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Buddsy

Blue Belt Tension

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Hi all

Ive just fitted a new blue Cam belt.

I know people used their phones to set the tension on this.

Was a satisfactory tension ever worked out?

All I can say is that it feels stiffer than a black belt from the start.

My auto tensioner plunger looks all the way in. Ive given the spring a few turns on the nut and Im slightly over tight on the gauge.

That said it feels about right to me by hand 45 degrees between cams 90 on long run.

Ive downloaded Easy Tune.

Just wondering peoples thoughts?

 

buddsy


 

"Belief is the enemy of knowing" - Crrow777

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Upgrade today to remove Google ads and support TLF.

Do a search, @gvy did some research and calculations to determine the frequency of the different belt types. Very interesting reading!

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I have made many mistakes in my life. Buying a multiple Lotus is not one of them.

 

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Buddsy,

It is physics infact:

frequency measurement vs tension on a belt has only a few variables :

And the only variable , that changes using a different belt (fe the blue gates belt) is the mass of the belt. So for the same lenght and tension frequency reading will only depend on the mass of the used belt.

Lotus specified for the HTD belt (OEM round toothed belt) between 100hz - 110hz spec on 30°BTDC .

I weighted this standard HTD belt at 164gr.

If you use another belt ( lighter or heavier) , weigh it and use the formula below

I have an old S2.2 with the older square toothed belts so in the example below I calculated the lower/upper frequency for two types square belt available : The trapezoidal belt lotus A907E191Z and the trapezoidal belt gates powergrip 5146. You will see the lighter the used belt the higher the frequency to become the same belttension. Think of it as a guitar snare. For the same tension a fat heavy snare will sound lower then a thin snare.

I never had a blue round toothed belt so I don't have its mass to calculate it for you, but you can do it yourself. To be honnest, I assume this blue belt will be heavier then the original HTD belt, so it will likely need around 95hz-100hz. I think the reason why people describe whining with the blue belt is because they tension it to high using the 110hz frequency, but this is an estimation. I have to weigh it to give an accurate answer.

Use a simple digital kitchen scale to weigh the belts. If you cant weigh it anymore because it is in and you don't want to take it out again. aim for 100h at 30°BTDC , listen if it does not whine when engine warm and you will be fine for the years to come.

image.png.c0b1ff429b08425c70145f029b7590bf.png

 

Edited by gvy
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Geert, are you sure about the weight of your HTD belt? I have just weighed one and it's 154 g (+/- 2 g) and other forum members have previously listed 154 g and 150 g (+/- 1 g) for their HTD belts.

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Derek,

That is an important question and I know I struggled with this one back in 2017 when I tried to get a solution to the ever returning question : what is the right frequency for a trapezoidal belt?

I had 3 old trapezoidal belts around to measure: one original , marked A907E191Z (157gr)  and two gates powergrip 5146.(121gr)

I weighted them on our digital kitchenscale and I doublechecked them at work on a  scale for measuring postpackages. I remember I got the exact same measurements out of those two digital scales, so we may presume these measurements  are right.

But I still had the problem that owning an old Esprit, I had no original HTD round toothed belt around to measure.  If I remember well ( it is 3 years ago) , I got a HTD belt from Escape and I measured it on the same kitchenscale at 164gr. That was 15 nov 2017 : https://www.thelotusforums.com/forums/topic/92186-timing-belt-tension-the-frequency-method/?do=findComment&comment=751067

Now I just have measured those 3 trapezoidal belts again with the same results, but I do not have Escapes HTD belt around to remeasure.

Edited by gvy

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I can add one comment to the above.

If you are sure you have an OEM lotus HTD belt ( marked B912E6697F ) and not an alternative version (fe the T188 or the T249, both 1mm smaller and lighter weight)  and you are about to change it for a new belt fe the gates blue belt. Measure them both on the same scale and use the formula above to determine the right frequency for your new belt.

The same goes for people that choose a cheaper T188 belt. I think that is ok, but be aware that this belt may ( infact will....)be lighter than the OEM B912E6697F and so it must be tensioned to a higher frequency then 100hz-110hz to end up with the same tension.

I had that with my lighter trapezoidal gates powergrip 5146 belt : I have to tension that one to around 120Hz.

Edited by gvy

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It's a genuine Gates B912E6697F belt I measured. The T249 is not a suitable replacement as the tooth profile is wrong.

Could you possibly have fat fingered a '6' instead of a '5' when typing the 164 g?

 

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6 minutes ago, sailorbob said:

It's a genuine Gates B912E6697F belt I measured. The T249 is not a suitable replacement as the tooth profile is wrong.

Could you possibly have fat fingered a '6' instead of a '5' when typing the 164 g?

 

I really don't know....It was 2017 ..... and I am becoming old😐

But again, for people who want to change : measure your own old B912E6697F and measure your new type belt on the same scale and use the formula's above and voila, you have your target frequency

 

Edited by gvy
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I just weighed a Gates T249 belt and it's 146 g (+/- 2g). Whilst the tooth profile differs slightly from the HTD belt I believe it's close enough for a reasonable comparison to be made. Therefore if we adjust the value for the belt width we get 146 / 25.4 x 26.4 = 152 g.

Any way, my apologies for going a little bit off topic.

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Derek,

since you have those round toothed belts available , ( I only have the two types of trapezoidal toothed belts ) I guess you are right about the weight.

I may add that incase of the trapezoidal belts, the difference between the OEM belt and the 1mm smaller Gates powergrip 5146,  I must add that the gates powergrip 5146 is not only 1mm smaller, but also 0.3mm thinner (2.5mm vs 2.2mm) so both dimensions are the reason for the difference 157gr/121gr between the two.

Edited by gvy

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Geert, I have recently changed the belt on my Excel as well, so I now have both the round and (several) HTD belts lying around. I'll put them on the digital kitchen scale, to add some more data. 😉

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I have made many mistakes in my life. Buying a multiple Lotus is not one of them.

 

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Thanks  Filip , always useful.

Most important is to define the OEM B912E6697F and its weight, since the only info we ever got from Lotus regarding target frequencies 100 -110 Hz is linked to this one. All other belts can be weighted and using the simple formulas above their target frequencies can be calculated .

I I read here in this topic that some people weigh it at 154gr , I must have made a mistake some years ago weighing it at 164gr

To be honnest ; In my case with an older style trapezoidal and lighter GATES powergrip 5146 type belt I am perfectly fine with a target frequency just above 120Hz, because I know this works.

But if people ask about it on the forum , It is just better to have the right info here. 

There are so many questions over the years about the right tension of the belt and about the frequency method on different fora and infact it is so simple with just the right starting point that it is word solving this for every one and every type of belt on a 910/912/907 type engine. The frequency method is by far the most accurate and for everyone with a smartphone and the right app the most accessable method to  measure that tension , but they have to know the target frequency that comes with the belt they are using

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If it is of any use to anyone, I have just weighed a Gates T104RB trapezoidal blue belt and it's 116 g (+/- 2 g).

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I tightened mine to what I thought felt about right.

Perhaps a little tighter than normal. It seems quite a stiff belt from the off so didnt need as much on the tensioner. 

When i checked the frequency it rang in at 117 as Geert suggested a figure of 120 I left it there.

 

buddsy


 

"Belief is the enemy of knowing" - Crrow777

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