Lower unsprung weight, what is the advantage?

Lower unsprung weight, what is the advantage?

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Sam_68

9,939 posts

200 months

Sunday 5th October 2008
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SlipStream77 said:
snotrag said:

[quote]

HiRich, is that antirollbar actually attached to the centre of the leading edge of the lower front wishbone? It must be putting quite a lot of stress on there.
I think the pic was originally posted by snotrag, but yes, WTF is that... apart from a textbook example of where not to feed a load into a wishbone?! yikes

Interesting thread though, I agree.

If I can find time to read it all I might join in later! boxedin

Edited by Sam_68 on Monday 6th October 12:16

HiRich

3,337 posts

217 months

Sunday 5th October 2008
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sminky said:
How come you don't see inboard brakes to save unsprung weight? i.e. discs on the driveshafts before the joints
In principle, yes, and there have been quite a few cars that have done it over the years (Alfasud? D-Type?)

First of all it's only really worth considering on the driven axle - the cost and/or weight of a brake-driveshaft is probably punitive.
Packaging is certainly an issue, whether on road or race car. Remember that modern wheels have a nice cubby hole capable of holding a disc & caliper with only minimal compromise of the suspension geometry. You also get a nice flow of air over it. Inboard, you might have problems fitting in the aero (critical around the tail end of an F1 car), a pigging great gearbox to wrap around, or perhaps costing you boot space.
Cooling shouldn't really be a problem, but there are problems putting hot discs so close to a hot gearbox or diff (particularly wrt bearings).

A final problem on race cars is that the brakes actually do an awful lot of work keeping the tyres hot. The most famous case fo this was the Lotus 72, which (if you built the Tamiya model) had inboard brakes at both ends. They had real trouble maintaining tyre temps and eventually reverted to outboard. Chapman tried again on the 77, with the same results.
I understand that the modern 'ducted' F1 brakes (and the whole Ferrari 'special gas')have paid much attention to getting consistent tyre temps and pressures.

And apologies to snotrag - I copied his image to reference and promptly forgot to do so. I'm also a bit surprised by that arb installation given that the mount it still there on the top arm.

Mave

6,371 posts

170 months

Sunday 5th October 2008
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Rocket Pepper said:
Mave said:
???? There is no way using lighter wheels gives you more power. If you see more power indicated on a dyno, then it's to do with how the dyno calculates drivetrain losses.
talking brake dyno's here......

Assuming you take out the alloy wheel then put in the carbon wheel (same tyre etc[weight]) and repeat a number of pulls consecutively, swapping back and forth between wheels until you're convinced the dyno is as accurate as can be, how would you explain a power increase, no matter how slight, shown with the lighter wheel?
Because the dyno uses the run down rate to calculate drivetrain losses. If it runs down quicker (because you've got less wheel inertia) then it inteprets this as higher drivetrain losses, and ups the indicated power accordingly.

Rocket Pepper said:
And just to stir the Hornets nest. Why can the dyno show different readings if you strap a vehicle down tighter and/or play with tyre pressures?
Because tyre losses are quite significant at higher speeds / higher powers, and strapping down harder / changing pressure has an effect on tyre losses.

Just think anout it, the power that your engine makes is down to just that, the engine. How can changing the weight of the wheels affect the performance of the engine?

Mave

6,371 posts

170 months

Sunday 5th October 2008
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catso said:
Mave said:
Rocket Pepper said:
Interestingly, we beat this up a bit a few years ago as my Buell has carbon fibre Dymags. We were dynoing a Land Speed Buell in Colorado readying for Bonneville and the conversation got around to even better unsprung weight brought about by a wheel such as mine giving a higher horse power. I believe we concluded that there may well be a small increase seen on the dyno due to a lighter rear wheel but essentially there would be no relevance to power gain unless the saving in weight was quite significant. As far as I'm aware there is no formula in race math to show such increase or not in power, though I'm sure a formula exists that would lend itself to such an equation.
???? There is no way using lighter wheels gives you more power. If you see more power indicated on a dyno, then it's to do with how the dyno calculates drivetrain losses.
But anything that gives less weight improves power to weight ratio so would feel more powerful on the road - where it matters. But I have heard that lighter components (wheels, flywheel etc) on bikes can show a small improvement on dyno figures probably due to being able to pick up quicker?
I'm sure you get more power / weight, but that wasn't my point. My point was that you don't get any more power.

Rocket Pep

12 posts

141 months

Monday 6th October 2008
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Mave said:
Because the dyno uses the run down rate to calculate drivetrain losses. If it runs down quicker (because you've got less wheel inertia) then it inteprets this as higher drivetrain losses, and ups the indicated power accordingly.
I was referring to a motorcycle rear wheel, where the de facto industry standard is a dyno that records peak power at the rear wheel, so does not factor in drivetrain losses. Such is calculated by the rear wheel turning a steel drum of which its mass and inertia required to rotate it are known. Such a dyno, like say a Dyno Jet 150 or 200, uses Newton's second law of motion which is basically Force = Mass x Acceleration. The dyno measures the time and rate of acceleration to a given RPM at full throttle. Torque and HP are calculated by software from the time and rate of acceleration. The more rapidly the steel drum is accelerated to a given RPM the greater the engines HP. Such a dyno could show an increase in power by the use of a lighter wheel as this would allow the dynos steel drum to be accelerated more rapidly.

I am not at all familiar with the black art of car dyno tuning. I think it's ridiculous to calculate an engines HP when rear wheel HP is what is needed to be known when the car is sat on its wheels at the track for example. Factoring in a theoretical drive train loss percentage seems somewhat pointless in lay mans terms. I find it equally ridiculous that there is no de facto industry standard like there is recognised worldwide for motorcycles. Even more so when the car is sat on rollers and a rear wheel figure can be given, why bother factoring in power train loss which is not a given for ever type of vehicle. But of course, the reason I mention this is simply, no a lighter wheel when measuring power for a car in the usual manner will not show a gain in power as the result is factored back to the motor thus ignoring the drivetrain.

HTH.

Mave

6,371 posts

170 months

Monday 6th October 2008
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I agree, it's wheel HP which is important. The problems come when you try to measure that with the engine accelerating, rather than steady state....

rsstman

1,918 posts

142 months

Monday 6th October 2008
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kambites said:
Gad-Westy said:
Slightly off topic, but with regard to wheels, you're also talking about rotational mass and the less of that the better when it comes to acceleration.
Is it measurable? I would have thought the rotational kinetic energy of the wheels paled into insignificance in comparison to the linear kinetic energy of the car.
on my old xr2i it was definately measurable. although it gripped better with the 17s it was definately slower to accelerate and i believe top speed was also affected, im sure on a car with much higher power the difference would be much less noticeable though.

Rocket Pep

12 posts

141 months

Monday 6th October 2008
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Mave said:
I agree, it's wheel HP which is important. The problems come when you try to measure that with the engine accelerating, rather than steady state....
But you can do that with a Dyno Jet. They can be had with an additional eddy current absorption unit for exactly this purpose.

Scuffers

20,887 posts

229 months

Monday 6th October 2008
quotequote all
Rocket Pep said:
Mave said:
I agree, it's wheel HP which is important. The problems come when you try to measure that with the engine accelerating, rather than steady state....
But you can do that with a Dyno Jet. They can be had with an additional eddy current absorption unit for exactly this purpose.
sorry, whist I know what you are saying, that's massively inacurate.

peoblem with all roller based systems is that the contact patch between the tyre and the roller is not a fixed item (and it's nothing liek the same as the contact patch between a tyre and the road).

  • all* roller based dyno work is a guestimate, some are cleary better than others, but they are still an approximation.
if you want imperical numbers, use an engine dyno at steady state (or if you must use power sweeps, at least make the RPM rise rate slow enough to eliminate inertia)

Next best thing is use a hub-dyno, (although then you have the drivetrain to cope with).

kambites

59,072 posts

176 months

Monday 6th October 2008
quotequote all
rsstman said:
kambites said:
Gad-Westy said:
Slightly off topic, but with regard to wheels, you're also talking about rotational mass and the less of that the better when it comes to acceleration.
Is it measurable? I would have thought the rotational kinetic energy of the wheels paled into insignificance in comparison to the linear kinetic energy of the car.
on my old xr2i it was definately measurable. although it gripped better with the 17s it was definately slower to accelerate and i believe top speed was also affected, im sure on a car with much higher power the difference would be much less noticeable though.
Are you sure they didn't have a different rolling diameter though?

Rocket Pep

12 posts

141 months

Monday 6th October 2008
quotequote all
Scuffers said:
sorry, whist I know what you are saying, that's massively inacurate.
Inaccurate because it relies on a software program to arrive at a figure? If that's what you mean, the software is simply calculating the inertia of a drum which the mass of is known, as is what it takes to spin the drum known too. Placing a force on the drum to rotate is is simply a calculation of the force it took to rotate the drum in a certain manner. Hence the software will establish such by use of a computerised mathematical formula which is then interpreted as a power figure derived at from the applied force - a rear wheel of a vehicle in this case. So I don't understand how you see that is massively inaccurate?

Scuffers said:
peoblem with all roller based systems is that the contact patch between the tyre and the roller is not a fixed item (and it's nothing liek the same as the contact patch between a tyre and the road).
Tyre slippage is always going to be a problem. All one can do is factor such in by familiarity with the same dyno. A good dyno operator knows about tyre pressures and how hard they pull down the straps, but at least it's likely a situation for nearly all and not just some vehicles would be my thought.

Scuffers said:
*all* roller based dyno work is a guestimate, some are cleary better than others, but they are still an approximation.
I think they're a little closer than a guestimate. If by approximation you mean same as above......?

Scuffers said:
if you want imperical numbers, use an engine dyno at steady state (or if you must use power sweeps, at least make the RPM rise rate slow enough to eliminate inertia)

Next best thing is use a hub-dyno, (although then you have the drivetrain to cope with).
Yes but we were talking about the effect or not of unsprung weight and its relationship if any to power. That is something not measurable on an engine dyno, though maybe a hub dyno could be brought into play for measuring the affect of wheel weight (if any) if you could leave the wheel in situ whilst driving the hub dyno. Such I imagine would introduce further losses to the dyno though through some weird contraption designed to drive it with the wheel still in situ.



Just like to add, I am not a dyno operator so please feel free to call me an idiot.

HiRich

3,337 posts

217 months

Monday 6th October 2008
quotequote all
singlecoil said:
Cars with live rear axles have significant unsprung weight at the rear. Not such a problem on smooth racetracks though. Nuisance on normal road when you hit a bump going around a curve.
This, or something similar, is a very good example of this "mass-between-springs" idea. If you think of an old, large, live-axle Transit, you have a great big mass in that cast iron brake hub suspended between the leaf spring and the springy sidewall of that high profile tyre.

You can imagine a situation where the hub is oscillating hugely between those two springs, with little visible effect on the body, and with little relationship to what's happening at the road surface. At certain frequencies it actually acts as a mass damper (as per Renault F1 2006, stabilising the chassis over the predictable input of the sawtooth kerbing). But at other frequencies it can magnify & totally freak out either the vibration going into the body or the loading on the tyre carcass.

300bhp/ton

38,615 posts

145 months

Monday 6th October 2008
quotequote all
Mave said:
I agree, it's wheel HP which is important. The problems come when you try to measure that with the engine accelerating, rather than steady state....
You don't actually measure HP for cars. You measure torque and then calculate HP. tongue outwink

smile

kambites

59,072 posts

176 months

Monday 6th October 2008
quotequote all
300bhp/ton said:
Mave said:
I agree, it's wheel HP which is important. The problems come when you try to measure that with the engine accelerating, rather than steady state....
You don't actually measure HP for cars. You measure torque and then calculate HP. tongue outwink

smile
I don't know how dynos work in practice, but it'd be easier to make one which did directly measure horsepower. You just need an electric motor which you use as a brake, then you keep increasing PD across it until the rollers are being driven at uniform speed, using some kind of feedback loop. Once you've done that, you just need to measure the current that you're feeding into the motor and you have your power usage and hence (assuming you know the efficiency of the electric motor) the power output of the car at that point in time.

How do you measure torque?

Edited by kambites on Monday 6th October 14:58

300bhp/ton

38,615 posts

145 months

Monday 6th October 2008
quotequote all
kambites said:
300bhp/ton said:
Mave said:
I agree, it's wheel HP which is important. The problems come when you try to measure that with the engine accelerating, rather than steady state....
You don't actually measure HP for cars. You measure torque and then calculate HP. tongue outwink

smile
I don't know how dynos work in practice, but it'd be easier to make one which did directly measure horsepower. You just need an electric motor which you use as a brake, then you keep increasing PD across it until the rollers are being driven at uniform speed, using some kind of feedback loop. Once you've done that, you just need to measure the current that you're feeding into the motor and you have your power usage and hence (assuming you know the efficiency of the electric motor) the power output of the car at that point in time.

How do you measure torque?

Edited by kambites on Monday 6th October 14:58
This, to my understanding is a huge and complex topic.

It starts with firstly what is horse power?

And then what type?

There many different types of HP, trains and boats are often measured in dHP, which is draw-bar horse power. This is actually a measure rating.

Motor car engines in this day and age have hp calculated.

To do this we measure torque (or resistance) via a brake machine. These take the form of eddy current dyno's as a rule, but could be in many forms.

But thus we get Bhp, as in brake horse power.

So we then measure torque and rpms, this then gives us work done or hp. If you like torque at speed.

This equation is used:

HP = torque x rpm / 5252

Some inertia dyno's (most Dynojets) use a static drag brake (a large weighted roller). because of this constant, they use a different equation and claim to measure HP. However, as I understand it, because of the constant load of an inertia dyno, they are still measuring torque but with a known weight. So the difference is the same.

kambites

59,072 posts

176 months

Monday 6th October 2008
quotequote all
None of that really answers my question. How does a dyno measure torque.

In order to do so, you need a way of applying a controllable rotational force to the rollers, I'm intrigued as to how they do it. Applying a constant (or at least easily a measurable) power seems easier to me.

Ultimately it makes no difference anyway. For a given engine speed, power and torque at a given point in the system are obviously the same thing (or, if you want to quibble, directly proportional to each other).

Edited by kambites on Monday 6th October 15:15

Scuffers

20,887 posts

229 months

Monday 6th October 2008
quotequote all
Rocket Pep said:
Scuffers said:
peoblem with all roller based systems is that the contact patch between the tyre and the roller is not a fixed item (and it's nothing liek the same as the contact patch between a tyre and the road).
Tyre slippage is always going to be a problem. All one can do is factor such in by familiarity with the same dyno. A good dyno operator knows about tyre pressures and how hard they pull down the straps, but at least it's likely a situation for nearly all and not just some vehicles would be my thought.
with all due, you have missed the point...

if you were working with the exact same tyre on the same tie-down loads, on the same roller at the same temp, then yes, you can make a pretty decent calculation, the problem is tyres are not uniform, and their charicteristics will change with temp/loads/etc.

add to this the small problem that the roller is round, (and with some dyno's there are two rollers/contact patches to contend with), then your in the land of guestimates.
Rocket Pep said:
Scuffers said:
if you want imperical numbers, use an engine dyno at steady state (or if you must use power sweeps, at least make the RPM rise rate slow enough to eliminate inertia)

Next best thing is use a hub-dyno, (although then you have the drivetrain to cope with).
Yes but we were talking about the effect or not of unsprung weight and its relationship if any to power. That is something not measurable on an engine dyno, though maybe a hub dyno could be brought into play for measuring the affect of wheel weight (if any) if you could leave the wheel in situ whilst driving the hub dyno. Such I imagine would introduce further losses to the dyno though through some weird contraption designed to drive it with the wheel still in situ.
point taken, drivetrain, engine, and wheel rotational intertia are all calculatable, if you know the accurate engine output for a given rev range, then do an acceleration run with the wheels in free air over that range, you can then work out the inertia of the drive train (as in based on rate of change).

back to where we got into this, less inertia (ie lighter wheels) does not get you more power as such, but it reduces the drivetrain inertia thus consumes less power accelerating.
Rocket Pep said:
Just like to add, I am not a dyno operator so please feel free to call me an idiot.
good for you, whist I am not employed as a dyno operator, I do work on dyno's 3-4 days a month doing engine calibration work etc.
kambites said:
None of that really answers my question. How does a dyno measure torque.

In order to do so, you need a way of applying a controllable rotational force to the rollers, I'm intrigued as to how they do it. Applying a constant (or at least easily a measurable) power seems easier to me.

Ultimately it makes no difference anyway. For a given engine speed, power and torque at a given point in the system are obviously the same thing (or, if you want to quibble, directly proportional to each other).
braked (as opposed to inertia) Dyno's measure torque, then calculate power from there.

How? well, the dyno brake is used to hold the engine back (either on the crankshaft in an engine dyno or on the roller of a rolling road etc), this brake will be mounted on a bearing/pivot such that the rotational force it's excerting on the ground (in reaction to the engines torque) is then mesured using a load cell, ie say the load cell is 1 foot away from the pivot point of the brake, then the load cell is going to read 1 lb/ft for evey 1lb of force it's being subjected to.

now, take the X ft/Lb's from the load cell, combine that with the brakes RPM and you have power (un-corrected).

for an engine dyno, that's 1:1 with the engine, this figure is the actual raw power and torque of the engine, for a rolling road (or hub dyno) you then have to either calculate the roller/wheel speed to engine speed or take it from a tacho feed off the engine and do the maths.

if you run from the roller speed, this is where the issue of slip comes in as the engine spped to roller speed will be a moving target as slip increases, and whist this does not directly affect the power read from the rollers (appart from loosing some in slip) it does make the calculated engine torque/power somewhat out of wack with the engines RPM.

300bhp/ton

38,615 posts

145 months

Monday 6th October 2008
quotequote all
kambites said:
None of that really answers my question. How does a dyno measure torque.

In order to do so, you need a way of applying a controllable rotational force to the rollers, I'm intrigued as to how they do it. Applying a constant (or at least easily a measurable) power seems easier to me.

Ultimately it makes no difference anyway. For a given engine speed, power and torque at a given point in the system are obviously the same thing (or, if you want to quibble, directly proportional to each other).

Edited by kambites on Monday 6th October 15:15
Well I'll have to pass, as I don't know frown

Mave

6,371 posts

170 months

Monday 6th October 2008
quotequote all
300bhp/ton said:
Mave said:
I agree, it's wheel HP which is important. The problems come when you try to measure that with the engine accelerating, rather than steady state....
You don't actually measure HP for cars. You measure torque and then calculate HP. tongue outwink

smile
You can't calculate HP if all you measure is torque wink

Frik

12,779 posts

198 months

Monday 6th October 2008
quotequote all
kambites said:
None of that really answers my question. How does a dyno measure torque.
I was under the impression that the torque is calculated from the power and speed...