How does castor trail relate to pneumatic trail ?

[wirralways]

I'M TRYING TO UNDERSTAND HOW CASTOR TRAIL AND PUEMATIC TRAIL COMBINE/CHANGE WITH VEHICLE SPEED AND HOW THEY WILL EFFECT VEHICLE PULL IN COMBINATION WITH CAMBER AND CASTOR SIDE TO SIDE VARIATIONS.

 

[GregLocock]

Castor trail is geometric, so it doesn't change with speed. Pneumatic trail decreases with speed.

Yes they'll all affect pull (depending partly on what you mean by pull- do you mean drift?). We quantify these effects by running a matrix of interactions, typically 81 runs for a given suspension.

BTW what's with the shouting?

Cheers

Greg Locock

 

[wirralways]

Can I assume that the castor trail and pneumatic trail add up or are they independant ? I would agree that the castor trail is geometric but it will change slightly with compliance.

We define pull/drift in 2 ways firstly the time to move a specific distance on a cambered road after we have released the steering wheel or secondly by the torque to keep the vehicle going straight on the same cambered road.

Currently I can improve vehicles by swapping tyres side to side and getting the tyre conicity's in the most helpful direction and tweaking the vehicles geometry helps a bit, but these are all a bit minor and everything is well within spec. Thus I'm trying to decide do I need to make a step change in geometry i.e more castor angle or is it really trail I'm after. I'm starting to look at the effects of toe settings,in over out. Or have I potentially got some suspension compliance going on giving me a steer effect.

 

[GregLocock]

OK, that's what we call it.

Yes, the straightening effect will be partly governed by the total of mechanical trail+pneumatic trail.

Gillespie says look at scrub radius as well. camber is important

There are several other important factors which should be considered, but unless you work for my employer then I can't list them for you.

SAE paper 2000-05-0253 Identification of a vehicle pull mechanism may help



Cheers

Greg Locock

 

[wirralways]

tried to find the sae paper but drew a blank. is the number correct.

i've been told that the springs in the struts could be giving a force around the king pin axis when they are compressed is this likely ???

 

[GregLocock]

Yes that is correct. Author(s): Oh, Sang-Hyun, Cho, Young-Hee, Gim, Gwanghun.

As to the other, you should draw a free body diagram for the wheel/hub.





Cheers

Greg Locock

 

[mloew]

wirralways,

Please see the collection of technical resources I listed in the thread shock friction (thread800-57725). The mechanism for side-load on the damper (strut) is well explained. Resolving the forces and moments from a FBD will be elucidating as Greg suggested.



Best regards,

Matthew Ian Loew
"Luck is the residue of design."
Branch Rickey

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[mloew]

Here is the list of SAE International (

http://www.sae.org)

technical papers that I referred to in my earlier post:[ul][li]2002-01-0317: The Effect of Rubber Seats on Coil Spring Force Line [/li][li]2001-01-0496: Modeling of Coil Springs Using Parallel Mechanisms [/li][li]2001-01-0497: Development of L-Shape Coil Spring to Reduce a Friction on the McPherson Strut Suspension System [/li][li]2000-01-0101: Optimization of Force Action Line With New Spring Design on the McPherson Strut Suspension for Riding Comfort [/li][li]2000-01-0727: System Modeling of a Damper Module[/li][li]970100: Optimization on Mac Pherson Suspension With a Spring[/li][li]960730: Approaches to Minimizing Side Force of Helical Coil Springs for Riding Comfort [/li][li]940862: Side Load Springs as a Solution to Minimize Adverse Side Loads Acting on the Mcpherson Strut [/li][/ul]



Best regards,

Matthew Ian Loew
"Luck is the residue of design."
Branch Rickey

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GregLocock]

Wirralways - The lightbulb lit, I've just realised who you are! I'll send you some pointers next week.

Cheers

Greg Locock