but not at higher speeds. Remeber the gear ratio will modify both torque and speed.

The objective of taller (higher) gear ratios is to reduce torque at low vehicle speeds and increase it at high vehicle speeds, this is precisely why they have lower acceleration but higher max-speeds.

Wheel torque vs. vehicle speed plots:

Having said that, there is the always-valuable cross-over point. Only if the speed-related losses are so high that the losses overcome the wheel torque before the cross-over point, then a taller ratio will have less max-speed than a shorter one. Another scenario could be super-duper tall gear ratios (you basically stall the car), a very weak motor or a very weak battery. I think all these scenarios are not something we experience since the cross-over points are quite low, 5.8 m/s (21 km/h) for the Super Stock TZ and 3.8 m/s (13.7 Km/h) for the Silver Can.

On the above graph, you can see that theoretical max-speeds are achieved when torque is zero (when each line crosses the horizontal axis), note speed is in m/s.

In reality, when you add losses, the max-speed will be when the losses equal the wheel torque. I anticipate the losses would look something like this (freely drawn, not calculated):

For a given motor, and provided we are past it's gear-ratio cross-over point (highly likely), a taller ratio will always result in higher max-speed, given a long enough straight and no heat issues.

When comparing between different motors with varying gear-ratios, you can see that considering a losses curve flips some results. E.g. 5.72 Torque Tuned has lower max-speed than 7.55 Super Stock if considering losses, which was not the case if there were no losses.