Cycling UK Forum

Thanks Brucey. Actually I wasn't talking about the velonews test, but the rolling resistance.com one that actually tests my tyre.

Any chance of how much say 10 watt difference is in layman's terms. Are we talking a lot? I could and happily will Google, but some of these discussions can get a bit erodite so a bit of layman's never hurts when reading through a thread

Thanks again.

simonhill wrote:Thanks Brucey. Actually I wasn't talking about the velonews test, but the rolling resistance.com one that actually tests my tyre.

Any chance of how much say 10 watt difference is in layman's terms. Are we talking a lot? I could and happily will Google, but some of these discussions can get a bit erodite so a bit of layman's never hurts when reading through a thread

Thanks again.

Well a tourist cycling working hard continuously produces something below 100 Watts (much less when pottering)* - so the rolling resistance difference would be >10% - I don't find that a credible figure myself, nor am I happy about most tyres taking at least 1/3 of my total power being in overcoming rolling resistance... If this sort of figure were accurate then tyre choice would make a massive difference to how fast you went. Something, somewhere doesn't feel right to me...

*To put this in perspective, a top pro cyclist will produce something between 200 and 300 Watts over an hour (the world hour record needed around 400) - no tourist is going to get remotely near that over a days cruising - my guess is that most of us are 50 Watt bulbs most of the time;-)

as per earlier in the thread the fietsersbond tests suggested that as a rule of thumb, 10W more drag might equate to 1kph difference in speed. I don't think that is at all unreasonable as an estimate, although the effect will vary with speed.

IIRC a typical touring bike needs about 100W to do ~12mph on the flat. Even at that speed the aero drag is appreciable so speed is a fair way off being linear with power.

BTW the rolling resistance losses are linear with speed; so when tests are done at ~20mph, the losses at 10mph are exactly halved (assuming the same load).

cheers

Brucey wrote:as per earlier in the thread the fietsersbond tests suggested that as a rule of thumb, 10W more drag might equate to 1kph difference in speed. I don't think that is at all unreasonable as an estimate, although the effect will vary with speed.

IIRC a typical touring bike needs about 100W to do ~12mph on the flat. Even at that speed the aero drag is appreciable so speed is a fair way off being linear with power.

BTW the rolling resistance losses are linear with speed; so when tests are done at ~20mph, the losses at 10mph are exactly halved (assuming the same load).

cheers

Well according to this calculator you require 60 Watts at 12 mph (http://bikecalculator.com/examples.html) - if that's true, and the resistance of a tyre (touring tyre from http://bikecalculator.com/examples.html) is 30 Watts at 18 mph that's 20 Watts at 12 mph. Having just read the test of the Vittoria Randonneur that figure is FOR ONE TYRE! So 2/3 of your energy at 12 mph goes into overcoming tyre drag? At 30 psi the tyre has a resistance of 50 Watts PER TYRE at 18 mph? Really??? Something doesn't add up here...

Brucey wrote:

simonhill wrote:I'm confused about tyre pressure on my new Marathon greenplus, 26x1.5. I am 65kgs on LHT with 10-15 kgs rear panniers.

I thought you should have the 15% bulge on your tyre and excessive pressure didn't reduce rolling resistance.

There is a graph often referred to on this branch that shows ideal pressure for tyre width and bike weight (sorry can't find it at the moment). On my old Marathons I followed it and reduced my tyre pressure to about 45 and 50.

Looking at this review, the lowest rolling resistance is at 90 psi. Should I be upping my pressures and ignoring that graph and all I've read about low RR not being dependant on high pressures?

Only if you want to sacrifice comfort for a little extra speed. Even then you may only get a little extra speed on perfectly smooth surfaces.

Remember that the velonews test is of lightweight skinny tyres and yours are not like that. IME with your tyres the slope of Crr vs tyre pressure is (unlike a lot of lightweight tyres) actually pretty steep, and the choice between drag and comfort is a very real one.

The best thing you can do is experiment and see what suits you. Do remember that the tyres will have a maximum pressure rating though; it may not be permissible to inflate your tyres much above 70psi anyway.

cheers

The problem with rolling resistance tests is that most ignore the single most important variable and that is the road surface. To go to extremes, if the surface was glass flat the lowest rolling resistance would be a rock hard or even solid metal tyre - no deformation losses - and the optimum pressure then reduces as the road surface deteriorates. I know of no study of this but my own personal experience is that a softer wider tyre will roll much faster on a typical cycletrack than a rock-hard tyre - however this is hardly empiricle and may be a function of the confidence and comfort allowing easier pedalling rhythm. My personal opinion is that the best compromise is closer to the graph you mention (there's a link to it here - http://www.bretonbikes.com/homepage/cyc ... y-tyres-be). For example my audax bike runs 28mm tyres at 70 psi and changes in tarmac have a noticeable effect (we all know the feel of really smooth tarmac - worth a gear!) - on my heavy tourer on 38 mm slicks and 50 psi changes in surface have far less effect, and on any grainy surface it's the quicker of the two downhill by some margin.

Quite how you would do a test like this over various surfaces (in the real world) I don't know but it's certainly something that needs doing by someone cleverer than me.

bretonbikes wrote:
Well according to this calculator you require 60 Watts at 12 mph (http://bikecalculator.com/examples.html) ...

hang on, what input parameters are you using there? You need to input realistic values for the various parameters. If you don't do that correctly you will be in a 'garbage in, garbage out' situation.

To represent a touring cyclist accurately you need to use the all numeric calculator here

http://bikecalculator.com/veloMetricNum.html

because none of the standard 'drop-down' values accurately represent the likely values in a real touring bike.

Remember also that the rolling resistance losses are proportional to both speed and load. If you don't allow for those correctly you will end up with some absolutely daft numbers coming out.

BTW I agree about the drag on rough surfaces; however it is likely that only a small fraction comes from the extra deformation in the tyre; a lot of the losses arise because the rider is being jiggled around in the saddle and that energy has to come from somewhere; these losses are more accurately referred to as 'suspension losses'.

cheers

These suspension losses would be hard to measure in a lab test. Perhaps you could attempt to measure the suspension losses in a real person at different levels of vibration (not easy), and then measure the vibration of the test wheel in the lab and apply the previously measured loss for that level of vibration.

Even then, different people are probably differently affected by vibration. How do the hysteresis losses in muscle and fat compare? What about flexed and relaxed muscle? State of hydration? So many unknowns!

All of this assumes you can even represent a variety of real road surfaces on a drum, itself a non-trivial task.

bretonbikes wrote:Well according to this calculator you require 60 Watts at 12 mph (http://bikecalculator.com/examples.html) - if that's true, and the resistance of a tyre (touring tyre from http://bikecalculator.com/examples.html) is 30 Watts at 18 mph that's 20 Watts at 12 mph. Having just read the test of the Vittoria Randonneur that figure is FOR ONE TYRE! So 2/3 of your energy at 12 mph goes into overcoming tyre drag? At 30 psi the tyre has a resistance of 50 Watts PER TYRE at 18 mph? Really??? Something doesn't add up here...

These figures are a bit off for various reasons (the calculator assumptions not applying to a touring position, the weights and therefore rolling resistance not corresponding, etc.).

But in general, rolling resistance truly is a large component of overall resistance to motion and especially so at the low speeds of touring. I am not sure why it has been discounted for so long. It never made much sense to me, and the difference in rolling resistance between two extremes of tyre is very obvious by simply riding them. You’d think people would therefore extrapolate that tyres may vary more subtly but still importantly between the extremes.

The slower you go, the more the tyres’ rolling resistance matters. The Schwalbe chart here makes that clear. It says that at about 18 km/h, half your energy fights aerodynamic drag and the other half rolling resistance, given several givens. Below 18 km/h, tyres very quickly become utterly dominant in overall resistance to motion.

My girlfriend, a casual cyclist, changed from cheap Kenda City tyres (beige ones into the bargain!) to Vittoria Voyager Hypers (black). The difference was colossal. I am sure she gained at least 2 km/h at our slow day-ride speeds, and the difference in comfort was huge. I no longer have to drag my brakes downhill when I’m freewheeling behind her; I can just sit up a bit to equalise our speeds. Her greater average speed has made cycling with her more enjoyable for me too, since I can no longer coast the entire way.

Similarly, the tyres that came on my Dawes Clubman (25 mm Vittoria Zaffiro) were shockingly bad. They felt like riding through treacle. When I changed them to the original 23 mm Michelin Pro4 Grip – not an especially fast tyre, but one with loads of grip – the speed difference was instantly noticeable and the comfort actually improved despite the width decrease.

Tyres matter a great deal.

you are right, lab tests for suspension losses would be difficult. They have bumpy rollers but they measure the tyre's losses, not the losses in the rider.

Jan Heine has at least demonstrated the nature of the effect, even if it doesn't allow real numbers to be put to it for a typical case. He used a power meter and/or changes in coasting speed to measure the effect of riding over rumble strips. IIRC his tests suggested that in extremis well over 200W could disappear in this way, and that fatter tyres meant that the losses could be greatly reduced.

Of course no-one expects to ride over rumble strips all the time, but it is easy to see how you could incur losses of tens of watts on an averagely crummy piece of tarmac.

BTW if anyone wants to play with the numeric calculator I'd suggest that for touring use you enter

- the correct weight
- a realistic Crr value (between 0.005 and 0.010) (cf 'clincher' value of 0.005)
- a frontal area of about 0.6m^2 (cf default value of 0.4m^2)
- a Cv value of between 1.0 and 1.2 (cf default value 'hoods' of 0.9)

this makes allowance for the (typically higher) riding position, mudguards, panniers, baggier clothing, tougher touring tyres etc. 100W sees about 19 kph or so this way (with Crr of 0.01 for averagely poor touring tyres), and at this speed you could easily save over 20W by choosing better tyres. Chances are that they would be less puncture resistant etc but there is no doubt that that less strong riders will benefit in group riding and that stronger riders will go a bit faster.

The calculator suggests that changing from bad tyres to really good ones (Crr 0.01 to 0.005) will increase speed by ~2.5kph for 100W power input. Obviously if your tyres are already pretty good or you wish to retain a fair amount of puncture resistance or longevity, there is less to gain.

cheers

Thanks Brucey, I think I see the light.

Although it would be very subjective if you could plot comfort against tyre pressure it would presumably slant the other way to RR and pressure. Would where they meet be the holy grail? Dream on.

As another poster wondered, I'm trying to see if my M greenguards feel like they are wearing in. It is hard to tell or whether I am just getting used to them.

When I first got them they felt like steel wheels but riding today they seemed pretty good, although still a bit harsh on rougher tarmac. I am riding in Thailand where most roads are made of silk, you back in the UK can only dream!

One thing I wondered about was how temperature affected the tyres. I started on these in UK late September, so probably high teens (°C), felt hard and and harsh but new. Then Korea, started mid 20s dropping off to mid teens, seemed less harsh. Now riding in mid 30s and they feel a lot better.

So is temperature an issue or are they wearing in, or am I just getting used to them. Will be back in the UK in a few weeks, so I will find out then.

rubber changes enormously with temperature, both temporarily (which is why rubber based suspension systems are less popular than they would be otherwise) and in some cases permanently. The latter effect can occur because of the short cycle time in the tyre mould; the rubber is only fully cured after a period of time + temperature and some of this always happens outside the mould.

[Car story alert; this happens with car tyres too; I once had a 'matched set' of tyres but they didn't seem matched to me. When I looked at them carefully it turned out that the tyres were identical apart from the manufacturing date. The one odd older tyre was harder and less grippy than the others and stayed that way for several years. I had to run it slightly softer than the others if I wanted it to grip similarly. Following a very nasty incident a few years ago I will choose not to use different tyres front and rear on a FWD car but I will do so if I have to on a RWD car.

More recently I wrecked one car tyre from a set of five on a FWD car, and another was worn anyway. The other three were OK still and one was unworn (a steel wheel as a spare, with a full-size tyre on it). However all three remaining tyres were several years old and I could no longer buy new ones of that exact type, leave alone the same manufacturing date. In a fit of inspiration I visited the local scrappies and by some miracle they had the exact same model car with the same spare wheel in the boot, also with an identical and unworn tyre fitted. The manufacturing date was within a few weeks of my good tyre too! So I am happy again; I have a perfectly matched set of tyres all round and the car handles properly as a consequence. Had I not found that I would perhaps have had to buy a set of four or five new tyres, and they would have been of a type that were never originally fitted to that chassis.]

cheers

After having had a think about this I guess that yes - rolling resistance is a major factor especially for us touring cyclists who potter at 10 mph - I mean, at that speed wind resistance is fairly low so all the energy you are putting into the bike must go in tyre resistance and mechanical friction of various cogs, chains, bearings etc - it's all got to be used in some way!

But the rolling resistance vs pressure and road surface is a complex subject and one that really needs investigating. One set of testers have the harder the tyre the less resistance, the other that there is an optimum setting - my gut feeling is that in the real world there is an optimal setting and that for a given tyre is dependent on weight and road surface with less weight = lower pressure and the poorer the surface the lower (within reason) that optimum will be.

But with us all using pathetically low powered engines it's well worth looking into;-)

I am a fan of flexible wide tyres for touring, so I have just bought a set of Compass Rat Trap tyres: 53 mm wide ultra flexible 26 inch tyres. I look forward to using them in the Spring, and I will report. In Winter I prefer the sturdier Conti Topcontact Winter ii, for better grip, and more comfort and speed once it gets cold and the rubber stiff.
As for the Marathon Greenguard, I once tested them for Schwalbe: I thought they were horribly stiff. The older Marathon HS368 were decidedly nicer (but not particularly long lasting). Companies like Schwalbe have to contend with the fact that the vast majority of people judge a tyre by two criteria: longevity and puncture resistance. Nice (comfortable and fast) tyres cannot possibly score very well on either. I think there is no point in getting a nice frame and then fit solid tyres.

Not all "rough" surfaces are "bumpy".

Many roads in Ireland are constructed with a top dressing of loose clippings rolled into the Tarmac. This is not terribly bumpy but is very rough and on occasion I have lost speed on shallow downhills. Under these conditions my ill informed guess is that lower pressure would considerably increase resistance.

willem jongman wrote:I am a fan of flexible wide tyres for touring, so I have just bought a set of Compass Rat Trap tyres: 53 mm wide ultra flexible 26 inch tyres. I look forward to using them in the Spring, and I will report. In Winter I prefer the sturdier Conti Topcontact Winter ii, for better grip, and more comfort and speed once it gets cold and the rubber stiff.

I too use the Topcontact Winter ii. This year due to getting fed up with other tyres I put them on early at the end of the summmer. I found that they didn't feel slower than the expensive version of Big Apples, haven't worn out and have generally been excellent. I would have to find a nice tyre to make me take them off.

I have tried all kinds of Schwalbe tyres and have developed a love/hate relationship for them. Several years ago I tried M+ and hated them at the beginning (slow, bumpy, not vey grippy and an odd shape for cornering) but by the time I had worn them out (with the blue showing through in a nice even line all the way round) they were fast and comfy. I have used Supremes and liked them but the side walls went soft in some places so the tyres went bulgy - ruining othersise good tyres. I have used big apples but found them almost impossible to seat properly on Andra rims despite trying all kinds of ways to let seat them but they were just about ok on other rims. I used marathon Mondials for off road touring because I got them cheap. They were so heavy that they made the bike feel dead. I have ordinary marathons on two bikes and they are OK but not as nice as the Conti Winters.

Anyway I am bored of Schwalbe and am going to experiment elsewhere!

I've just taken delivery of a couple of the new 2016 Ridgeback Expedition tourers for possible use at Breton Bikes - and they come with some Schwalbe Marathon Cross tyres. How anyone thinks that a tyre that audibly hums on the road is suitable for general touring - even expedition touring - is beyond me. Otherwise utterly brilliant bikes - I'll be posting a review soon...