Energy required - knobbles vs slicks

[Bodhbh]

Anyone have an idea how much energy is required - all things been equal -to push these around?

I'm using 50kcal/mile to crudely estimate how much energy I'm burning on local rides. It's not so far off imo, based on what I'm shoving down my throat and what I gain/put on in a given week...with slicks.

However, been messing about a bit swapping out wheels for ones with knobblies to take in some off-road routes and what I'll say, is the difference is 'non-trival'.

First off, they batter your legs. Resistance is always there, no letup. A couple of the mixed road and off-road rides I've had around 40 miles have felt more like 60-70 on the legs.

Secondly, I was stuck with them for 3 weeks due to some problems with the wheels the slicks were on, for all riding, long weekend ones and including the commute. Legs were battered. In two of those weeks I lost 3kg. I had 45 mile ride on knobblies yesterday and I'm absolutely ravenous atm, think I ate around 3000-3500kCal yesterday.

On of the spreadsheet calcs for thsi stuff estimates I would be burning approx 420 kCal/hr for a 15mph ride on the slicks, but 1560kCal/hr for the knobblies on gravel - there are no figures for knobblies on tarmac. Surely that's OTT, but what's the real situation? Anyone have any practical experience via heartrate monitors or such, or any difference calculations. I am in the process of loosing a few kg, but tbh just interested anyhow.

/edit knobblies even in title!

 

[ColinJ]

I can't give you any precise figures, but I can give you an example of how much harder riding an MTB with wide knobblies is compared to a racing bike with narrow slicks...

I live about 17 moderately hilly miles from a bike shop in Padiham. I've ridden there and back many times on my road bike and it isn't much of a problem. I can tell that I've done a reasonable ride afterwards but I don't feel particularly tired by it.

I decided to buy a new road bike frame after my old one developed a crack. I didn't want to ride the road bike because of the cracked frame so I rode my MTB to the bike shop. That felt about as hard as the round trip would have done on the road bike. By the time I got home, I felt like I'd done about 75 hilly miles on the road bike rather than the 34 miles I'd actually ridden.

I was really shocked at how much difference there was. Riding offroad on the MTB is another matter - I rarely ride a road bike offroad and I don't expect riding offroad to feel easy so the MTB feels fine despite it being heavier and slower.

 

[jimboalee]

Have you got both pairs of tyres?

Yes. Do an experiment.

Find a long steepish hill and measure the slope in % terms. 0.05 = 5%, 0.08 = 8% etc.

Wait for a windless day and fit the slick tyres.

Weigh the whole vehicle (including yourself).

Freewheel down the hill recording the maximum speed you get to ( equilibrium velocity ) in mph.

The equation is :- Power Pf = Slope x Weight x (Velocity / 375)

Where slope is 0.05 for 5%, Weight is in pounds and speed is in mph.

The result is in Horsepower.

Now fit the knobblies. Re-weigh and do the trial again.

You have two results, one for slicks and one for knobblies.

The next step is to find CdA. For this you need the Coefficient of rolling resistance of the tyres.

CdA = ( (Pf-Prr) x 391 x 375 ) / V^3

Where Prr = Crr x (W/375) x V

Crr ( Slicks might be 0.0048 and MTB knobblies might be 0.008 )

V^3 is Velocity speed in mph cubed.

Next is to calculate power requirement to attain any speed.

Pv = ( Crr x (W/375) x V ) + ( CdA x (V^2/391) * (V/375) )

Crr is rolling resistance, W = weight (lb), V = velocity (mph)

So bung in 15 mph and see how much more power ( Horses ) are required.

kWatts = Hp x 0.746.

 

[Bodhbh]

I can't give you any precise figures, but I can give you an example of how much harder riding an MTB with wide knobblies is compared to a racing bike with narrow slicks...

By the time I got home, I felt like I'd done about 75 hilly miles on the road bike rather than the 34 miles I'd actually ridden.

For me it's not like I'm riding anything lightweight to start - a MTB with 2" balloon slicks at about 55psi - but the difference is still huge (road bike incoming when I can afford one). Even pushing my tourer loaded with camping gear and 2" Marathons seemed easier on balance around the local lanes, although the climbs were harder.

I know I'm only stating the obvious, but I found suprising how much difference it makes and numbers would be interesting.

 

[Bodhbh]

Have you got both pairs of tyres?

Yes. Do an experiment.

Well, it took 10minutes, but I understand it at least well enuff to get the numbers to split out at the end.

I'll have a crack for the hell of it and see what it gives. There's a long, steady tree-enclosed decent about 18 miles away I can have a play on which should be perfect and give me a focus for local rides.

However, it depends upon accurate rolling resistances - these liable to be accurate enought to get sensible results? I'll google some and try anyhow, although be a week or two till I get over there with each set of wheels.

 

[Globalti]

It's a subject which has interested me ever since I started doing Polaris events and realised how slow knobblies are. I still have some old grey tyres called Kona Crossroads, which have a narrow band in the middle and which are much faster, I used to use them for Polaris and they're on the MTB at the moment. My league table of energy wasters goes something like this:

2.1 knobblies under-inflated on warm dry tarmac
2.1 knobblies well inflated on warm dry tarmac
...on cool tarmac
....on wet tarmac
2.1 knobblies on cinder or dry mud track
1.8 knobblies (same sequence as above)
1.6 commuter
1.6 slick
road tyre

 

[Bodhbh]

My league table of energy wasters goes something like this:

2.1 knobblies under-inflated on warm dry tarmac

Last time they were on, when I checked pressured b4 swapping they had run down to about 25psi - which might explain a thing. I'm not used to the knobblies enough to estimate pressure from feel. I know this is too low to be running about on tarmac, but this is not a bad pressure to run off-road?

 

[ColinJ]

Last time they were on, when I checked pressured b4 swapping they had run down to about 25psi - which might explain a thing. I'm not used to the knobblies enough to estimate pressure from feel. I know this is too low to be running about on tarmac, but this is not a bad pressure to run off-road?

It depends on how much you enjoy crashing, fixing punctures or replacing rims!

In my early days on my MTB, I didn't pay much attention to my tyre pressures and I got numerous snakebite punctures. Worse than that, I dented my rims. And worse even than that, I managed to peel an under-inflated tyre off a rim! I was very fortunate not to crash or end up under a car because I was descending at speed into a tight bend at the time.

I still use pressures lower than many people, but they'd typically be in the range 40-55 PSI.

 

[Bodhbh]

It depends on how much you enjoy crashing, fixing punctures or replacing rims!

I was very fortunate not to crash or end up under a car because I was descending at speed into a tight bend at the time.

I still use pressures lower than many people, but they'd typically be in the range 40-55 PSI.

I've not had any problems with snakebites, but yes, it's not a very good feeling cornering when they are very low and there's a bit of movement back there. They're up to 40psi now and trying out a bit of that Stans stuff in the inner tubes to keep them there.

 

[ColinJ]

I've not had any problems with snakebites, but yes, it's not a very good feeling cornering when they are very low and there's a bit of movement back there. They're up to 40psi now and trying out a bit of that Stans stuff in the inner tubes to keep them there.

I suppose it depends on what type of offroad we are talking about! If it is canal towpaths and dusty bridleways then you might well get away with 25 PSI but you wouldn't last long doing 25 mph down steep rut-and-rock-infested bridleways or badly worn ancient stone packhorse trails!

 

[Globalti]

Nice pic of the packhorse trail Colin. I have saved it. Did you know that these stones were rotated to present a fresh face when they became too smooth? Or that they were laid sideways rather than lengthways, which would have been more efficient but could have allowed them to rock, which would have un-nerved the pack ponies and slowed them down? They were the motorways of the hills before canals then railways then roads came along, you'd have been shoved off into the bog if you met 40 loaded pack ponies belting along and there were often violent disputes over the right of way. The original packhorse bridge in Hebden Bridge has refuges built into the cutwaters for pedestrians to avoid being knocked over the parapet.

When I first started MTBing I used to ride with my tyres so soft that they crept around the rims when braking and even tore out the valves. I would also get pinch punctures and rim damage. Nowadays I pump my tyres to 45 lbs, which seems about right.

 

[ColinJ]

Nice pic of the packhorse trail Colin. I have saved it. Did you know that these stones were rotated to present a fresh face when they became too smooth? Or that they were laid sideways rather than lengthways, which would have been more efficient but could have allowed them to rock, which would have un-nerved the pack ponies and slowed them down?

I didn't know that - interesting!

The original packhorse bridge in Hebden Bridge has refuges built into the cutwaters for pedestrians to avoid being knocked over the parapet.

I noticed them a few years back and worked out what they must have been for. The triangular shaped bit sticking out of the side of the bridge in the next photo is one of them. I don't have any pictures of the top of the bridge but I'll take some next time I'm out with my camera.

When I first started MTBing I used to ride with my tyres so soft that they crept around the rims when braking and even tore out the valves.

Ooh, I had that happen! I descended a 17% local hill at speed and got away with it, but as soon as I got onto the main road there was a sudden hiss and the tyre instantly went flat, came off and jammed my front wheel. I slewed across the road and fell off in front of an oncoming vehicle. Fortunately the driver was paying attention and managed to stop in time!

 

[jimboalee]

Well, it took 10minutes, but I understand it at least well enuff to get the numbers to split out at the end.

I'll have a crack for the hell of it and see what it gives. There's a long, steady tree-enclosed decent about 18 miles away I can have a play on which should be perfect and give me a focus for local rides.

However, it depends upon accurate rolling resistances - these liable to be accurate enought to get sensible results? I'll google some and try anyhow, although be a week or two till I get over there with each set of wheels.

For some info on Rolling resistance, Google 'John Lafford'.

 

[Bodhbh]

I suppose it depends on what type of offroad we are talking about! If it is canal towpaths and dusty bridleways then you might well get away with 25 PSI but you wouldn't last long doing 25 mph down steep rut-and-rock-infested bridleways or badly worn ancient stone packhorse trails!

I wish we had some of that down here! (just NW of London, edge of the Chilterns). Just riding off-road a bit on local loops for the sake of variation, what I've come up against is a bit of slippy chalk, some tree roots and a whole load of mud. I didn't even bother with knobblies at first, but fishtailed a couple of times coming down wet, muddy banks and thought tools for the job and all that.

 

[plank]

Its fine to run some MTB tyres at 25psi, for example ones with double ply side walls. I run my downhill bike at 25psi and I'm 15stone, however running the same pressure on lighter single walled cross country tyres would be asking for trouble. The compound of the rubber also makes a big difference, eg 70a is a lot faster roling than 42a!