reciprocating mass / spun weight, torque, top end - page 1 - IDI Engine - VWDiesel.net The IDI, TDI, and mTDI source.

• reciprocating mass / spun weight, torque, top end by subsonic on 11 May, 2007 17:26
• I was wondering how much the wheel and tire combo people use effect performance.  I see people with 15" 17" rims and big fat tires on thier vw's.  Mine has 13" plain jane steelies.  It would seem that the rims and tires would be one of the larger areas, poundage wise that effect spun weight.  I have heard a ratio in the past , but can't remember it.  It was somthing like removing one pound of spun weight is equal to removing ten pounds of dead weight. Any truth to that?
  I am also wondering if any weight savings you pick up going to a aluminum rim is offset by the fact that it will probably be bigger, and you will now also be spinning a bigger tire.
  If overall weight stays the same, will the fact that the new tire has a larger circumfrence cause you to be slower off the line?
  Do you need little 13" tires for low end speed, and heavier taller tires for the highway runner?  Anyone ever figure out how much they could manipulate there 0-60  times, or top end by tire and rim swaps?
  What else is simple spun weight to remove?  I see some high priced titanium stuff out there.  Could you achieve the same results by just switching to a lighter lug nut? Say 1 once lighter ber lug nut X 8 = half a pound of spun weight.  Just some out loud wondering going on.
• #1 by westcoaster on 11 May, 2007 17:57
• Well, what little I can tell you is that 31x10.5" mud tires aired down to 20 psi with a 3.73 rear diff and an unknown reduction in the transfer case has pretty pathetic performance on the highway. As in barely able to keep up with the flow of traffic with my foot right to the floor.....
  Then again, the motor has massive blowby and the injection pump/cam  timing is questionable, all on a suzuki samurai platform. So I'm thinking this post really isn't helping you answer your question.  
• #2 by xud9te on 11 May, 2007 18:43
• oops
• #3 by xud9te on 11 May, 2007 18:58
• itchy trigger finger
• #4 by xud9te on 11 May, 2007 18:59
• oops
• #5 by xud9te on 11 May, 2007 19:01
• Lots of different issues in that question!
  
  Think of the wheel/tire/gearbox/driveshaft mass as additional mass on the flywheel (and rolling friction to some extent).  If you have a heavier mass to accelerate then you nedd more torque to do so, but you will store more momentum, energy in the 'spun' mass.  
  
  The really applicable issue for performance in the case of wheels and tires   is unsprung mass.  Lower unsprung mass results in a better ride and better handling due to the fact that the tyre will be in contact with the road more of the time as the suspension can more easily do its job due to lower reciprocating forces.
  
  Grip is the other factor.  If you spin your skinny tyres under power, then you need more grip.  So, you get a wider tyre or taller wheel (or both).  If you cannot spin this tyre then you have more grip than torque time the gearing times the radius of the rim times tyre friction.  
  
  Once you have theoretically found the critical grip, then you can reduce unsprung mass all you like, it will only help.
  
  You can damage acceleration if you have too big a wheel/tyre diameter as the moment is too large.  You could damage grip if the tyre contact patch is too small.  
  
  Finding a balance between grip and acceleration is the key.  17" x 9" wheels and tyres that weigh 30 kilos each corner will only blunt the performance, as you nedd about 400hp in the dry in 3rd to break traction , but also 12" wheels and 155 tyres will not have the grip to get you going!!
  
  Ideal scenario is having wheel/tyre size matched to torque in the critical gear (usually 2nd or 3rd - can always spin in 1st), then reduce as much unsprung weight as possible.  'Spun' mass is pretty much the same as unsprung.  
  
  Its another thing altogether when considering going round corners though  
• #6 by jtanguay on 11 May, 2007 19:13
• deja vu?
• #7 by subsonic on 11 May, 2007 20:09
• that sounds like a line out of the sound of music :lol:
  
  
  Thanks for the input.  For reciprocating mass, is there some kind of basic formula like the remove 1 pound car acts if you have removed 10 pounds of dead weight thing?
  When you say sprung and unsprung, do you mean sort of the diff between the front and rear tires?  Meaning fronts are under power, rears rotate but are not under power?
• #8 by burn_your_money on 11 May, 2007 20:55
• I beleive the wheels pictured in the front left are the lightest 13s VW ever made, and they are defintily light. I don't have an exact weight though
  
  
• #9 by Vincent Waldon on 11 May, 2007 21:17
• "Sprung" refers to the weight the springs support... pretty much the entire car, with the exception of:
  
  "Unsprung": the stuff the springs *don't* support: the tires, rims, rotors, calipers, outboard CV joint, etc.
  
  Then there's "rotating mass"... stuff the engine has to spin... wheels, rims, rotors, axles, CV joints, flywheel...
  
  Different reasons to optimize the different parts.
• #10 by jimfoo on 11 May, 2007 21:18
• Quote from: "subsonic"

  that sounds like a line out of the sound of music :lol:
  
  
  Thanks for the input.  For reciprocating mass, is there some kind of basic formula like the remove 1 pound car acts if you have removed 10 pounds of dead weight thing?
  When you say sprung and unsprung, do you mean sort of the diff between the front and rear tires?  Meaning fronts are under power, rears rotate but are not under power?

  
  I don't remember the exact number, it could be 10, but yes, removing mass from the engine/drivetrain acts like removing more weight from the vehicle. Unsprung weight  is wheels, tires, axles, anything without a spring between it and the pavement.
• #11 by rabbid79 on 11 May, 2007 22:42
• The ratio I heard was more like 3:1 unsprung/dead.  I think there's an article on tirerack.com where they tested 2 or 3 wheel/tire combinations that used the same width, style, and compound tire, but with different size wheels.  Their point was to prove how the extra unsprung weight in the wheels and tires affected handling.
• #12 by MaxHedrm on 12 May, 2007 11:40
• Basically, get the lightest wheel & tire combo you can. It will improve:
  

  Handling
  Mileage
  Acceleration
  Braking

  
  
  All by different amounts, but all at least a little.
• #13 by RabbitJockey on 12 May, 2007 11:57
• i know that 14x6 alloys are way better than 13" steelies.  a very noticable difference in handling.
• #14 by Black Smokin' Diesel on 12 May, 2007 19:14
• Quote from: "Trev0rbr"

  i know that 14x6 alloys are way better than 13" steelies.  a very noticable difference in handling.

  
  
  True. I run 14x6 Ronal R8 alloy wheels from an old FWD Audi (4x100) with 195/60R14 Falken Azenis tires during summer. The whole combo is fairly lighter than the 13x5 "tarantula" alloy wheels with 155/70R13 tires I run in winter.
  
  My summer setup has significant grip, more than I need in day to day driving. Performance wise they handle great and also provide better self centering due to the wider tires.
  
  Summer wheels/tires on my old NA golf (you can see a bit of what happened to the car) Note the massive ammount of brake dust
  
  
  Winter wheels/tires on my current TD jetta (looks a bit different now)