Will switching an ebike to mullet cause problems with speed sensor?

My wife is looking at the Transition Relay as her next bike. The standard one is full 29er but Transition say that you can switch to mullet using their flip chip (like the more expensive PNW spec) but won’t the wheelsize change affect how fast the motor thinks it’s going as the little wheel will rotate faster? And won’t this mean the bike’s top assisted speed will be lower?

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Yes it will be slightly lower, unless whatever motor it uses has the option to change wheel size.

FWIW I ran a 27 wheel in the back of my Rise for a while and didn’t really notice any difference, I guess the difference will be more pronounced on a full power bike as it’ll be easier to hit the cut out speed.

I asked about putting a 29er on a Specialized Turbo Levo SL, the answer was that it required a configuration change that can only be done by a dealer. Didn’t really think too much about it then, but I can’t see any effect other than on the speed limiter.

Yes, the speed sensor will be slightly out & the motor will cut out at a slightly lower speed.

Have you checked the bikes software app to see if there is an option to select rear wheel size? If there is then that should correct any issues.

I did some calculations (based on the circumference of a 29″ vs 27.5 tyre which put it at between 1 and 1.5mph.

FYI, the Trek Fuel Exe can be run as a mullet and in Treks FAQ they state:

Is it compatible with other wheel/tyre sizes?
Yes, this bike is mullet-compatible. Riders can run a 27.5” rear wheel for better bike-to-body clearance and more agile handling. This requires the Mino Link to be in the High position. Note, the smaller wheel circumference will affect the accuracy of the speed reading and cannot be adjusted. For example, switching to a 27.5 x 2.5 rear wheel and tyre combination will reduce cut-off speed by about 0.6 mph (1 km/h).

As with anything YMMV depending on tyre sizes etc. Having run my Fuel Exe as mullet the differences are very subtle but TBH I really didn’t notice any change to the speed/assist cut off

switching to a 27.5 x 2.5 rear wheel and tyre

without knowing what tyre and rim width before and after is vague as anything.  I used same rim width, tyre width and model. 2213 vs 2333 mm for a 2.4 tyre so a 5% change = 1.3kmh

Cheers. I assumed as much.

When I put a 27.5 on the Rail I had the shop adjust the wheel circumference to suit.

without knowing what tyre and rim width before and after is vague as anything.

That’s blurb direct from Trek for the Fuel Exe so the previous tyre and rim size is known.

With my plus wheels I’m about 0.5 mph slower than my watch says.

The flip side, of course, is the bike will accelerate faster…

…and become more of an annoyance as the motor cuts out sooner so more effort is required to go fast…

Ebikes are great, but there have been several threads about making the cut out a higher speed so I suspect a lot of people will just dismiss the idea knowing the cut out is now lower.

I don’t mind ebikes – they all passed me going several times faster than I was going all weekend, so it isn’t going to make any odds to me!

Specialized e-bikes can all be configured for the correct wheel size at any S dealer.

I would suppose others would have this option too, but do not know for sure. Ask a dealer of your bike brand about it.

“ Specialized e-bikes can all be configured for the correct wheel size at any S dealer.”

I’m hoping this is the case because I just ordered a (bargain) Hope hub and DT Swiss rim to try mulleting my Levo!

When I put a 27.5 on the Rail I had the shop adjust the wheel circumference to suit.

Is there any known system reason for doing this?

All I can think of is that manufacturers advise it so they don’t marginally stray out of the law with the cutoff speed limiter.

On the Bosch Flow app you can set the circumference yourself… no need for dealer intervention.

“ All I can think of is that manufacturers advise it so they don’t marginally stray out of the law with the cutoff speed limiter.”

It’s either legal or it’s illegal. If an ebike assists beyond 25km/h then it’s illegal in the EU and UK. I believe manufacturers have made it harder to hack motor cut-out speeds so they’re not liable for accidents caused by bikes causing injury when powered to higher speeds.

Yes, so not a system reason.

As an individual, perhaps you’d take your bike in for reconfiguration to ensure you’d not be in trouble should you be involved in an accident on your “illegal ebike” which can do 1.3km/h too fast.

Cheers guys.

It wouldn’t be an “illegal bike” – putting a 27.5″ rear wheel on will mean the motor cuts out before it hits 25km/h (not after).

The only way it might be deemed “illegal” is if the software was configured for a 27.5 wheel but a 29 was actually fitted.

“will reduce cut-off speed by about 0.6 mph”

“…and become more of an annoyance “

Really? I mean, according to the Kiox my bike cuts out at around (can’t say I’ve made a proper record of it) 16.6mph. Would do my head in if it cut out at 16mph, oh man, the thing just wouldn’t be worth riding! lol!

It was the latter situation I was thinking of despite quoting the opposite.

This thread is worth a read….   https://www.emtbforums.com/threads/wheel-circumference-calibration%E2%80%A6.28027/

The bottom line seems to be that only the speed indicated on the display (and distance travelled?) is affected by changing the cirumference but doesn’t affect the “actual” speed at which the assistance cutoff occurs.  Can’t properly get my mind around it tbh but I did change my circumference value in the Bosch Flow app to reflect the correct value for my rear tyre.