Starting and braking chain tension

[nvpF1crazy]

By intuition I think that the starting and braking chain tensions should be larger than the normal operating chain tensions. I could not find anything on how to calculate either so I made up my own formula by simple physics:

Starting Tension = mu*(Static weight on rear axles)*(Tyre radius)/(Sprocket pitch radius)

Braking Tension = mu*(Dynamic weight on rear axles just before braking)*(Tyre radius)/(Sprocket pitch radius)

Using these I'm getting values much smaller than that during normal running conditions. Something is wrong but I can't figure it out.

[nvpF1crazy]

By intuition I think that the starting and braking chain tensions should be larger than the normal operating chain tensions. I could not find anything on how to calculate either so I made up my own formula by simple physics:

Starting Tension = mu*(Static weight on rear axles)*(Tyre radius)/(Sprocket pitch radius)

Braking Tension = mu*(Dynamic weight on rear axles just before braking)*(Tyre radius)/(Sprocket pitch radius)

Using these I'm getting values much smaller than that during normal running conditions. Something is wrong but I can't figure it out.

[whiltebeitel]

What mu values are you using? Also, the dynamic weight transfer onto the rear wheels is probably a better case to look at, since most cars are perfectly capable of spinning the tires at near a g of longitudinal acceleration.

[nvpF1crazy]

I'm using a mu value of 1.1 and dynamic weight is calculated using a formula mentioned in Fundamentals of Vehicle Dynamics by Gillespie (pg 39).

Chain tension under running conditions is torque at wheels by sprocket radius OR power at wheels by (omega*r) for sprocket. Strangely the dyno charts I'm following are giving me drastically different values by both formulae.

[shark.ashwa]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by nvpF1crazy:
I'm using a mu value of 1.1 and dynamic weight is calculated using a formula mentioned in Fundamentals of Vehicle Dynamics by Gillespie (pg 39).

Chain tension under running conditions is torque at wheels by sprocket radius OR power at wheels by (omega*r) for sprocket. Strangely the dyno charts I'm following are giving me drastically different values by both formulae. </div></BLOCKQUOTE>

I have not gone through gillespie.....could you just post the equation used?
Refer to this thread....this is what we do
http://fsae.com/eve/forums/a/t...10394241#79410394241 (http://fsae.com/eve/forums/a/tpc/f/125607348/m/21810684241?r=79410394241#79410394241)

Sharath

[whiltebeitel]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by nvpF1crazy:
I'm using a mu value of 1.1 and dynamic weight is calculated using a formula mentioned in Fundamentals of Vehicle Dynamics by Gillespie (pg 39).

Chain tension under running conditions is torque at wheels by sprocket radius OR power at wheels by (omega*r) for sprocket. Strangely the dyno charts I'm following are giving me drastically different values by both formulae. </div></BLOCKQUOTE>

I would not use a mu less than 1.5. Read SAE Paper 2009-01-0643 By the Honda F1 team for some insight on the relative shock loading in a well-developed circuit car. They found that the drivetrain could see approximately 2.5 * maximum engine output torque loads under hard shifts and standing starts. That's where a FOS of 2-3 comes in when you base the design on the maximum expected torque to the system.

[Pico]

I think that looking at the problem from another point might prove more straightforward. If you know what the maximum torque output of your engine is in 1st gear you assume that it induces the max chain tension you would ever see, assuming no slip so you don't guess a mu value.

[shark.ashwa]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Pi*co:
I think that looking at the problem from another point might prove more straightforward. If you know what the maximum torque output of your engine is in 1st gear you assume that it induces the max chain tension you would ever see, assuming no slip so you don't guess a mu value. </div></BLOCKQUOTE>

and then you can add a considerable safety factor to be sure of its robustness http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

Sharath

[nvpF1crazy]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> If you know what the maximum torque output of your engine is in 1st gear you assume that it induces the max chain tension you would ever see, assuming no slip so you don't guess a mu value. </div></BLOCKQUOTE>

That would give the max chain tension under normal operating conditions.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> and then you can add a considerable safety factor to be sure of its robustness </div></BLOCKQUOTE>

True! What is the range FOS you decide on? The part of Gillespie used states that "For a solid rear axle with a locking differential the maximum tractive force is

Fx=u*(W*b/L)/(1-(h/L)*u) " (pg.39)

[shark.ashwa]

I would stick somewhere between 3 and 4......do some testing and make changes accordingly...

Regards,
Sharath

[Paul Achard]

This is an occasion where strain gauges on the driveshafts come in handy...

[Pico]

With what i proposed the safety factor I would use is 1. It's so over conservative, it's an ultimate load case and you'll never reach it. But strain gauges will tell you what you're really seeing...

[Gruntguru]

A 1 kW engine with suitable flywheel and clutch will break traction on a one ton car.
You need to consider mu, weight transfer under acceleration, inertia of all components downstream of the chain and angular acceleration of those components. (Some video analysis should give a reasonable estimate of that acceleration). Otherwise read the Honda paper.